Download The earliest datable observation of the aurora borealis

Aurorae
The earliest datable
observation of the
aurora borealis
F Richard Stephenson, David M Willis and Thomas J Hallinan document
observations of the aurora borealis from a Babylonian clay tablet from
more than 2500 years ago – probably the earliest reliable account.
A
s many as 3000 Late Babylonian astronomical texts (LBAT) are preserved
today. These texts, which range in date
from about 650 BC to AD 75, are all in the
form of baked (or sun-dried) clay tablets
inscribed on both faces with a cuneiform syllabic script. This writing was produced by
pressing a pointed reed into moist clay, thus
forming a series of triangular wedges. All of the
extant tablets are damaged – some extensively
so. The largest pieces are more than 20 cm
across, but many fragments are scarcely larger
than a typical postage stamp. The text that is
the subject of this paper (see figure 1) measures
about 10.9 cm by 10.6 cm.
Much damage probably occurred when the
tablets first came to light. Most of the preserved
texts were unearthed accidentally during the
1870s by inhabitants of the nearby town of
Al-Hillah, who were in the process of digging
over the site of Babylon for bricks to use in new
building work. By comparison, only a few
tablets were uncovered as the result of planned
excavations. Most of the texts were acquired by
the British Museum, although a few LBAT are
in other collections around the world. In particular, the tablet from the 37th year of King
Nebuchadnezzar II is in the Vorderasiatisches
Museum, Berlin (VAT 4956). In the rest of the
paper this tablet will be frequently referred to
as the “Berlin text” for brevity.
Abstract
The Late Babylonian astronomical texts,
discovered at the site of Babylon (32.5°N,
44.4°E) more than a century ago, contain
what is probably the earliest reliable
account of the aurora borealis. A clay
tablet recording numerous celestial
observations made by the official
astronomers during the 37th year of King
Nebuchadnezzar II (568/567 BC) describes
an unusual “red glow” in the sky at night;
the exact date of this observation
corresponds to the night of 12/13 March
in 567 BC. The most likely interpretation
of the phenomenon is an auroral display.
This event occurred several centuries
before the first clearly identifiable
observation of the aurora from elsewhere
in the world, namely China in 193 BC.
The Babylonian auroral observation is
remarkable in the sense that it is one of a
series of carefully recorded astronomical
observations, for each of which the year,
month and day are known precisely. This
observation occurred at a time when the
geomagnetic (dipole) latitude of Babylon
was about 41°N compared with the
present value of 27.5°N, suggesting a
higher auroral incidence at Babylon in
567 BC than at present.
Babylonian astronomical diaries
The largest proportion of the LBAT is in the
form of astronomical diaries and it is in this category that the Berlin text may be placed. In their
pristine condition, such diaries probably contained almost daily reports of various celestial
phenomena by the Babylonian astronomers for
at least 800 years, commencing around 750 BC.
However, a statistical estimate based on the
content of the extant diaries indicates that only
about 5–10% of the original material is preserved today. For a useful illustration of the
degree of preservation at different periods, see
figure 2 in Sachs (1974). The earliest known
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astronomical diary dates from 652 BC (Sachs
and Hunger 1988). At the other end of the scale,
no diaries are known to be more recent than
61 BC. Several later almanacs, containing predictions based on the data in what must then
have been recent diaries, have survived. The
most recent almanac, from AD 75, is the latest
datable text written in cuneiform (Sachs 1976).
When complete, a typical diary summarized
several months of day-to-day (and night-tonight) observation. Most of the records related
to astronomical phenomena, although there are
also frequent meteorological reports. It appears
that the various events were recorded as they
were observed, not retrospectively when a few
months of data had been compiled. Most of the
celestial occurrences that were noted involved
the Moon and planets: e.g. eclipses; times of rising and setting of the Moon relative to sunrise
or sunset; conjunctions of the Moon with planets and certain stars; planetary conjunctions;
and heliacal risings and settings of planets. At
an early period, these various phenomena were
recognized to be cyclical and thus approximately predictable if sufficient observational
material were accumulated. In addition, other
remarkable events of a sporadic nature – for
example comets and bright meteors – were also
occasionally noted. Weather information
included incidences of overcast conditions,
mist, rain or severe cold, while lunar and solar
haloes frequently attracted attention.
Until 1955 very few of the LBAT had ever
been published, while extensive translations
have only become available within the last 15
years or so. Sachs and Schaumberger published
drawings (but not translations) of some 1500
texts (Sachs and Schaumberger 1955). Later,
Sachs gave a detailed summary of the general
form and content of the LBAT (Sachs 1974).
However, it was not until 1985 that the importance of the Babylonian astronomical observations became widely known. This resulted from
the discovery of Babylonian records of Halley’s
Comet in both 164 and 87 BC (Stephenson et
al. 1985, Stephenson and Walker 1985). Very
recently, photographs, transliterations and
translations of all the datable astronomical
diaries from 652 to 61 BC have been published
by Hermann Hunger, who has incorporated the
unfinished work of the late Abraham Sachs
(Sachs and Hunger 1988, 1989, 1996).
The Babylonian auroral record
The Berlin text is the second oldest Babylonian
astronomical diary in existence. Unlike so
many other LBAT, its date is well preserved.
The first line of the obverse begins: “Year 37 of
Nebuchadnezzar, King of Babylon”, a date that
is confirmed at the end of the reverse side of the
text. From historical considerations, the 37th
year of this famous king is well established as
extending from April 568 to April 567 BC on
the Julian calendar (Parker and Dubberstein
1956). The tablet originally covered a full year
of observations, but only data from lunar
months I–III (obverse) and X–XII (reverse) are
now preserved. (Most years consisted of 12
lunar months, each containing either 29 or 30
days; every two or three years a 13th month was
added so that the year always began around the
time of the vernal equinox, although there were
only 12 months in the year 568–567 BC.)
The red glow in the sky – a phenomenon that
has no parallel in later Babylonian astronomical diaries – is stated to have occurred on the
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Aurorae
night of the 29th of the 11th month. According
to the calendrical tables (Parker and
Dubberstein 1956), this date corresponds to
some time between sunset on 12 March and
sunrise on 13 March in 567 BC (the Babylonian
day began at sunset). We have confirmed that
this Julian date is precisely correct by computing some of the lunar phenomena recorded in
the same diary during the 11th and 12th
months. These phenomena include measured
time-intervals between sunset and moonset at
new Moon and conjunctions of the Moon with
reference stars (Stephenson and Willis 2002).
A transliteration and translation of the text
that describes the phenomenon on the night of
12/13 March in 567 BC can be given as follows
1: The reverse of the Babylonian tablet (VAT 4956)
that contains the record of the aurora in 567 BC.
(Staatliche Museen, Berlin/Olaf M Teßmer)
(Sachs and Hunger 1989):
“GE6 29 a-ku6-ku6 -(ku6)-tu4 ina S̆Ú KUR 2
DA[NNA....]”
“Night of the 29th, red glow flared up in the
west; two double-[hours...]”.
It is unfortunate that the remainder of the entry
is no longer preserved. The next entry, two days
later, records the sighting of the young crescent
Moon; this began the 12th month (there were 30
days in the 11th month). However, although
lunar observations are also reported on the
evenings of the 2nd and 3rd of the new month
– suggesting that clear sky prevailed – there is no
further mention of the red glow. Presumably it
was a relatively short-lived phenomenon.
In the above transliteration, the ku6 placed in
parentheses should be omitted. As was recognized long ago (Neugebauer and Weidner 1915),
in a similar transliteration and translation (into
German) of the passage, a scribal error has
occurred here. The Chicago Assyrian Dictionary
(Gelb et al. 1964), which also comments on the
6.16
same text, interprets the glow in the sky as
occurring “when dusk was falling”, rather than
in the west. However, there is strong evidence
that the key expression ina S̆Ú should be translated as a direction (“west”), rather than a time
(“dusk”). According to H Hunger (private communication), the diaries have a very rigid terminology, and it is clear from many parallel texts
that ina S̆Ú is used to indicate the approximate
direction to sunset, i.e. “in the west”.
Furthermore, time indications in the diaries
always stand at the beginning of a sentence
describing an observation. In the auroral text, ina
S̆Ú follows the term for “red glow” (akukutu),
implying that it is not a time indication.
In principle, the time interval of two double
hours (i.e. four hours) in the above quotation
could have related to either
the duration of the phenomenon or the start of
some subsequent event. If
the latter alternative
were correct, the second
event must have
occurred during the
same night as the red
glow; following established practice, the date
would have been repeated
for a daytime phenomenon
that took place during the
same 24-hour period. It is difficult to suggest any other
night-time occurrence that could
be referred to. Certain events
involving the Moon (e.g. eclipses)
were timed, but on the night when
the red glow appeared in the sky the
Moon would have been very close to the Sun
and thus invisible (there was no solar eclipse at
that time). Our computations based on the crescent visibility criteria of Neugebauer (1929)
indicate that in the 11th month the last visibility of the crescent would have occurred on the
morning of either 11 March or 12 March; by
the morning of 13 March the Moon would have
already passed conjunction with the Sun and
would reappear in the evening sky on 14
March.
Inspection of a large number of astronomical
diaries shows that planetary phenomena that
did not also involve the Moon were never timed
– even at much later periods in Babylonian history. Similar remarks apply to ordinary meteorological phenomena (such as rain showers or
thunderstorms) and also the occasional human
events noted in the diaries. In view of these
remarks, there seems little alternative to supposing that in the above quotation the duration
of the red glow is referred to.
Since there is no other example of the use of
akukutu in other extant Babylonian diaries,
which cumulatively cover about 100 years of
data, a very rare atmospheric phenomenon
seems to be referred to. However, it is noteworthy that the same expression is found in several more ancient omen texts (some of which
are perceived to be earlier than 1000 BC, while
others extend up to the 7th century BC). These
involve akukutu in conditional statements
(Weidner 1912, Neugebauer and Weidner 1915,
Gelb et al. 1964, Hunger 1992), which indicate
that the term akukutu could apply to either a
daytime or night-time phenomenon. The following examples may be cited:
(i) “If in Sivan (month III) an akukutu blazes,
there will be hostility in the land.”
(ii) “If the day becomes overcast and there is an
akukutu in the sky…”
(iii) “If the night (sky) is tinged with a fiery light
and an akukutu flares up…”
(iv) “If an akukutu is high up in the south…”
In the Chicago Assyrian Dictionary (Gelb et
al. 1964), it is asserted that “since akukutu
denotes an exceptional meteorological phenomenon of bad portent, it probably refers to the
aurora borealis”. On the contrary, the term
akukutu has been interpreted as nothing more
than the red dawn or twilight glow (Weidner
1912). In view of the diversity of the above
examples, it would seem better to decide each
case on its own merits, rather than making a
generalization. The second example quoted
above would seem to refer to some phenomenon of the lower atmosphere – e.g. a dust
storm. However, it would be difficult to explain
the third example as anything other than an
auroral display.
Scientific interpretation of the auroral
record
Since the event described in the Berlin text
occurred close to the vernal equinox, the Sun
would have set almost due west. However, the
recorded interval of as long as four hours precludes the interpretation of the phenomenon as
an extended sunset, caused by scattering of light
from volcanic dust in the stratosphere.
Moreover, since the colour of the light is stated
to be “red”, the observation cannot refer to the
zodiacal light. The duration of an auroral display
at low geomagnetic latitudes can vary between
about an hour and a few days (Yau et al. 1995)
but, on the basis of more modern observations,
the longer durations are almost certainly associated with intense geomagnetic storms. It should
also be noted that the really intense geomagnetic
storms tend to occur most frequently near the
equinoxes (Taylor et al. 1996).
The first of the visions experienced by the Old
Testament Prophet Ezekiel (I: 1–28) has been
interpreted as an auroral display (Eather 1980,
Silverman 1998, Siscoe et al. 2002). The date of
this event is recorded in a precise form (year,
month and day) but the interpretation of the
year (possibly 593 BC) is questionable
December 2004 Vol 45
Aurorae
(Greenberg 1983). Moreover, it is arguable that
a geophysical interpretation of Ezekiel’s vivid
imagery is highly speculative. By contrast, the
Babylonian text from 567 BC is contained in a
diary devoted to the recording of astronomical
phenomena (mainly involving the Moon and
planets), which was compiled by observers with
specific responsibility for watching the sky.
In (later) Chinese history we find several
instances of auroral reports resembling the
description in the Berlin text. Two examples are
as follows (Yau et al. 1995):
AD 567 May 31: “Early in the night, in the
northwest, there was a red vapour extending
across the sky. At midnight, then it was extinguished.”
AD 1137 February 14: “At night, in the
northern direction, there was a red vapour; it
lasted until dawn.”
The most common colour in the high-latitude
aurora is green. However, the relatively rare
aurorae that are seen at low latitudes are most
commonly red and consequently are known as
great red aurorae. Such aurorae are observed
most frequently in a general northerly direction
from a particular site, but under very intense geomagnetic activity the main auroral display
moves even further towards the equator and can
sometimes be seen to the south of the observing
site. The most usual form of auroral display is a
long luminous band that arcs from horizon to
horizon in a magnetic east–west direction. If the
band is overhead, an observer looking west or
east observes a long path length through the
luminosity and therefore sees a brighter glow.
This would be described as a red glow in the west
or east, depending on which region is brighter.
The aurora borealis has been a permanent feature of the night sky in the northern polar
regions for many millions of years (Eather
1980). Because auroral displays are usually visible in largely frozen, unpopulated regions of
the Earth, however, ancient and medieval
astronomers, chroniclers, philosophers and
writers did not witness them frequently.
Occasionally, spectacular auroral displays have
appeared at more temperate latitudes but the
comparative rarity of such occurrences is
reflected in the relative paucity of historical references to ancient auroral displays (Eather
1980, Schröder 1984, Brekke and Egeland
1994). Nevertheless, the historical auroral
records that do exist provide a potentially valuable resource for the study of secular solar and
auroral variability over about two millennia
(Siscoe 1980, Silverman 1992). Although the
Babylonian auroral observation is essentially an
isolated event, at a relatively remote epoch, it
is important because it can be dated exactly and
it occurred at a time when the orientation of the
geomagnetic field was substantially different
from that pertaining to the present epoch
(Constable et al. 2000).
December 2004 Vol 45
Auroral isochasms (lines of constant auroral
occurrence frequency that circumscribe the
magnetic poles) based on auroral data acquired
during the interval AD 1700–1942 (Fritz 1881,
Vestine 1944, Chapman 1957, Oguti 1993) suggest that the expected frequency of bright aurorae at Babylon (32.5°N, 44.4°E) should be
significantly less than once per decade. Indeed,
the lowest-latitude isochasm (annual frequency
of 0.1) in the published figures is about 12.5°
north of Babylon (Fritz 1881, Vestine 1944).
However, the auroral isochasms for the interval
AD 1700–1942 are not applicable to 567 BC,
since average spherical harmonic models (100year means) of the geomagnetic field imply that
the geographic co-ordinates of the north geomagnetic pole (centred dipole axis) were about
82°N and 35°E around 567 BC (Constable et
al. 2000). Therefore, the geomagnetic (dipole)
latitude of Babylon was about 41°N in 567 BC,
whereas it is currently 27.5°N. Moreover, since
the north geomagnetic pole was essentially
tilted towards Babylon in 567 BC, the lowestlatitude auroral isochasm (annual frequency
0.1) may have been no more than just a few
degrees north (or possibly even south) of
Babylon. Consequently, the occurrence frequency of bright aurorae in Babylon in the 6th
century BC could easily have been approximately once per decade, even without allowing
for any questionable short-lived geomagnetic
excursion at that time (Raspopov et al. 2003).
There may well have been further reports of
aurorae on the original LBAT, but such is the
fragmentary state of extant texts that no other
reports have survived. The references to
akukutu in the omen tablets suggest that the
phenomenon was known considerably earlier
than the observation reported in this paper.
Conclusion
In recent years, studies of the LBAT have
revealed many remarkable observations that
have proved to be of considerable scientific
interest. These include some of the oldest
known records (164 and 87 BC) of Halley’s
Comet (Stephenson et al. 1985, Stephenson and
Walker 1985) and an extensive series of lunar
and solar eclipse reports that are of great value
in studying long-term variations in the Earth’s
rate of rotation (Stephenson 1997). Our discovery of what we believe to be the oldest reliable
– and readily identifiable – observation of the
aurora borealis suggests that the scientific relevance of the LBAT may be far from exhausted. ●
F Richard Stephenson, Dept of Physics, University
of Durham, Durham DH1 3LE, UK; David M
Willis, Dept of Physics, University of Warwick,
Coventry CV4 7AL, UK and Rutherford Appleton
Laboratory, Chilton, Didcot, Oxon OX11 0QX,
UK; Thomas J Hallinan, Geophysical Institute,
University of Alaska, Fairbanks, AK 99775, USA.
Acknowledgment: The authors thank Prof. David
Hughes of Sheffield University and Dr Christopher
Walker of the British Museum for reading the
manuscript and offering several valuable comments.
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