Download Archaeoastronomy, Astronomy of Celts, A. Gaspani

Astronomy in the Celtic Culture
Image: Stonehenge megalithic site at sunset, December 2008.
Copyright: Simon Wakefield. Source: Wikimedia Commons.
In the present paper we discuss how the Celts practiced Astronomy at a high level both for
speculative and for practical purposes. This is attested by the calendric finds that show an excellent
knowledge of the apparent motion of the Moon and of the Sun. The astronomical knowledge of the
Druids was to a large degree inherited from preceding cultures that built the megalithic monuments.
It is highly probable that an appreciable influence was also exercised by the Greeks philosophy and
cosmogony, in particular the Pythagorean, but a number of independent discoveries were also
probably made by the Druids themselves.
1
Introduction
The investigation of the transmission of the knowledges in the fields of mathematics and astronomy
is one of the important means of establishing the cultural level reached by an ancient civilization.
This is because stylistic motives, religious or philosophical doctrines may be developed
independently by a culture and can travel great distances through a wide number of diffusion
processes, while generally astronomical methods, usually complicated, require the use of scientific
treatises in order to be diffused from one culture to another.
In the Celtic culture there was a tendency to concentrate all the knowledge within a restricted class
of persons, the Druids, who transmitted orally a great number of mnemonic rules to their scholars
over many years of training. The Celtic society compared with other ancient civilizations was
characterized by an astonishing number of persons devoted to the teaching of "Natural Philosophy".
No written documents or scientific treatises were produced except for some calendric tables
compiled much later mainly for ritual purposes when the process of Romanization of the Celtic
people was well advanced. Some fragments of these calendric tables were found during the last
century. The best example was found at Coligny (AIN) in 1897. With the discovery of the Calendar
of Coligny it became clear that astronomy was currently practised by the Celts, at least among the
druidic class.
The existence of megalithic monuments widely spread in Europe suggested that astronomical
observation was currently practised also in more ancient times, certainly before the Celtic culture
flourished.
The astronomical knowledge encoded in the Calendar of Coligny seems to be very sophisticated, at
least limited to the apparent lunar and solar motions on the celestial sphere useful for genuine
calendric purposes, but after a deeper analysis it becomes quite evident that this find is only a limited,
rather important, testimony of the existence of astronomical knowledge in Celtic culture.
From the time of the discovery, various attempts were made in order to decode the features observed
on this archaeological find (see the detailed historical review given by Duval and Pineault in 1986
(RIG III)), but despite the able contributions of a great number of researchers, many facts remain at
present unclear and unexplained.
First of all, the rules of its practical use deserve a number of questions so far unsolved.
This paper is not devoted directly to the interpretation of the Calendar of Coligny, work being at
present in progress and the publication of the results being planned for the near future, but the aim
of the present work is to consider the Celtic culture in a wider context in which astronomy plays a
very important part.
2
The existence of a find like the calendar of Coligny raises a number of very interesting questions
about the actual astronomical as well as the mathematical knowledge among the Druidic class; also
about the existence of speculations about the cosmogony arising from a need to find some models
explaining the apparent motion of the Sun, the Moon, the visible planets, and the stars.
Another set of questions about astronomical knowledge arises from the need to carry out some
measurements of the position of the observed celestial bodies in order to collect the basic data
required to compute the calendars.
It is very likely that astronomy as well as calendric practice were exclusively reserved to a privileged
religious social class that mastered the culture and the knowledge of natural philosophy, so it is very
likely that the ability to make predictions about the occurrence of the periodic astronomical
phenomena, such as the eclipses or the visibility of the planets, has strengthened the belief in the
magic power of the members of this class, the Druids. It is well known that the role of this class was
not just consultative, it also dominated the local political life among the Celtic community.
An interesting hypothesis is that the astronomical knowledge of the Celts was pre-existent, having
been inherited from some more ancient cultures which built various megalithic monuments that can
be still seen in Ireland, Wales, Brittany and other countries in the northern Europe.
This must surely be true at least partially. In our opinion the astronomical knowledge of the Celts
was not limited to the domain of the calendric practice (involving the Lunar, Solar motions and related
fields), it also encompassed the domain of the stellar astronomy. This was obviously limited to the
data acquired by the naked eye observation and monitoring of the sky visible at that time.
The aims of the present paper are various, the main one is to show that astronomy was currently
practised by the Celts; another is to suggest that the astronomical knowledge diffused among the
Celt was concerned not only with the Sun and the Moon, but also with the stars.
Since the knowledge of the exact physical mechanisms ruling astronomical phenomena were
obviously unknown to the Druids, we suggest that the methodology for the processing of the
information obtained by continuous observations follows the so called recursive learning based on
stochastic estimation. This methodology is required because the gained information is corrupted by
inherent errors ("noise", in technical jargon).
In this work we stress out that Druids were able to predict eclipses as well as the tides as natural
consequences of their knowledge of the lunar motion. Additionally, we propose that stellar astronomy
would rule the seasons as well as the times of the religious Celtic festivals.
Another interesting problem we solved taking into account stellar astronomy is the real season in
which the Celtic year started or, in other words the correct placing of the Trinox Samoni along the
tropic year. It is easy to think that perhaps the Trinox Samoni festival can not only be related to the
lunar cycle, but also that the correct choice of the most relevant lunation can follow some additional
astronomical circumstances of stellar significance.
3
In order to verify our hypotheses, extensive computer simulations were performed in order to
simulate the sky visible by Celtic places along the year in a geographical location placed roundabout
the virtual barycentre of the distribution of the Celtic peoples during a time span ranging from the
2000 B.C. down to 500 B.C.
Such accurate dating has no sense from the archaeological point of view, but the computer program
does not accept fuzzy dates to carry out numerical simulations, so we adopted round figures only for
the sake of the numerical satisfaction of the computer input.
The first date can be considered as the date in which various famous megalithic monuments existed
and were in all probability being used for astronomical work. In fact, the megalithic monuments seem
to have been used since up to 4000 B.C. The second date represents a period in which the Celtic
culture was most certainly well developed.
Astronomical knowledge required in order to establish a lunisolar calendar
Before starting on our discussion it is useful to make some preliminary considerations about the
minimum necessary astronomical knowledge required in order to establish a calendar of the kind
that was found near Coligny.
First we will briefly describe the general problem of to establishing a lunisolar calendar like the
Coligny calendar. This problem will be attacked from the point of view of mathematical astronomy
without taking into account any social implications.
Nature provided the ancients with three time units:
The solar day
The lunar month
The solar tropic year
Unfortunately, these three natural time-units have the drawback of not being easily commensurable
with one another.
The computation of a reliable calendar is equivalent to identifying three integer numbers A, B, C
capable of satisfying with reasonable accuracy the following simple mathematical relation:
A years = B months = C days
(1)
over a reasonably long time lapse.
4
This mathematical relation is very general and represents the basis of every lunisolar calendar
established through the centuries by all ancient cultures.
The degree of accuracy of a given calendar depends upon the efficiency of the choice of A, B, C.
The relation given above has a cyclical character so that it can be interpreted in terms of the number
of complete revolutions of the Earth round the Sun and of the Moon round the Earth.
The mathematical relation (1) can be easily split into the following simple equations:
C
1 year = - days
A
(2a)
C
1 month = - days
B
(2b)
B
1 year = - months
A
(2c)
The relations (1) and (2,a,b,c) are arithmetically equivalent, but they represent two very different
historical views of the problem of designing a lunisolar calendar.
The relation (1) can be obtained by simply counting the elapsed years and months without requiring
high accuracy.
The tern of number A, B, C may be simply estimated and they can remain reasonably valid for a lot
of cycles without requiring correction.
The solution represented by the equations (2a,b,c) requires, to be obtained, a long time of accurate
astronomical observations with the object of determining the numerical values of the terms of the
fractions C/A, C/B and B/A accurately for them to remain valid for a reasonably long time span.
Obviously accurate positional astronomy can be achieved by relevant as well as accurate
observational methodology. In addition, accurate positional astronomy requires accurate instruments
which at that time were also the megalithic monuments with their alignments. In fact, it is necessary
to establish accurate times for the solstice in order to get a reasonable value of the length of the
tropic solar year and a deep insight into the empirical rules of the complicated apparent motion of
the Moon that can be obtained by measurements of its position in the sky over a long period.
5
In our opinion good methodologies were known by the Druids and good values of the constants A,
B and C were determined by continuous observational efforts.
In this context we must remark that the calendar of Coligny embodies a mathematical model that
require an excellent, but empirical, knowledge of the geocentric theory of the Lunar motion as well
as some very good empirical ideas on the laws regulating the apparent motion of the Sun in the sky.
Obviously such knowledge is of a totally empirical nature and based very probably on the
experimental search for some regularity among the apparent configurations of the visible celestial
bodies.
The regular observation of the celestial bodies over century after century can suggest some
empirical, but very useful models in making predictions, such as for example, the Meton and the
Saros Cycles proper to the lunar motion and very useful in the prediction of lunar and solar eclipses.
We must also take account that in Celtic times there was no mathematical formalism useful in
developing computations so that all speculative activities in the astronomical domain had to be made
by purely empirical reasoning.
We propose that if the Celts were able to build the Calendar of Coligny, then they were also perfectly
able to make stellar positional astronomy.
At this point a very interesting question arises i.e. which method could be used by an ancient
population to identify a given star or a given group of stars in the sky throughout the years.
We can find an answer, later in this paper, in the science that studies the biological neural network
ruling the human visual system.
Why to observe the Moon?
The Moon is, after the Sun, the easiest observable celestial object, so many ancient cultures based
the planning of their social life on the time scan regulated by the apparent motion as well as the
phases of the Moon.
At first, the duration of the lunar synodic month, nearly 29.5 solar days, was a useful intermediate
time unit between the year and the day for practical applications. Additionally, the time required to
go from one phase to the next is about a quarter of the lunar month, about seven solar days, this
suggests another unit for the computation of this short lapse of time: the week.
Another interesting feature is that about 13 lunar synodic revolutions correspond approximately to a
solar tropical year. This ensemble of time divisions suggested an easy and useful technique for the
scanning of time and the planning of future events.
6
The observation of the Moon in addition to the calendric, agricultural and social use, made it possible
to discover some correlations between its cycle and a number of natural phenomena; for example,
the female fertility cycle apparently closely linked on the lunar cycle, and the correlation between the
tides and the lunar phases.
Obviously the Druids were not able to give a reasonable physical explanation of these phenomena,
but it is very likely that they were able to use them. They limited themselves learning, by repeated
observation, the existence of the correlation between the lunar cycles and the natural phenomena
and to thinking of them as rules to apply in practical situations.
Druids use stochastic estimation
In the domain of Statistics, the expression 'Stochastic Estimation' identifies a branch dealing with the
identification of a set of parameter describing a given system (or a natural phenomenon) at a given
time by successive observations of their outputs usually obtained working under some uncertainty
conditions.
It is interesting to draw a parallel between the way in which the Druids learnt from the observation of
the sky and the development of today's statistical models describing a very complicated
phenomenon.
Starting with a preliminary description of the phenomenon under study the Druids would for a long
time observe its repetition gaining ever newer experience about it.
The information obtained in this way is not clean, but dirty because it could be incomplete, distorted,
misinterpreted or misunderstood, or because the phenomenon might be badly observed, and so on.
So, there is the need to recursively average the information accumulated by long time monitoring of
the phenomenon under study in order to recognize and identify its principal characteristics.
This enabled the Druid to make predictions about the behaviour of the natural phenomenon basing
them on past experience. Past experience can be a number of predictions, formulated on the basis
of partial knowledge of the phenomenon, and corrected by comparing them with the observations.
Since the observations usually were of limited accuracy there are always inherent errors in the newly
acquired information, so the process will produce perfect and complete knowledge of the
phenomenon in time virtually infinite, in practice very long.
This technique is the typical learning process that goes on in the biological neural network of the
human brain. Mathematically this "modus operandi" is called Recursive Learning based on
Stochastic Identification.
7
At present, about 2500 years later, when it is required to build a mathematical model of a natural or
artificial phenomenon so complicated that a relevant physical theory is not available, the unique way
to go about it is to observe it repeatedly measuring the output when a given input is entered.
After a sufficient number of trials, we are in a position to search for statistical correlations between
the outputs and the corresponding inputs.
If the degree of correlation is relevantly high, an empirical law can be formulated that would be able
to predict the response of the system under observation without knowledge of the true physical laws
ruling it. In a few words, past history enables extrapolation for the future.
The Druids worked in the same manner: the repeated constant observation of the sky after a
sufficiently long time suggested the existence of cycle and periodicities.
At first the knowledge was very approximate, so the Druids discovered timescales (pseudo
periodicities), after further observations had made it clear that the timescales were genuine
periodicities, so that predictions could be made.
This knowledge is "a posteriori", but this enables successful "a priori" prediction for future cycles. In
this sense we may affirm that the Druids reasoned by unconsciously applying the so called Bayesian
Statistics in studying the natural phenomena.
It can be instructive, at this point, to give two examples.
The tidal cycles of the sea could be correctly correlated with the motion and the phases of the Moon
in the sky following the reasoning technique outlined above. So by observing the Moon it is possible
to predict the sea level without explicit knowledge of the gravitation theory, and this can be useful for
practical purposes of navigation and fishing.
But recursive learning sometimes has some problems as the following example shows. It is very
likely that the female cycle, coupled with some mystic beliefs about fertility was related to the Moon,
but it has no physical relation with the Moon's motion!
This example is very significant because it shows that stochastic estimation can generate a reliable
prediction technique also for physically uncorrelated and independent phenomena. The funny thing
is that it works....
The first example deals with an existing physical relation between two natural phenomena, the Moon
cycles and the sea tides, that was unknown to the Druids, and also too complicated for them to
understand. The first correct explanation of the tides was given by I. Newton about 2200 years later.
The second example deals with a non-existent links between the two observed phenomena, but from
the point of view of recursive learning based on stochastic estimation, the mechanism is the same,
so the method works well in both cases.
8
It is useful to remember here that there may be some physical connection between these two
phenomena, unknown in the present state science.
In our opinion the science of the Druids was composed mainly of a great number of empirical
correlations, discovered and improved through the centuries, between the behaviour of the natural
phenomena. These rules although empirical worked well in practical situations so there was a
tendency to believe also in the existence of true physical relations between unrelated phenomena.
The motion of the Moon
In order to better understand the problematic connected with the observation of the Moon we will
briefly describe the main features of the motion of the Moon today.
The apparent motion of the Sun in the sky occurs on an apparent trajectory, that is the projection of
the orbit of the Earth on the celestial sphere, named Ecliptic.
The Ecliptic is inclined on the celestial Equator by a slowly varying angle that, at the time which
concerned our simulations (500 B.C.), assumed a value of 23.8 degrees.
This particular angle is called Obliquity of the Ecliptic and at present (1995 A.D.) its value is 23.4
degrees.
Its varies cyclically with a period of 26000 years due to the precession of the axis of the Earth.
The Moon circulates around the Earth running on an elliptical orbit at an average distance of about
60 times the radius of our planet.
The eccentricity of its orbit is about 1/18 and varies through the times.
This distance increases with a rate of 4.4 cm/year due to the transfer of angular momentum from the
Earth to the Moon as a consequence of the tidal drag.
At the time of the Celtic culture the Moon was about 110 meters closer to the Earth. This coincides
with the slowing down of the speed of rotation of the Earth.
The minimum and maximum distances between the Moon and the Earth are respectively 55.4 and
66.1 times the average radius of our planet. This variation is due to the eccentricity of the lunar orbit.
The points of intersection of the lunar orbit with the Earth's are named "Nodes" and they move due
to the combined effects of the gravitational attractions of the Sun and of the Earth.
This motion is a retrogradation i.e. the nodes move in the direction opposite to the direction of the
orbital motion of the Moon.
9
A complete rotation of the nodes requires 6793.39 solar days (18.66 solar years, which is a
remarkable number from the historical point of view).
The retrogradation of the nodes involves a great excursion between the minimum and maximum
values of the declination of the Moon in the sky.
The orbit of the Moon is inclined by 5.15 degrees on the Ecliptic with a periodic variation of 0.15
degrees in modulus within a period of 173.3 days.
This is half the so called Eclipse Year which is the synodic period of the Moon's ascending node, i.e.
the lapse of time between two successive conjunctions of the node with the projection of the Sun on
the celestial sphere. The duration of the Eclipse Year is 346.62 solar days.
Since the obliquity of the ecliptic is 23.44 degrees and the inclination of the lunar orbit is 5.15
degrees, the oscillation in declination will vary from 18.19 to 28.59 degrees.
When the ascending node of the lunar orbit coincides with the ascending node of the orbit of the
Earth (i.e. the position of our planet at the Vernal Equinox) and the descending node coincides with
the Autumn Equinox, the Moon reaches its maximum boreal declination, in the sky, that is 28.44
degrees.
In the period during one sideric revolution the declination of the Moon will oscillate between 28.44
degrees above the celestial equator to 28.44 degrees below.
Nine years and a third later the Descending Node will coincide with the Vernal Equinox then the
declination of the Moon will have the smallest maximum boreal value (as well as the austral)
This extreme declination will be 18.3 degrees i.e. during a sideric revolution the Moon oscillates from
18.3 degrees above the Celestial Equator to 18.3 degrees below.
It can be interesting to point out that during a complete synodic revolution (a cycle between two
successive full moons), the Moon covers about a full circle on the celestial sphere inclined by 5.15
degrees over the Ecliptic.
At full Moon, the Sun and the Moon are located the former one in the austral hemisphere and the
latter in the boreal or vice versa.
This situation explains why at the latitudes typical of the Celtic world sites during winter time, at fullMoon, the Moon is located very high on the local horizon (it has a positive declination because the
Sun has a negative one) and the situation is the opposite during summer time.
During summer time, at full moon, the Moon is low on the local horizon, it has a negative declination
because the Sun has a positive one.
10
From the calendric point of view, it is important also to take into account the Draconitic and the
Anomalistic revolutions of the Moon around the Earth.
Under the effect of the retrogradation, the ascending node of the lunar orbit moves towards the
Moon, so the interval between two consecutive passages at the same node is shorter in relation to
the sidereal revolution period. This is the Draconitic Revolution that is 27 days 5 hours 5 minutes
and 35.8 seconds of mean time.
The Anomalistic revolution is the interval between two passages of the Moon at the Perigees, the
points of the orbit closest to the Earth.
The minimum distance between the Earth and the Moon is 55.4 times the mean radius of our planet,
this is the Perigee Distance. The maximum distance, at the Apogee, is 66.1 radii of the Earth.
The line connecting the Perigee with the Apogee is called Line of Apsides. This line rotates under
the combined gravitational effects of the Sun and the Earth in the direct sense, so that the Perigee
performs a complete rotation on the celestial sphere in relation to the stars requiring 3232.59 average
solar days involving an increase of the longitude of the Perigee of 6 minutes and 41.054 seconds of
arc. The duration of the Anomalistic revolution is 27 days, 13 hours, 18 minutes and 33.1 seconds.
Summarizing, there are five kinds of lunar revolutions: 1) the sidereal 2) the tropical 3) the synodic
revolution (lunation), 4) the draconitic revolution and 5) the anomalistic.
It becomes clear that the motion of the Moon is so complex that a number of famous mathematicians,
including Newton, Gauss, Euler, Delaunay and others, have spent a relevant fraction of their lives
trying to describe with the greatest accuracy attainable the apparent motion of the natural satellite of
our planet.
But if the mathematicians of the last few centuries have challenged this problem equipped with the
best computational techniques available at the time, the same thing would be done by Druids, as
well as by other ancient populations, by empirical computation without analytical mathematics in
order to obtain the best agreement possible between the lunar and the solar motions for the purpose
of devising a calendar.
Another task that must have been performed by the Druids is the elaboration of appropriate
computing techniques able to predict the occurrence of the eclipses.
Since eclipses in order to happen in a given geographical place, require a relevant reciprocal position
of the Earth and of the Moon in relation respect to the Sun, a number of dynamical constraints have
to be complied with.
As a first approximation, we can say that a given eclipse can happen when the Sun, the Moon and
the Earth return after a given time to the same reciprocal location in space.
11
It is our opinion that the Druids knew that in a time span of 6585 days (18 years plus 11 days)
eclipses would repeat in the same cyclical order.
The reason for this phenomenon, called today "The Saros Cycle", is very simple if we consider the
motion of the Moon as described above.
Working with the limited computational resources available to the Druids and applying recursive
learning, we can infer the correct length of the Saros Cycle by monitoring the sequence of the
eclipses observable in a given geographical location for at least, say, five or six cycles, about a
century. In other words, starting without previous knowledge, after two generations of Druids
monitoring the lunar motion, the third generation was able to predict eclipses.
After a synodic lunar revolution (29.5306 days) the Sun and the Moon return to be in conjunction
with each other. After every draconitic revolution (27.21222 days) the Moon returns to the ascending
node of its orbit. After every anomalistic revolution (27.5545 days) the Moon returns to the Perigee
of the orbit.
So 223 synodic cycles are equivalent to 242 draconitic cycles and have the same duration of 239
anomalistic revolutions, to within a few fractions of a day, constituting the 18 years and 11 days of
the Saros Cycle.
After one Saros Cycle the Sun, the Moon and the Earth are located with high approximation at the
same reciprocal positions in space then the eclipses repeat in the same order, but not on the same
days. It is easy to show and also easy to demonstrate that in order to predict the occurrence of both
solar and lunar eclipses with relevant accuracy knowledge of the Saros Cycle alone does not suffice.
Taking into account the features of the lunar motion, it becomes clear that the Moon returns 255
times to the ascending node of its orbit as well as 235 times to the Zigyzies in 19 average solar
years. This implies that after 19 average solar years the Moon has covered 255 draconitic
revolutions, but also 235 synodic ones, so that after 235 lunations equivalent to 6940 days the
eclipses will repeat on the same day.
The accumulation of some slight fractions of time can, sometimes, involve the missing of the
predicted eclipse, but this happens very rarely.
This cycle was discovered in the fifth century B.C. by a Greek astronomer named Meton.
Since the Meton Cycle contains a no integer number of anomalistic revolutions, the apparent relative
diameters of the Moon and the Sun vary and so does the occurrence total or annular eclipses.
In our opinion also the Meton Cycle was known by the Druids at least as a consequence of a degree
of Greek influence, but there is a reasonable assumption of an independent discovery resulting from
their long monitoring of the lunar apparent motion in the sky.
12
Taking into account the relative frequencies of the lunar and solar eclipses in a single Saros Cycle
we can infer that over the whole world 72 eclipses will happen, usually 43 solar and 29 lunar ones.
Every year happens a number of eclipses intermediate from a minimum of two to a maximum of
seven with the constraint that when only two eclipses happens in a given year, they forced to be
solar ones. Due to the cyclical variation of the inclination of the orbit of the Moon, the lunar and solar
eclipses will be spaced out by a time span of 173.3 days or integer multiples of this value.
The correct prediction of eclipses could be very important for ritual purposes. The predicted day may
have borne some particular sign meaning either ill-omened or propitious day. In fact, for a number
of ancient people eclipses are announce of some misfortunes. At present we do not know the real
Celtic ritual interpretation of eclipses.
How the Druids could predict eclipses
The calendar of Coligny clearly suggests that the Druids were aware of the apparent motion of the
Moon in the sky as well as of its characteristic periodicities. The ability to predict eclipses is a natural
consequence of this knowledge, thus it is difficult to refrain from concluding that the Druids were able
to predict eclipses.
The method apparently used could be as follows.
If the orbit of the Moon lay on the Ecliptic, the declination of our natural satellite would change, in
500 B.C., from about -23.8 to +23.8 degrees, the value of the Obliquity of the Ecliptic at the time of
our simulations, and back again in one sidereal lunar month.
Because of its orbital inclination of 5.15 degrees, the declination of the Moon could change from 28.95 to +28.95 degrees, at one extreme, or from -18.65 to +18.65 degrees at the other, depending
upon the position of the Moon's nodes.
The period of oscillation from northerly maximum to southerly maximum and back again was still one
sidereal lunar month. The northerly maximum and southerly declinations changed during a sidereal
month from 28.95 degrees in modulus to 18.65 degrees in modulus, and back again, in 18.6 tropical
solar years. In fact, this involved the existence of four turning points in the moonset azimuth
corresponding to the position on the local horizon of the following four turning points in the declination
of the Moon: -28.95, -18.65, +18.65 and +28.95 degrees.
Continuous astronomical observation carried out over scores of years made it possible to fix, with
high precision, these positions on the local horizon as well as the period of 18.6 years. But continuous
observational efforts could, in our opinion, imply that the Druids had also discovered the oscillation
of the inclination of the orbital plane of the Moon in relation to the Ecliptic due to the Sun's
gravitational disturbing effect.
13
This oscillation is, as listed above, of 0.15 degrees in a period of half an Eclipse Year i.e. 173.31
solar days. It is this oscillation that enables predictions about to be made eclipses.
Every 173.31 days the Sun is placed at one of the nodes of the lunar orbit and at this time the
oscillation of the orbital inclination reaches a maximum. It is only at such times that eclipses can take
place. If the Moon is new, a solar eclipse will occur, if the Moon is full, a lunar eclipse will occur.
If the Druids could keep track of the period of 173.31 days for long enough, and observe its
correlation with the eclipses, they would be able to predict successfully future eclipses.
Knowledge of the Saros and the Meton cycles involved the attribution of consultative importance to
a class of people able to predict eclipses which could be seen as some transaction between the deity
and nature.
What druids knew about the shape of the Earth
An interesting fact is that the observation of the shape of the moving shadow of the Earth visible
during an eclipse of the Moon suggested clearly the round shape of the Earth, but this proof of the
nearly spherical shape of our planet is due to Aristoteles, at a later date.
Also the continuous measurement of the height of the stars on the local horizon makes it possible
after some reasoning, to detect the roundness of the Earth.
The affirmations of Posidonius as well as Caesar's about the knowledge the Druids had of the shape
and the measure of the world open the mind to some interesting extrapolations about the Celtic
cosmogony.
We do not here explicitly affirm that the Druids knew for certain that the Earth was round, but this
information could easily be reached in the Celtic places brought by the Greeks living in the colonies
situated on the southern shores of Gaul. It is a well-accepted fact that there were cultural exchanges
between the Druids and the Pythagoreans, so we could put forward the following considerations.
Theofrastus, disciple of Socrates, reports that Parmenides was the first to propose the idea of the
roundness of the Earth. Parmenides, a Pythagorean philosopher, lived around the V and VI centuries
B.C., about the time of the beginning of the Celtic culture. His nice demonstration of the roundness
of the Earth was based on the consideration that "The spherical shape is the only physical shape
capable of maintaining equilibrium".
This is an unusually argumentation modern taking into account the fact that he does not know
anything about the shape of equilibrium of the self-gravitating bodies. This intuitive thought will be
the result, 2000 years later, of the works of mathematicians such as Radau, Clairaut and Roche who
lived in the XIX century after Christ and Chandrasekar in the XX. The apparent spherical shape of
the celestial sphere might have also suggested the hypothesis of a spherical shape for the Earth.
14
From the experimental point of view, the hypothesis of a spherical shape for the Earth was very
attractive because it was able to explain, in a simple and natural way the mechanism of the setting
of the Sun, the Moon, the Planets and the stars below the western horizon and their corresponding
rising the following day above the eastern horizon.
The hypothesis of the sphericity of the Earth was accepted by all only a century or so later, in Plato's
time, so the notion of a round Earth was universally accepted in the Greek astronomical community
about 400 B.C. This date corresponds to the full development of Celtic culture.
The cultural exchange between the Druids and the Pythagorean philosophers in Greek colonies, like
Massilia and others, must surely have involved the diffusion of this idea among the Celtic
astronomical environment. Obviously an independent discovery by the Celts is also possible.
because also a number of other astronomical and terrestrial phenomena, easily observable by the
Druids, could have suggested the roundness of our planet, but this is at present undocumented.
Another interesting problem concerns the rotation of the Earth.
The daily rotation of the celestial sphere was at first supposed to be only apparent by Philolaus,
another Pythagorean philosopher, and due to the rotation of our planet around its axis, but the exact
solution of the problem was due to Heraclides in the 4th century B.C. Although this was the right
solution, it was not universally accepted in the Hellenistic world, but the idea may have been diffused
at least in the Pythagorean cultural environment and so diffused also among the Druids.
... and about the measure of the Earth
The problem of the shape was not the only one dealing with the description of the Earth; there is
also the question of it measure.
This problem was elegantly solved by Herathostenes, about 230 B.C. by measuring the difference
in the height of the Sun at the Summer Solstice at Siene (Aswan, Egypt) and Alexandria. The results
of the computation were a diameter of 12629 Km, with an error of only 113 km as compared with to
the figure accepted today. The same author was also able to get an accurate determination of the
Obliquity of the Ecliptic; his value was 23 degrees 51'. The true value for that time was 23 degrees
and 43', the error being thus only about 0.56 %.
This accurate measure of the diameter of the Earth was obtained during a period when Celtic culture
was developed. We may estimate that the diffusion of these ideas took about a century to reach to
the Celtic world, but in our opinion they did reach it and the Druids assimilated them.
Taking our reasoning further we must bear in mind that Aristarcus lived in the 5th century B.C,
Appollonius in the 3rd century B.C and Hipparcus in the 2nd century B.C, i.e. during the full
development of Celtic culture. This means that a relevant fraction of Greek astronomical science
was available and could be used by the Celts.
15
How the Druids could predict tides
The knowledge of the movements of the Moon could be useful in a wide domain of practical situations
Interesting, for example, would be the possibility of taking advantage of the ability to make reliable
predictions of the time of the new Moon for planning purposes in a war situation. This is of
fundamental importance for a people of warriors.
In the same war-like context the correct knowledge of the correlation between the tidal cycles of the
sea and the lunar configuration in the sky can be very useful.
The defeat of the Caesar's ships swept away by an exceptional unpredicted tidal wave, in 55 B.C.,
on their way to Britain is well known. This exceptional tidal wave was unpredictable for the Romans,
but in our opinion it could have been easily predicted by a native Druid who would correlate lunar
with the tidal cycles.
Having assumed from the start that the Druids had good empirical knowledge of the cycles of the
apparent movements of the moon it is natural to say that they were also able to predict the periodic
variations of the sea level correctly.
The main tidal phenomena are that the successive high waters (high tides) are separated by about
12 hours and 25 minutes, as are the times of successive low waters (low tides) occurring half-way
between two consecutive high tides. In addition, particularly high tides, named spring tides, occur at
new Moon and full Moon. Half-way between these spring tides, at the first or the last quarter, the
day's high tides are at their lowest (neap tides).
This exposition is strictly true if we suppose the Earth uniformly covered by the sea without land
masses and varying depths in the oceans. In the Mediterranean Sea the amplitude of the tides is
greatly diminished by the land masses and the reduced depth of the sea, which is an internal sea.
The Romans predicted the oceanic tides on the basis of their past experience with the Mediterranean
Sea so they greatly underestimated the height of the tidal wave during the journey to Britain in 55
B.C. In addition, the journey was planned at the time of the spring tide.
As a consequence of the correlation between the lunar phases, the Moon's daily position in the sky
and the sea level, the practical algorithm of prediction of the tide adopted by a Celtic Druid would be
very simple and immediate.
The tidal wave will occur twice during a day with a time-lag of about 1 hour according to the
retardation of the Moon's culmination (transit at the local meridian i.e. the maximum height on the
local horizon). Moreover, the tidal waves were higher when the Moon was full or new and lower at
the first and the last quarters. These two simple statements are sufficient for the successful prediction
of tides.
16
The role of Astrology
In the present paper we deal exclusively with Celtic astronomy, but it is natural to think that
astronomy and astrology were far from being two distinct activities.
Nevertheless, Julius Caesar (De Bello Gallico, VI, 14) credits the Druids with great experience in
Astronomy, but not in Astrology. On the contrary Astrology was practiced during the Romanization
of the Celtic places under Hellenistic influence.
It has been demonstrating that Astrology has a non-Celtic origin and that the Druids neither practiced
nor taught it at least as it is commonly conceived today.
The observation of the starry sky would be made mainly for astronomical as well as calendric
purposes not for divinatory purposes.
How to recognize, identify and remember stellar grouping
In this section we will give some brief and simple explanations about the way in which a man sees,
recognizes, memorizes and remembers a given stellar pattern visually observed in the sky.
The human visual system is composed, roughly speaking, of a number of elements, first of all the
eye which is able to receive the faint light flux coming from the stars and to transform it into the
corresponding stimuli operated by the fovea, the innermost part of the eye. The process,
electrochemical in nature, is able to code in same way some single definite identical patterns.
Obviously, since the underlying processes are of electrochemical origin the coding is fuzzy and
mathematically can be described by stochastic rather than deterministic processes. More simply a
given stars' configuration observed a number of times would be coded as a pattern of stimuli that
are nearly the same, but not exactly the same depending upon the instantaneous performances of
the fovea.
The electric stimuli are transmitted to the brain along the optic nerve then the brain, by means of its
biological neural network, will process them deciding on the related behaviour to adopt. The
biological neural network ruling the human visual system is able to store in a semi-permanent
electrochemical way a pattern obtained by averaging all the observed configurations. After
appropriate training it is able to recognize new and unknown configurations, nearly of the same kind,
as closely related to those previously observed.
This phenomenon is known in the community of the researcher in the domain of artificial neural
networks by the name of Associative Memory.
Obviously this is possible by concomitant work of some billion neurons, the neural cells, that
continuously exchange information with each other in real time.
17
An interesting feature is that the optimizing tendency of nature rules the gain of the information so a
given configuration of stars would be stored into the memory in a way that requires the minimum
amount of employed resources. This is called "The Minimum Energy Principle".
This means that a man observing a given group of stars randomly distributed will recognize a shape
virtually connecting the stars with imaginary straight lines from one star to the other, but satisfying
the minimum energy principle. In practice that the total length of the connecting lines may be
minimized. This would involve that a number of persons independently observing the sky without
exchanging information would assign nearly the same shape to the same observed group of stars.
This was experimentally demonstrated by great many tests conducted by the neurophysiologists in
the domain of the medical research, but there exists also some ethnological evidence i.e. the
graphical representations of the constellations visible in the sky by a number of ancient populations
belonging to very different cultures and living in countries geographically a long way apart.
To take a single, but meaningful example we can look at the representation of constellations on the
ancient Chinese astronomical maps as reproduced by Ho Peng Yoke (1962).
The maps produced by Ho Peng Yoke show that the major constellations (i.e. the widest spread in
the sky) are connected in almost the same way as they are in the modern representations on a starmap as they are also on the mediaeval European maps as well as Arabian maps.
Some differences become evident when we consider the smaller groups of stars, except for the most
compact and characteristic ones as the Pleiades, the Hyades and some others. These differences
in connecting the single stars belonging to the smaller groups can be explained by the latent
fuzziness typical of the processing of the information by the biological neural networks.
To come back to our Celtic astronomers, we are allowed to think that after the stars were grouped
in constellations and the observers had been trained to recognize them, no doubts or errors were
possible either in the identification of a given star belonging to a given constellation after a long time.
It might be very interesting to add a final consideration i.e. the fact that once a constellation has been
memorized with a given imaginary pattern of connecting lines it is impossible to forget it and after
many years during which someone has not observed the same sky, because for instance he was
living in the opposite hemisphere, a simple glance again at that pattern will bring about the almost
instantaneous memory of the original set of virtual connecting lines.
This is due to the performance of the most efficient information retrieval technique known at present
in the absolute: that operated by the human brain in the act of remembering.
This process works now, but it worked in the same way at the time of Celtic culture because a few
number of millennia is negligible compared with the timescale of the evolution of man.
18
Observing the stars
In this survey we put forward the view that the Celts practiced the observation of the position as well
as of the apparent motion of the stars throughout year.
In the interest of the argument a number of preliminary considerations about the fraction of the sky
observable and the number of visible stars must first be gone into.
The fraction of the sky visible during the Celtic Age
Different cultures lived in different places on the planet, and so lived in different geographical
locations. It is well known that the visible sky is different as seen from one location or another
depending upon the geographical latitude. So the declination of the stars that are at the zenith at a
given date depends on the latitude of the observation points.
Also the apparent positions of the Sun and of the Moon in the sky depend on the latitude of the
observer. In general, it can also be demonstrated that the fraction of the sky visible through the year
varies from place to place depending on the geographical latitude.
Taking into account the average distribution of the Celtic peoples we can affirm that the range in the
latitude concerned varies from +40 to +60 degrees north. Relevant computations show that only a
fraction between 75% (at +60 degrees of latitude) and 88% (at +40 degrees) of the overall sky was
visible in the belt of geographical latitude which concerns Celtic culture.
For the sake of comparison, the fraction of the sky visible in Egypt (at about +25 degrees of average
latitude north) varied between 97% and 99%. This fact implied that in the land concerned by Celtic
Culture there was a tendency to devote their observations to the Sun, the Moon and the stars with a
high declination.
On the contrary in the lands where the latitude was lower (Egypt, Mesopotamia etc.) the astronomical
observations mostly dealt with the zodiacal stars and the constellations that were visible higher in
the sky. This may be one of the reasons for the non-astrological character of Celtic astronomy.
It is interesting to advance the hypothesis that in high-latitude lands, where the visible fraction of the
sky is reduced, solar and lunar astronomy, and so calendric studies, were privileged. On the contrary,
in the low latitude lands, zodiacal and planetary astronomy, and astrology as a consequence, were
preeminent.
Another interesting fact is that. In the lands in which horizon astronomy is preeminent, as in Celtic
territories, there is a necessity for having sights for alignments. This requires the construction of
observatories which is what the megalithic monuments are. In the lands in which zodiacal astronomy
was privileged, the latitude being low, the location of the zodiacal constellations would be higher in
the sky. In this situation megalithic monuments were useless.
19
Number of visible stars
Having determined the fraction of visible sky it could be instructive to try to estimate the number of
the stars observable by a Druid during a complete rotation of the Celestial Sphere.
In order to do this, we must take into account the fraction of the sky visible computed in the preceding
section and the average number of visible stars up to a given magnitude per squared degree on the
celestial sphere.
By adopting an average value for the geographical latitude, say +47 degrees (the same value will be
used for the simulation of the sky, later in this article) we can compute that the 84.1% of the whole
celestial sphere would be observable at the time of the Celtic culture.
By using appropriate computing methods, we can obtain the following table showing the average
number of stars visible to the naked eye up to a given visual magnitude.
By adopting an average value of 0.6 cm for the iris diameter of the observer's eye, we get the data
collected in the following table:
Average Number of Stars Visible to an observer's naked eye
=====================================================================
Geographical Latitude: L= +47.0 Degrees
Fraction of visible sky: F%= 84.1 %
=====================================================================
Limit of the
Average Number of
Visual Magnitude
Visible Stars
+1.0
+2.0
+3.0
+4.0
+5.0
+6.0
+6.5
+6.6
10
35
124
420
1381
4398
7754
8677
=====================================================================
Formally adopting 0.6 cm for the iris diameter during the nightly visual we can obtain a limit of the
visual magnitude of 6.6, so more than 8600 stars can be totally seen by a visual observer during the
entire year in Celtic territories. It is likely that the noticeable stars were only the brighter stars of, say,
magnitude 3.0, so the Druids monitored a number of stars ranging from about 30 to 120.
20
It is interesting to remark that it is possible to observe their heliacal rising and to use this method to
get an accurate time-scan through the year as well as to mark the dates of particular importance. In
fact, a given time for the heliacal rising of a given star was almost the same in all Celtic places thus
providing an excellent method for fixing dates of festivals, gatherings and other remarkable occasion
among the various Celtic communities.
The Heliacal rising of the stars
It is worth noting that a very important astronomical phenomenon frequently observed by the
astronomers in ancient civilizations was the so-called heliacal rising of the stars.
The heliacal rising of a star can be roughly defined as the rising of the star when it coinciding with
that of the Sun. When we watch the stars rising above the eastern horizon, we see them appear
night after night at about the same position on the horizon. But when we extend the observation into
the period of twilight fewer and fewer stars will be recognizable when they reach the local horizon,
and close to sunrise all stars will have faded out altogether.
Let us suppose that a given star was seen just rising at the break of dawn but vanished from sight
within a very short time because of the rapid approach of daylight, then we define this phenomenon
as the "heliacal rising" of that star. This designation comes from Greek astronomy.
The Sun has a slow motion of its own in relation to the stars in the opposite direction of the daily
rotation of the celestial sphere. The eastward motion of the Sun delays the rising of the sun from day
to day in relation to the rising of the star so, its rising will be more and more clearly visible and it will
take more and more time before fading away in the light of the coming day. After a while, during the
year, it no longer makes sense to take this star as the indicator of the end of the night, but there are
new stars which can take its place and this procedure can be repeated throughout the year until the
Sun comes back again to the region of the sky occupied by the given star. Obviously, one could be
very strict and every day choose a different star which is just in the phase of heliacal rising so that
every day of the year can be identified by a given star heliacally rising on that very day.
This identification procedure is independent of the seasonal characteristics of a given day from one
year to another and it can supply a very useful astronomical method for a rigorous identification of a
given day throughout the year. Obviously the relevant arc of the local horizon is the sector ranging
from the North to the South passing across the East. It is well known for instance that it was the
heliacal rising of Sirius that heralded the yearly Nile Flood in ancient Egypt, and by consequence the
beginning of the year.
In the present paper we propose that the Celts, as some other ancient populations did, would monitor
the eastern horizon, searching for the heliacal rising of a set of relevant stars through the year. A
Druid could monitor the horizon every day, watching the stars rise a short time before the Sun,
waiting for a given star in order to identify a particular day throughout the year.
21
Another fact seems to be of great interest.
As various authors have pointed out there was an important, and, at practically present unexplained,
climatic change in Europe about 1800 B.C. The climate worsens after this date limiting observation
of the overall stars, then stellar astronomy as practiced in northern Europe became principally
horizontal i.e. special effort were devoted to the observation of the rising and the setting of the stars.
The remarkable stars
It is our present purpose to claim that a number of stars were held on to as important by the Druids
who observed the sky.
To begin with we must give some criteria to justify why a given star could be considered as important.
A conservative point of view is first to assume that a degree of importance bore some relation to the
brightness of the star, so we will only apply our argumentation to the stars of first magnitude i.e. the
brightest, adopting the classification that we owe to the astronomical visual photometry.
This sampling will only take in to account only the brightest stars visible in a clear sky.
This is meaningful also take into account that if the observations are carried out made under a sky
which is not perfectly clear and not without clouds, the first magnitude stars can still be seen.
Another parameter that could define the importance of a star for a Celtic observer could be the colour
visually perceptible.
It is well known from Astrophysics that the visually observable, colour of the stars depends on their
temperature and their stage of evolution.
If the star has a low temperature, its colour is nearly red, if the temperature is high, the colour is blue.
The visual system of a human observer that uses only its eyes as a tool is very sensitive to the red
light, then, as it is well known in the community of amateur astronomers who perform the visual
monitoring of the long period variable stars, a red star is seen apparently brighter than it should be.
This is a typical consequence of the performances of the biological neural network that controls the
visual system of the human who carries out his observation in a dark environment (see Gaspani,
1994).
It is very likely that the remarkable stars for the Druids unless of abnormally high brightness as in
the case of Sirius, would be red stars or at least strongly yellow ones.
22
This can be due also to some psychological effects since the majority of the very bright stars easily
visible in the boreal sky are white or blue or slightly yellow, thus less evident to the human eye, so a
small number of red bright stars can become very noticeable for a primitive observer that has no
knowledge at all of the physics of the interior of the stars.
If we search for a search of a set of stars likely to become remarkable stars for a Druid, carrying out
astronomical observations, we can obtain the following table I.
TABLE I: Some remarkable stars visible in the boreal sky
Star Visual Magnitude
Constellation Colour
================================================================
Antares
0.96
Scorpius
Red
Betelgeuse 0.50
Orion
Red
Aldebaran
0.85
Taurus
Red
Arcturus
-0.04
Boote
Orange
Capella
0.08
Auriga
Yellow
Regulus
1.35
Leo Major
Yellow
Sirius
-1.46
Canis Major White
Rigel
0.12
Orion
White
Procyon
0.38
Canis Minor White
Spica
0.98
Virgo
White
Vega
0.03
Lyra
White
Altair
0.77
Aquila
White
Deneb
1.25
Cygnus
White
Pleiades
various
Taurus
Blue
================================================================
(The reported numerical value of the visual magnitude means that a first magnitude star has a visual
magnitude of located between 0.0 and 1.0, a second magnitude star has a visual magnitude placed
of between 1.0 and 2.0 and so on.
The negative value reported for Sirius and Arcturus means that their luminosity is higher than that
typically required for a first magnitude star.
The peculiar visual magnitude scale associates higher luminosities to smaller numerical values so a
star with a low or negative numerical magnitude is brighter than one with a numerically high visual
magnitude.
The peculiar magnitude scale was calibrated on the peculiar response of the biological neuronal
network governing the human visual system, so it is highly nonlinear so that a difference of 1
magnitude in the luminosity of two stars requires a ratio of 2.512 times their brightness.
23
This strange scale has its roots in the antiquity, when the first stellar photometric catalogues based
only on visual estimates of the brightness of the stars were written. The Pleiades being a stellar open
cluster have various magnitude values depending on the star being considered.
It is interesting to remark that Vega, Altair and Deneb constitute the Summer Triangle well known far
from the antiquity.
Another couple of relevant stars known in the antiquity are Spica and Regulus that enabled
Hipparcos to discover the phenomenon of the Precession of the Equinoxes.
Adopting as relevant the hypothesis of stellar observations practiced by the Celts, it is natural to
assume that the dates of occurrence of the various ritual events important in social life can be
associated with some astronomical events concerned with the stars, not just the Sun or the Moon.
To make some field considerations we should be able to observe today the sky that was visible at
the time of the Celts. Obviously this is not possible in practice, but it is virtually possible to make
appropriate computer simulations, for arbitrary dates and also for an arbitrarily chosen geographical
location in the Celtic world.
Another fact that requires to be borne in mind is the alteration that the sky visible at a given place
undergoes through time. This is the obvious consequence of the proper motions of celestial bodies
in outer space following their own motion laws under the effects of gravitational force.
For a brief time span the alterations are small and can be estimated by linear methods, but for time
spans of the order of 3000 years or more, the problem requires a very sophisticated mathematical
apparatus in order to be solved.
Factors that alter the patterns of the visible sky through time
Every celestial body observable, in the sky, from the Earth has a geocentric position that can be
defined within the frame of the Altazimutal System, the Equatorial, the Ecliptic and the Galactic.
For each system a celestial body is identified, for a given time, by a couple of numbers, its
coordinates according to the adopted reference system.
In all cases the coordinates of a celestial object are subject to nonlinear variations according to laws
of the Precession, Nutation, Aberration, Proper Motion and so on. This is a matter of basic Spherical
Astronomy.
For the purposes of this work, the most relevant Coordinate System to adopt is the Altazimutal
because it is the most natural system, for a technologically primitive (but not inexperienced!)
observer.
24
The two coordinates involved in the Altazimutal System are the Azimuth and the Height on the
Horizon. The basic reference circles on the Celestial Sphere are the fundamental meridian of the
given place, that is the projection on the Celestial Sphere of the local geographical meridian and the
local horizon.
The azimuth of a given star, indicated by the symbol A is the angular distance of the star from the
North reckoned on the arc of a circle parallel to the local horizon and passing for the star. The azimuth
will be 0 degree at the cardinal point North, 90 degrees at the cardinal point East, 180 degrees at
the South, 270 degrees at the West and again 360 (or 0) degrees at the cardinal point North.
The height on the horizon, denoted by H, is the angle of the vertical elevation of the celestial object
on the horizon. The height H varies from 0 degree, identifying an object placed on the local horizon,
to 90 degrees identifying an object placed at the local zenith.
Obviously the altazimutal coordinate system is a local system, so the numeric values of the
coordinates of a given celestial object change according to the geographic position of the observer
as well as through time.
The variations affecting the coordinates through time will be short-term ones in relation to the
different positions of the Earth in its orbit round the Sun, and long term ones in relation to the
precessional periodic oscillation of the Earth's axis due to the gravitational combined influences of
the Moon and the Sun.
The precession cone, covered by the axis of the Earth in nearly 26000 years is also perturbed by
the combined gravitational effects of the other eight planets constituting the Solar System.
This is obviously a minor effect, limited in practice to the gravitational influence of the more massive
planets, but it is clearly measurable and its effects on the apparent geocentric places of the stars are
consistent, and remarkable over a long lapses of time.
The historical period concerned in the context of the present survey dates back to about 2500-3000
years ago when a consistent portion of the precessional cone was covered by the axis of the Earth.
The practical consequence is that the only way to obtain a relevant image of the sky visible at that
time is to make computer simulations using some sophisticated computer codes which incorporate
all the required nonlinear equations as well as some relevant techniques which help to minimize the
propagation of the numerical errors coming back in the time.
This is a very difficult problem completely solved, so the deviations can be forced under some
reasonable limits.
25
The Celtic “Polaris”
Under the name "Polaris" we identify the star Alpha Ursae Minoris that is at present the star nearest
to the North Celestial Pole. This star is of second magnitude and is placed at less than of one degree
from the ideal intersection between the virtual direction identified by the axis of rotation of the Earth
and the celestial sphere.
The conic motion of the axis of the world due to the lunisolar precession with a period of 26000 solar
tropical years forces the position of the North Celestial Pole to change through the time. In particular,
this point covers a circle with semi amplitude equal to the value of the Obliquity of the Ecliptic during
the 26000 years. The trajectory covered across the circumpolar constellations is not closed because
the value of the Obliquity of the Ecliptic changes slowly through time.
It is interesting to note that the North Celestial Pole runs across the foreground stars so that through
time various stars become candidates for the "Polaris".
It is obvious that in the Celtic period say 600 B.C. the star Beta Ursae Minoris (Kochab) was the star
closest to the North Celestial Pole, so this star marked the astronomical North to the travellers of
that time. In more ancient times, about 2000 B.C. when the megalithic monuments were erected,
"Polaris" was Alpha Draconis (Thuban). In 7000 B.C. at the times of the Egyptians, the star Tau
Herculis marked the position of the North Celestial Pole.
The simulation of the Sky
Although our object, here, is rather descriptive being concerned with the qualitative formulation of
some hypotheses about stellar astronomy practiced by the Celts, we have moved towards the
highest precision obtainable in our computations.
The simulations were executed in various stages with a number of targets, briefly described as
follows.
First stage simulations (exploratory purposes)
The first simulation stage was exploratory with the following aims.
a1) Simulation of the sky in 500 B.C. in order to reconstruct the visible sky during Celtic ages.
Additionally, the simulations were carried out at the dates of the four principal festivals typical of
Celtic culture with the object of inferring the occurrence of some remarkable astronomical
phenomenon that could be adopted as reference in order to fix the dates of the festivals along the
year.
b1) Simulation of the sky visible for those same years at the dates of the Solstices and the Equinoxes
assessing whether there were some remarkable astronomical facts in addition to the peculiar
positions of the Sun.
26
Second stage simulations (quantitative purposes)
The computer simulations at the second stage were carried out with quantitative purposes.
The targets of the second stage are summarized as follows.
a2) To search for the stars that showed heliacal rising at the four most important festivals in Celtic
social life throughout the year.
b2) To search for the date in the year on which the festivals could be fixed on the assumption that
a given festival would be associated with the heliacal rising of a given star (or stars).
c2) To search for some correlation between the star in heliacal rising at a given festival and the deity
celebrated at that festival.
d2) To search for a correlation between the heliacal rising of the stars and the dates making the end
of agricultural activities.
e2) To search for the dates of the beginning and the end of the seasons based on observable
astronomical evidence.
The simulations were performed for a Geographical Latitude of +47:00:00 degrees North and a
Longitude of +5:00:00 degrees West.
The adopted Geographical Latitude typically passes as near as the centre of the places in which the
Celtic culture flourished.
We must remark that the adopted value for the geographical longitude is of little importance here,
because it only affects the local time of occurrence of the astronomical phenomena.
The time difference implied by a variation of a few degree of longitude is rather small and negligible
taking in view of the purposes of the present work.
The time precision required for our purpose is barely a day so the value of the Longitude adopted in
the simulation can be chosen within wide limits without affecting the final result.
27
The adopted simulation program
There are various computer programs that are able to perform simulation of the sky through time.
Good performances in precision and low propagation of numerical uncertainties have fundamental
importance in a simulation work of the kind required by our purpose.
For the work described in the present paper we opted to use the PLANETARIO program developed
at the Astronomical Observatory of Catania by P. Massimino. The program was interactively run on
various personal computers equipped, at least with an Intel i80386 microprocessor, in order to get
reasonable computational speed. The program accepts in input a set of items i.e. the geographic
coordinates of the site of observation, i.e. the place for which the simulation must be done, as well
as the local time at which the sky must be simulated.
The results of every single simulation is showed on a SVGA graphic display and represents the
visible sky as a Cartesian diagram with azimuth on the abscissa and height on the horizon as
ordinate. An appropriate option enables one to obtain the hardcopy of the diagram on the printer.
In the diagrams are represented, in their correct positions, the Sun, the Moon in its relevant phase,
all the visible planets with their appropriate symbols, the stars, the nebular objects like the galaxies
and the star clusters whose brightness is below the ninth magnitude. The diagram also shows the
projection of the Ecliptic and the Celestial Equator, the Local Meridian, the Line of the Horizon and
the Cardinal Points
The default settings consist in having the X-axis to coincide with the Local Horizon and the Y-axis to
coincide with the arc of the Local Meridian from the North Cardinal Point on the local horizon up to
the Local Zenith on the Celestial Sphere. The database of the program includes stars up to the ninth
visual magnitude, but for the present purposes we have plotted only the stars up to the third
magnitude included.
From a strictly formal point of view we must remark that a visual observer, operating under a clear
sky in a historical epoch in which both light and chemical pollution were absent, could see stars up
to the visual magnitude of 6.5 or more, depending on the performances of its visual system.
Nevertheless, in our opinion the faint stars have not received high consideration from the visual
observers at that time, except for some peculiar stellar patterns like the Pleiades, the Hyades or
some other open stellar clusters also registered in classic Latin literature.
The stars were represented in the program by different marks according to their visual magnitudes,
additionally the brightest stars have their proper names, so the program writes them, sometimes
abbreviated if too long, nearly the respective marks. Additionally, a set of data about the local sidereal
time, the height of the Sun on the local horizon, some data about the position of the Moon and so on
are given at the top of the graph.
The program also enables the animation of the celestial bodies by real time repetition of the
computation of the visible sky for a set of ages separated by an arbitrary time span.
28
The four Celtic Festivals
The existence of four ritual festivals, celebrated every year by the Celts, is a well-known and
accepted fact. These festivals are: Trinox Samoni, Imbolc, Belteine and Lugnasad, listed in
chronological order, starting with the first which identifies the beginning of the Celtic year. Each of
these festivals, excluding perhaps Trinox Samoni, seems to be devoted to an individual deity, as
listed in the following table.
==========================================
Festival
Celebrated Deity
-----------------------------------------Trinox Samoni
-
Imbolc
Brigh
Belteine
Belenus
Lugnasad
Lug
==========================================
About the actual dates of the celebrations there is at present great uncertainty because if we adopt
Duval's hypothesis, the festival of Trinox Samoni takes place in the month of November of the
Gregorian calendar, but if we adopt the hypothesis developed by Mc Naill, the Trinox Samoni would
coincide with the Summer Solstice. The uncertainty, in practice lies with the season in which the
Celtic year started. The association of the Irish traditional festival of Samhain seems to support
Duval's hypothesis.
In order to get an alternative approach to the problem we decided to make a number of astronomical
consideration about this, in particular we tried to simulate the sky observable on the days on which
the four festivals took place. The object being to infer some relevant astronomical phenomenon
which would occur every year on such days.
We propose that the dates on which the festivals were celebrated were related to some astronomical
evidences, and also that the various astronomical evidences could be related to the deities to which
the festivals were dedicated. We are particularly interested in the stars heliacally rising on the festival
days. In our opinion the heliacal rising of certain stars would identify the days of the festivals.
Having carried out the required simulations of the visible sky and after a very close analysis of them,
we noticed a number of very interesting facts. The first of them is that if we assume valid the seasonal
placing of the festivals according to Duval, i.e. Trinox Samoni located in the month of November of
the Gregorian calendar, we can observe that there were four stars of first magnitude that had their
heliacal rising on the days of the four festivals.
29
Table II lists the stars as well as their visual apparent magnitude and colours. These stars were
retained as remarkable in a precedent section of the present paper and constitute a subset of those
listed in Table I.
TABLE II: Stars in heliacal rising at the four Celtic Festivals
====================================================================
Festival
Heliacal Rising Star Visual Apparent Magnitude Colour
-------------------------------------------------------------------Trinox Samoni
Antares
0.96
Red
Imbolc
Capella
0.08
Yellow
Belteine
Aldebaran
0.85
Red
Lugnasad
Sirius
-1.46
White
At this point it is natural to come to think that the actual dates of celebration of the four festivals could
be close to the days of the minimum apparent difference of height on the horizon of the listed stars
from the sun.
Having carried out an additional set of computer day by day simulations in search of the minimum
heliacal distance at sunrise under the conditions of visibility of the stars, we obtained four dates that
could be the actual days of celebration of the festivals by the Celts. Table III lists the dates resulting
from the analysis of the results of the computer simulations.
TABLE III: Dates of heliacal rising of the stars typical for the four Celtic festivals.
====================================================================
Festival
Heliacal Rising Star Date of Minimum Apparent Distance from the Sun
(Gregorian calendar)
-------------------------------------------------------------------Trinox Samoni
Antares
November 7 (*)
Imbolc
Capella
February 13
Belteine
Aldebaran
May 18
Lugnasad
Sirius
July 28
===================================================================
These dates are calculated for 500 B.C., however if we consider other century the dates change of
one week more or less.
30
The days listed in the following table refer to the last days on which a given star can be seen just
before sunrise. A day before, the stars will not be visible because they are immersed in the sunlight,
on the days after the listed dates, the stars would be high in the sky, far from the Sun, then their
rising would no longer be heliacal.
In the table above we marked Trinox Samoni with an asterisk (*) because we think that the Samon
month starts with this heliacal rising, but the choice of date for the festival of Trinox Samoni, the
beginning of the new Celtic year, must also be subjected to some lunar constraints. This and bearing
in mind the words of Caesar in the "De Bello Gallico", the listed day can be taken to be the date
closely preceding the lunar event (beginning of the new Moon), as required for the Trinox Samoni.
In other words, the Trinox Samoni festival would be celebrated at the new Moon immediately after
the heliacal rising of the star Antares.
The other festivals would be celebrated, according to our hypothesis, on the day of the heliacal rising
of Capella, for the festival of Imbolc; on the day of the heliacal rising of Aldebaran, for the festival of
Belteine; and Lugnasad, the most important festival, was celebrated when Sirius rose with the Sun.
Correlation between the stars and the importance of the deities
From the analysis of the simulations carried out, and from a glance at Tables I and II we can remark
that there seems to exist a correlation between the brightness of the stars in heliacal rising and the
importance of the deities celebrated during the various festivals.
The most obvious fact is that the star associated with the festival of Lugnasad is Sirius, (Alpha Canis
Majoris), the brightest star of the whole sky. Lug was also the most important deity of the Celts, so
the association between the brightest star visible and the most important deity seems to us far from
being a pure coincidence. If we remark that the etymology of the name Lug means "bright" or also
"luminous", the association with a very bright star like Sirius seems to be very appropriate. As natural
consequence we come to think that the festival of Lugnasad would be celebrated when the Sun and
Sirius rise together and this happens only at beginning of August.
Another interesting observational evidence is that, from the simulated sky, we can remark that at the
festival of Lugnasad, the night sky is dominated by the Summer Triangle (Deneb-Altair-Vega) that is
one of the best known typical wide stellar configuration, different from the antiquity, from a standard
constellation.
The association between the noticeable stellar pattern as the Summer Triangle and the most
important deity like Lug does not necessarily stand to reason, but in our opinion the appearance in
the sky of the Summer Triangle could be used to inform in advance of the approach of the
Midsummer festival. Another natural consequence involved by placing Lugnasad on the first day of
August is to consider this as an independent confirmation of the correctness of the hypothesis of
Duval for placing the beginning of the Celtic year on the first days of November, thus rejecting the of
Mc Naill's hypothesis.
31
Another very interesting fact is the celebration of the festival of Imbolc when the heliacally rising star
is Capella (Alpha Aurigae) which is the second one in order of brightness among the stars listed in
Table II. On the other hand, Brigh was in order of importance, the second deity in the Celtic pantheon.
Moreover, this association seems far from being only a random coincidence. In our opinion the
Druids or perhaps the members of the sacerdotal class belonging to a more ancient population, if
we accept the possibility of an inheritance of the astronomical knowledge from a pre-existing culture,
have closely calculated the ritual dates of the festivals on the astronomical events with a yearly
periodicity, perhaps for magic or propitiatory reasons.
It is natural to imagine that the astronomical events were also closely related to the agricultural
events, so important for the survival of the population. From this point of view we could also imagine
that the interval between the heliacal rising of Antares and the celebration of the Trinox Samoni could
be used at the sowing of the corn and the celebration of the festival would have propitiatory purposes
so as to have a good crop after the Winter.
The sky visible at the four fundamental Celtic festivals
In this section we shall describe in detail the main astronomical manifestations that were visible to
the naked during the four fundamental Celtic festivals.
The sky at Trinox Samoni
The sky simulated for the date of the festival of Trinox Samoni shows a number of very interesting
stellar configurations. First as before, we consider the sunrise. At this date we can see the heliacal
rising of Antares, but it is clear, from the results of the computer simulations, that formally the date
of the heliacal rising goes back a few days. At the Trinox Samoni, the morning sky is dominated by
the stars Vega, Antares and, higher in the sky, Arcturus and Spica. The western horizon shows
Regulus, Castor, Pollux, Procyon, Capella and setting Sirius and Betelgeuse. At sunset the
dominating stars are those belonging to the Summer Triangle, Deneb, Vega and Altair, that set early
in the evening. The eastern horizon is dominated by Capella and on the western horizon Arcturus is
shining and will set a short time after sunset.
The sky at Imbolc
The sky simulated for the date of the festival of Imbolc shows a set of interesting stellar
configurations. First as usual we consider the sunrise. The first fact is the heliacal rising of Capella;
is remarkable is the appearance of the Summer Triangle high in the sky in the south-east. Also very
remarkable are the setting stars Antares, Spica, Kiffa and Arcturus shining in the western sky. The
sky at sunset is richer in bright stars that it is in the morning. Over the eastern horizon we can see
Regulus, Procyon, Castor, Aldebaran and the entire constellation of Orion with Betelgeuse and Rigel.
At Imbolc the great constellation of Orion is placed vertically at south in the centre of the sky.
32
Near the local meridian can be seen Aldebaran as well as the Pleiades and, on the western horizon,
we can observe Deneb and Vega sets a short time after the sun. The night sky is dominated by the
light of Aldebaran in the winter season when this festival takes place.
The sky at Belteine
The sky simulated for the date of the festival of Belteine shows a set of interesting stellar
configurations in the sky. We first consider the sunrise. Taking into account the eastern arc of the
horizon we can note that the most remarkable phenomenon is the heliacal rising of Aldebaran (Alpha
Tauri). At Belteine this star would move across the sky in daylight and set at sunset. The red star
Aldebaran would be more important for the Celtic observers. Another remarkable fact is that when
the star Aldebaran is in heliacal rising, there is the small asterism of the Plejades that rises a short
while before Aldebaran. The peculiar shape of the Pleiades cluster would not pass unnoticed to a
Druidic observer. Another relevant star that is just risen is Capella (Alpha Aurigae). Keeping an eye
on the western arc of the horizon we can note the setting of another remarkable star, Arcturus (Alpha
Bootis), an orange star known from the antiquity as useful in an agricultural context. High in the
Belteine's morning sky is visible the Summer Triangle composed by Deneb, Vega and Altair. The
morning sky, at this festival, shows no other bright stars of remarkable importance. At sunset there
are a number of remarkable astronomical manifestations that characterize the aspect of the night
sky visible at this date. First we have the Sun that sets with Capella. The western horizon is
dominated by Procyon (Alpha Canis Minoris), Castor and higher in the sky there is Regulus (Alpha
Leonis) The eastern horizon is dominated by the rising of Vega (Alpha Lyrae), Deneb (Alpha Cygni)
and Antares that will shine in the sky during all through Summer Time. Another couple of first
magnitude stars shine higher in the sky: Arcturus (Alpha Bootis) and Spica (Alpha Virginis).
The sky at Lugnasad
The sky simulated for the date of the festivals of Lugnasad shows a set of very interesting stellar
configuration. In this case also we first take into account the time of sunrise. The most remarkable
astronomical evidence is the heliacal rising of Sirius. Regulus (Alpha Leonis) rises very close to the
sun. This star is located at about the same height on the horizon as Sirius, but its close proximity to
the sun would made difficult its observation. At the same time the constellation of Orion, with
Betelgeuse and Rigel is near the local meridian. This constellation has so peculiar a shape that it is
very evident to a naked eye observer that other ancient cultures, such as the Egyptians, have based
some rituals on the heliacal rising of Sirius and the presence of Orion in the sky. At the same time,
the white stars Vega and Deneb are setting. The higher southern sky is dominated by Castor,
Capella, Aldebaran and by the stellar cluster of the Pleiades. The night sky is dominated by the
Summer Triangle that begins to become observable high on the eastern horizon. This pattern is
composed of three white bright stars, Vega, Altair and Deneb, that will characterize all the summertime sky. The Summer Triangle is the unique remarkable stellar configuration visible in the eastern
sector of the sky at Lugnasad. The western horizon is dominated by Arcturus, high in the sky, Antares
Kiffa, and Spica setting a short time after the Sun.
33
The Seasons
Considering our stellar astronomical view of the Celtic social and ritual life the location of the seasons
can also be discussed.
According to D. Laurent (Laurent, 1990) it seems very probable that the Celtic year was composed
of only two seasons: Winter Time and Summer Time. The Winter Time would include Spring and
Summer Time would also include Autumn. It is certain that the Druids would know about the
Equinoxes and the Solstices and were perfectly able to fix and to predict their dates with high
accuracy, but from the point of view of the climate such astronomical data are of little use in defining
the dates of the beginning and the end of seasons and of little practical help for agricultural purposes.
In this case stellar astronomy can also help us (and in all probability, helped the Druids).
If we take into account two of the four stars listed in Table III, i.e. Antares and Aldebaran, respectively
Alpha Scorpii and Alpha Tauri, we can remember that Cleomedes wrote in his "De Motu" (On the
circular motions of the celestial bodies) written around 370 A.D. Cleomedes reports that the rising of
Antares coincides with the setting of Aldebaran and vice versa, in mathematical terms these two red
stars differ by nearly 180 degrees in longitude.
The same fact was noted in another treatise which goes under the name of "Anonymous of the Year
379". The diametrical position of the two stars is once more also mentioned by Rethorius for about
500 A.D. This fact implies that when Antares is high in the sky, then Aldebaran will be completely
unobservable because its apparent arc on the celestial sphere is covered during daylight hours. On
the contrary, nearly six months after, the night sky will be dominated by Aldebaran and Antares will
be unobservable.
From the climatic point of view, the periods dominated by the visibility of one or the other star
correspond to the cold and the warm seasons respectively, so it is natural to think that the two
seasons would correspond to the presence in the sky of one or the other of the two above mentioned
red stars.
At this point it becomes natural to propose that Winter Time would start at the heliacal rising of the
star Antares, since the star Aldebaran dominates the sky during the nights of the cold season.
Obviously Summer Time would be characterized by the presence in the sky of Antares, so the date
for the beginning of summer would be fixed at the heliacal rising of Aldebaran.
Table IV clarifies the situation as follows:
34
TABLE IV: Estimated dates of starting of the Seasons
====================================================================
Heliacal Rising Star Date (Gregorian calendar) Starting Season
Festival
-------------------------------------------------------------------Antares
November 7
Winter Time
Trinox Samoni (*)
Aldebaran
May 18
Summer Time
Belteine
===================================================================
(The asterisk refers to the shift required in order to satisfy the Lunar constraints on the festival of
Trinox Samoni). The division of the year in two periods is a fact not completely new for us.
From the study and the interpretation of some megalithic monuments, dates back to the Eneolithic
(Copper Age) period, the tendency to divide the solar year in two equal segments when the average
declination of the Sun is, for certain geographical latitudes, about 0.44 degrees in modulus,
corresponding to Vernal and Autumnal equinox becomes obvious (The relative dates in Gregorian
calendar are 11 of April and 13 of October).
A division based on this criterion is an artefact implying a duration of 182 and 183 days for each
segment and is only justified if we conceive a circular orbit for the Earth around the Sun.
This division is not matched by the occurrence of the true equinoxes. In fact, the time lapses between
the vernal and the autumnal equinoxes is 186 days and 179 from the autumnal to the following vernal
equinox.
This difference of a few days is very great compared with the accuracy reached by the measures of
the positions of the Sun carried out at the megalithic monuments. This is due to the ellipticity of the
orbit of the Earth.
After our analysis of the results of the computer simulation for 500 B.C we propose to take 7
November of the Gregorian calendar as date of the beginning the Winter season and to take 18 May
as the date of the beginning of Summer.
This division implies an unequal duration of the two seasons, in particular we have a duration of 168
days for the Summer and of 197 days for the Winter. These different durations agree better with the
yearly climatic cycle of northern Europe.
It is interesting to remark that if we follow our results and take in account the structure of the calendar
of Coligny, we obtain Table V computed according to an average Celtic year. Obviously the "exact"
dates listed in Table V are the result of a number of computations, so the true dates are an
approximation. A certain degree of fuzziness is unavoidable in ethnoastronomical results.
35
Table V: Remarkable dates throughout the Celtic Year.
=======================================================================
Celtic Month Gregorian dates
Festival
Astronomical Recurrence
Samon
7/11
(*)
Trinox Samoni
Duman-Rivros
26/12
Winter Solstice
Anagantio-Ogronn
13/2
Imbolc
Ogronn-Cutios
26/3
Vernal Equinox
Giamoni
18/5
Belteine
Simivisonna-Equos
26/6
Summer Solstice
Equos-Elembiu
28/7
Lungnasad
Beginning of Winter Time
Heliacal rising of Antares
Heliacal rising of Capella
Beginning of Summer Time
Heliacal rising of Aldebaran
Heliacal rising of Sirius
Summer Triangle high in the Sky
Edrini-Cantlos
28/9
Autumnal Equinox
=======================================================================
(*) The date of Trinox Samoni is correct only as a first approximation because the Lunar constraint
imposed on the occurrence of this festival involves some oscillations about an average date through
the five-year cycle typical of the Celtic Calendar. These dates are calculated for 500 B.C, as we have
said before for other century dates change of one week more or less.
The two season model developed here implies the splitting of the Celtic year according to the
repartition of the months as showed in Table Va.
Table Va: Seasonal repartition of the months of the Celtic year.
=======================================================================
Winter Months
Summer Months
Samon
Giamoni
Duman
Simivisonna
Rivros
Equos
Anagantio
Elembiu
Ogronn
Edrini
Cutios
Cantlos
Giamoni
=======================================================================
36
Agricultural occurrences during the Celtic Year
In this section we suggest the existence of some agricultural occurrences related to the astronomical
events along the year. Table VI summarizes the situation (the corresponding average Gregorian
dates are reported in brackets).
Table VI: Agricultural occurrences related to astronomical events
=======================================================================
Month
Sky
Season
Festival
Agriculture
Samon
(28/X)
First quarter Moon. Heliacal Starting
rising of Antares Setting of Winter
Aldebaran
Trinox Samoni
Sowing
Sun rising at its maximum Azimut Winter
Solstice
Druids Yearly
Assembly
Ogronn
(26/II)
Heliacal rising of Capella. Setting
of Spica, Pleades at the Meridian
Imbolc
Hop Pruning
(Thanksgiving)
Cutios
(23/III)
Giamoni
Heliacal rising of the Pleades
Vernal
Equinox
Nearly Harvest
Simivisonna
(13/V)
Heliacal rising of Aldebaran,
Setting of Antares and Arcturus
Beltane
Wheat Reaping
Equos
(21/VI)
Minimum Azimut of the rising Sun Summer
Solstice
Lughnasa
Gleaning,
Gathering
Dumann
Riuros
(21/XII)
Anagantio
Starting
Summer
Elembiu
Edrini
(1/VIII)
Heliacal rising of Sirius, Arcturus
high in the sky at Sunset,
Summer Triangle at the Meridian.
Cantlos
(23/IX)
The Plejades rise at the Sunset
Autumn
Equinox
Wheat
Wheat
Stocking,
Hop
Plucking,
Firewood
Cutting
and Stocking
=======================================================================
37
Dynamic management of the intercalary months in the Celtic Calendar
The months in the Celtic calendar start with the first quarter Moon visible in the west soon after
sunset, i.e. as soon as the Moon becomes visible.
The lunisolar structure of the calendar of Coligny, designed so as to obtain agreement between the
Sun and the Moon, is very far from being easy to use.
The table of Coligny suggests the existence of an intercalary month of 30 days to add after 2.5 lunar
years within the cycle of five years.
The true method for the insertion of the intercalary months is at present unknown.
There is the conservative hypothesis of a rigid insertion following the sequence engraved on the
table, but in our opinion this is not the right way to go about in order to conciliate the Sun and the
Moon's cycles.
The conservative solution is, in our opinion, very rigid and scarcely useful from a practical point of
view. Moreover, it makes the agreement with the stellar phenomena observable in the sky very
difficult to reach.
A similar static use of the intercalary months existed also in the calendar adopted by the Romans
before the Julian reform. In fact, the overall discordance between the civil and the astronomical time
rose up to 3 months in 45 B.C. when Sosigenes, under Julius Caesar, was assigned the task of
reforming the calendar system.
We are at present studying; the results obtained so far are very encouraging, so they should be
published as soon as possible.
An interesting fact is that our dynamic algorithm seems to agree well with the words engraved on
the table near the days of the intercalary months.
The dynamic management seems to get better results and better agreement also with the stellar
cycles visible in the sky.
The Planets
The planets were certainly observed by the Druids because of their peculiar motions among the
stars. The five planets visible to the naked eye, i.e. Mercury, Venus, Mars, Jupiter and Saturn, move
on the zodiacal belt as well as the Sun.
The Druids monitored the sky in order to observe the heliacal rising of the stars, but the heliacal
rising of the planets must also have been remarked.
38
It is easy and natural to believe that the Druids were able to remark that the planets did not match
the usual yearly periodicity of their heliacal rising as happens for the stars.
As an example we will take the planets Venus which is very bright and visible as it anticipates or
follows the Sun throughout the year. When Venus anticipates the sun it is visible as a very bright
morning object and it is an evening object when it follows the sun. Its limited maximum elongation
from the sun would surely have been noted by the ancient observers of the sky.
Mercury, being an internal planet too, shows a similar harmonic motion in relation to the Sun, but its
maximum elongation is narrower.
Observing successive heliacal risings of the planets those observers were surely able to determine
their synodic periods. The synodic period of a planet is the time required for it to return to the same
phase in relation to the Earth.
In example from an opposition to the following one. The synodic period of a planet is related to the
period of revolution of the Earth and that of the planet.
Table IX shows the synodic periods for the five planets visible to the naked eye, thus known also in
the antiquity.
Table IX - Synodic periods of the planets:
============================================
Planet
Synodic Period
-------------------------------------------Mercury
115.9 days
Venus
583.9 days
Mars
779.9 days
Jupiter
398.9 days
Saturn
378.1 days
-------------------------------------------Considering Venus again, we observe that a synodic period of 583.9 days implies that this planet is
visible for 263 days as a morning object rising before the Sun, then it is invisible for nearly 50 days
due to the heliacal conjunction, and it appears again as evening an object, following the sunset, and
remains visible for another 263 days, finally disappearing for 8 days before becoming again a
morning object.
The Druids would also know the sidereal periods of the planets because this information could be
easily obtained by measuring the lapse of time required for a planet in order to cover the entire circle
of the Ecliptic returning near the same stars. This is the sidereal revolution or, in other words, the
orbital period around the Sun.
39
Table X lists the sidereal periods of the five planets known at the time of the Celtic culture.
Table IX - Siderical periods of the planets:
============================================
Planet
Sideric Period
-------------------------------------------Mercury
88.0 days
Venus
224.7 days
Mars
687.0 days
Jupiter
4332.0 days
Saturn
10759.2 days
-------------------------------------------We need to remember that the apparent motion of the planets in the sky is rather complex due to
the combination of the orbital motion of the Earth and that of the planets themselves. The Druids'
interpretation of the apparent motion of the planets in the sky is at present unknown.
There is an interesting fact related to the movements of the planet Saturn across the constellations
of the zodiac. The time this planet takes to complete its orbit is nearly as long as the "Celtic
Saeculum"; then after a saeculum this planet can be observed again in about the same position in
the sky in relation to the stars.
These correlations are only empirical but, in our opinion, the observation of the motion of the planets
along the years could give some additional indications on the passing of time and of its measure.
Conclusion
In the present paper we propose that the Celts practiced Astronomy at a high level both for
speculative and for practical purposes.
The astronomical knowledge of the Druids was to a large degree inherited from preceding cultures
that built the megalithic monuments. It is highly probable that an appreciable influence was also
exercised by the Greeks philosophy and cosmogony, in particular the Pythagorean, but a number of
independent discoveries were also probably made by the Druids themselves. It seems quite obvious
that the astronomical level reached by the Celtic culture was reasonably high. This is attested by the
calendric finds that show an excellent knowledge of the apparent motion of the Moon and of the Sun.
In our work, we have particularly stressed that the observation of the stars could also be efficiently
practiced together with the monitoring of the Sun and the Moon. In our opinion, stellar astronomy
was of fundamental importance both for agricultural and for social purposes. In particular, we suggest
connecting the dates for the occurrence of the four typical traditional festivals of the Celtic culture
with the heliacal rising of certain stars. In this framework, is also suggested a relevant date for the
beginning of the Celtic year.
40
Stellar astronomy seems to be also able to provide the dates for the beginning and the end of the
two seasonal periods. In fact, in our opinion, there were two stars, in opposite positions in the sky,
that would provide a good indication for the typical seasonal periods of Celtic agricultural life. The
use of the astronomical observation of the stars serves as high-precision tools for agricultural
purposes. This is not possible if we are to rely solely on the apparent solar and lunar monitoring.
Finally, were also noted some correlations between the importance of the deities of the Celtic
pantheon and the stars visible or rising with the Sun on the days of the celebration of the traditional
festivals.
From the calendric point of view, we have taken into account the calendar of Coligny and have noted
that the agreement between stellar astronomy, lunar and solar constraints can be reached only by
having recourse to a dynamic use of the intercalary months present in the calendar. The object of
the present work was not the close analysis of the calendar, itself, but to show the importance of
Astronomy in general for the Celtic culture. A detailed, and very promising, study is in progress and
will be published in the near future.
We must, nevertheless expose here a number of interesting considerations imposed by the
hypothesis of the dynamical use of the calendar.
The first is that in order to make a successful dynamic use of the calendar it is necessary to make
systematic, and not just occasional, astronomical observations throughout the time.
The second consequence is the necessity of determining the positions of the Sun and of the Moon,
in the sky, so as to obtain the accuracy required to decide when and how to insert the intercalary
days.
A third interesting consequence is that the accuracy required to check continuously the apparent
motion of the Sun and the Moon as well as their positions with respect to the stars implies the
necessity to having access to devices capable of achieving the required accuracy.
We are well aware, of course, that the Celts were able to perform only naked eye astronomical
observations, but the required accuracy can be achieved by making use of natural lines of sight
based on the projection of distant natural configurations, like mountains' profiles for instance, on the
celestial sphere.
This process was ordinary working situation for the astronomers belonging to the pre-celtic cultures
that built the megalithic observatories; we think that the Celts inherited this astronomical
observational methodology and used it for calendric purposes. A natural consequence of this is that
the Celts could also have used the pre-existing megalithic observatories for their own astronomical
observations.
Adriano Gaspani
41
References
C. J. Caesar : "De Bello Gallico", VI, 14.
Cleomedes: "De Motu", I,11 p. 106, 25 to 108, 5 Ziegler.
A. M. Duval and G. Pineault: 1986 "Requeil des Inscriptions Gauloises Vol III : Le Calendriers
(Coligny, Villards d'Eria)", XLV Suplement a "Gallia", Editions du CNRS.
A. Gaspani: 1994 "On the visual sensing of the faint light sources"
Poster paper presented at the International Workshop on Cometary Astronomy,
Selvino - February 1994.
Ho Peng Yoke: 1962, "Ancient and Mediaeval Observations of Comets and Novae in Chinese
Sources", Vistas in Astronomy, pag.127.
D. Laurent : 1990, "L'Histoire, l'espace et les milieux de la tradition - Le juste milieu. Reflexion sur
un rituel de circumambulation millenaire: la tromenie del Locronan", extract de "Tradition et histoire
dans la culture populaire", Doc. d'Ethn. Reg. No.11, C.A.R.E.
P. Mela : III, 2.
Plinius: "Historia Naturalis", XVI, 249.
42