Download Theme 3.1 Astronomy of the Ancients Stonehenge Most people

Theme 3.1
Astronomy of the Ancients
Stonehenge
Most people instantly recognize pictures of Stonehenge, seen here from
the air and at ground level. The famous stone circle in England has
indeed become iconic. It's worth asking, however, whether it is unique
or whether there are other examples like it in the world before we
address the question of any astronomical content. In fact, even if we
restrict our attention just to the United Kingdom there are very many
stone circles, as shown in the map on the upper right. You'll notice
that the lower right, the south-eastern part of England, has few stone
circles. This is the area that was inhabited by the Romans. But in the
outlying regions there are many of them to be found historically. Some
of them are quite large. Look at the one on the lower left which is in
the village of Avebury, not in fact very far from Stonehenge itself.
Others are less imposing, as shown on the lower right, a stone circle
from Scotland.
Arrangements of standing stones can be found in many parts of the
world, not always in circular patterns. For example, we find them in
Sweden on the upper left picture shown here; long arrays of them in the
Carnac stones in Brittany and France as shown in the right hand panel;
and in Africa, as shown on the bottom left. Stonehenge is unique in at
least two ways. The first is the size of the stones, which you can see
stand many feet tall. It would have been a great challenge to raise
these into position using primitive technology. The second is that the
stones are themselves shaped. As you can see in the lower right hand
figure where a mortise and tenon structure is used to help hold the
stones together. The raised lintels are sitting on top of the upright
stones in a way that locks them together like Lego pieces so that there
is more stability to the structure.
In addition, we're equally impressed by the fact that the stones used
in the construction of Stonehenge were brought in from such a long
distance away. We know this from the geology of the stones. We can
track their sources. The upright stones, the sarsens, were brought in
from the Marlborough Downs, a considerable distance north of Stonehenge
and must have been dragged across country. The bluestones were brought
from Wales and were presumably floated on rafts along the coast, as
shown in the right hand side. These are enormous engineering efforts
and represented the fact that the construction of Stonehenge must have
had real significance to the people of the day, presumably largely for
religious reasons.
The interpretation of Stonehenge is a little confused by the fact that
the site was not undisturbed over millennia. It had fallen down in
part, and has occasionally been re-raised as shown in the picture on
the left. This means that the alignment and arrangement of the stones
may not be precisely as it was at the time that it was standing. On the
upper right, you can see a picture that shows various speculations
about the way in which the ancients might have managed to raise the
stones, which, as you can see from the picture on the left, are indeed
very massive. The link at the bottom of this panel leads you to a
recreation of the human efforts that might have been involved in
dragging stones across the country with a set of volunteers who decided
they would try to show that such a big stone could indeed be moved by
simple person-power.
Thanks to archeological investigation and age-dating we now understand
a great deal about the history of construction of Stonehenge, which was
spread over many phases occupying many centuries. Stonehenge is larger
than just the obvious set of stones and began with a large circular
moat surrounding the set of so called Aubrey holes named after the
investigator, 56 of them. The raising of the stone circle itself came
rather late in the process. But in fact the complex is very much larger
than just the Stonehenge site itself. In the vicinity for miles around
there are other sites-- burial mounds and other stone circles such as
that found in Aubrey to the north -- and there appears to have been a
very elaborate complex interplay between all of these sites, not yet
fully understood.
One thing we do know is that there is no real connection between Druids
and the Stonehenge site, despite the popular mythology to that effect.
The idea originated with an investigator names Stukeley who became
quite obsessed with this notion and saw himself as a latter day Druid
as you see in the woodcut to the left. Nowadays, especially at
Midsummer Day in late June, modern day Druids often occupy the site to
have ceremonies of various sorts celebrating this connection, which in
fact is not historically supported.
Various arguments have been made about the astronomical significance of
some of the features in Stonehenge. But it is complicated by the fact
that if you stand within the circle and look in almost any direction
you are bound to see an alignment with various stones that will point
towards the direction of the rising of various stars. It's very easy
coincidently to discover a lot of such connections, perhaps without any
significance at all. One thing that seems fairly certain though is if
you stand at the centre of the trilithon, the largest of the upright
standing three stones, in the middle of Stonehenge and look out along
what may have been the main entrance to the circle, past what is called
the heelstone, you're looking in the direction of the sunrise point at
the summer solstice, the farthest northern point of the sun in its
yearly cycle. That's shown in a diagram on the left and the picture on
the right shows the point.
This may be the only astronomical connection in Stonehenge whatsoever,
and may be analogous to the way in which we built cathedrals with the
apse pointed in the eastward direction, symbolic and nothing more. It's
been suggested at various times that the Aubrey holes could have been
an eclipse predictor. There are 56 Aubrey holes, and you may remember
that the cycle of the repetition of eclipses is in the mid-50 year
range. But that is probably not the case. The Aubrey holes were so
early in the construction of Stonehenge that they presumably were too
primitive in intent to have done anything so elaborate as to be used
for the prediction of eclipses. So the astronomical content of
Stonehenge, as fascinating as it is, may be rather limited.
The Pyramids
Let's turn now to the Egyptian pyramids, of which more than 120 are
known in all, across the land of Egypt. The most famous of these of
course are the Giza pyramids shown on the bottom right here, as they
stand right next door to Cairo. Pyramids are not unique to Egypt, of
course. We know them in other cultures as well. For example, here is
one in Chichen Itza in Mexico, this is from the Mayan people; and
another one shown from Cambodia.
We'll focus our attention on the Giza pyramids that lie just outside
Cairo as they are very well studied and have interesting internal
structures that seem to have a clear astronomical intent. The picture
on the upper left reminds us of the appearance of the pyramids just
outside the capital city and the other two pictures point out the fact
that they are made of elaborate piles of huge blocks of stone that were
brought in from a site on the far side of the Nile, one that's been
well explored archeologically.
The archeological record of pyramid building in Egypt is well
understood, and in fact shows the development of building skills and
techniques as the centuries progressed. Some early pyramids actually
fell down and were modified in various ways. Construction techniques
changed with time to provide greater stability. The link at the bottom
of this panel will lead you to an interesting article in which there is
a discussion of how the pyramids were actually built -- not
technologically, but rather the social structure. It's clear that they
were not built with pure slave labour in the way that is often depicted
in the movies.
Let's now consider the interior structure of one of the great pyramids
of Giza, shown in cutaway view in the upper left. You can see the
King's chamber where the King's mummified body would have been laid to
rest, along with treasures and so on. The grand gallery, the Queen's
chamber, various passages for access and air and so on: quite an
elaborate structure buried deep within that huge pyramid of stone.
On the lower right we see various alignments represented, and we see
that they seem to have an astronomical connection. From the Queen's
chamber for example there is a thin access shown that points in the
direction of Sirius. From that location within the Queen's chamber
looking through that tiny vent hole, one would have seen the light of
the star Sirius as it passed across the field of view during the
rotation of the starry realm at night. Likewise in the King's chamber,
there is an alignment toward Orion's belt and a couple of other vents
lead in the direction of well-known stars to the north as shown off to
the right. These are presumably by design and there seem to be clear
astronomical connections. We also know for example that the star Sirius
and the constellation of Orion were of particular significance to the
Egyptians for religious reasons.
One last interesting point is that one of the vents within the pyramid
points directly toward the position of the current pole star, and in
the days of the Egyptians, glancing through that vent would have drawn
one's attention to a pole star -- but not, interestingly, Polaris. As
we saw earlier, the earth is slowly changing in its orientation in a
phenomenon known as procession that's shown in the left-hand panel
where we see that currently the spin access to the earth, the North
Pole points towards Polaris. But over the passage of many centuries
that direction slowly changes as the Earth's spin tips.
In about 14,000 years we'll be pointing towards the star Vega, which
will then be the pole star. On the right hand side, we see a
representation of how this changes over time. To the right of that
figure you can see that the current pole star is indeed Polaris, lying
above our North Pole. And if you look a little bit clockwise of that
you will see that about 4000 years ago, in the time of the ancient
Egyptians and the construction of the pyramids, the pole star would
have been Thuban, which would have sat directly above the North Pole of
the earth and to them have been a conspicuous North Star. Notice that
in between, at about zero AD, the time of the birth of Christ, there
was no pole star at all. If you follow the circle around you can see
that Vega will be close to the pole, although not so well aligned as
Polaris in about 13 or 14 thousand years. And the cycle repeats.
Greek Cosmology: the Ptolemaic Model
Let's now turn our attention to ancient Greek cosmology, their
understanding of the nature of the solar system. Needless to say there
were many thinkers and many expositors in this subject over numbers of
centuries, but we'll focus our attention on the Ptolemaic model. That
is to say the model of the nature of the solar system as interpreted by
Ptolemy himself, as this became the dominant mode of thinking about the
solar system until about 1500 AD in European scientific circles.
Ptolemy began with a number of working assumptions. First, that the
earth was at the centre of all motions. Secondly, that all motions were
in circles which are the perfect geometrical forms as befitted motions
in the heavens. And finally that all motion is at uniform unchanging
speed. Very simple assumptions but these lead quickly to several
complications.
Remember of course that the ancients were able to see the planets only
as moving dots of light across the sky, and it is remarkable therefore
that they were able to correctly deduce that Saturn was the most remote
of those planets. Next to it is Jupiter and then the next most remote
is Mars, for instance, as shown in the figure here. The details break
down when we consider the location of the nearer planets, Mercury and
Venus and the sun. But that's because of the incorrectness of the model
that has the earth as the centre and everything revolving around it.
Still they did remarkably well in understanding the relative distances
of the outer planets at least.
We noted the very simple assumptions that Ptolemy had made about the
motion of the planets and the shapes of their orbits, but this leads to
immediate problems. For instance, he insisted that the planets had to
be moving in circular motion centred on the earth at uniform speed. How
then do we explain the retrograde motion of the outer planets -- the
fact that they occasionally seem to come to rest, move backwards for a
time and then move forward again? This is a remarkably difficult
complication.
Secondly, if Mercury and Venus are orbiting the earth, as do all the
planets, why is it that they never stray very far from the sun from our
point of view? We never see Mercury or Venus overhead at midnight, but
the other planets can be in that location. Why the distinction?
Thirdly, we sometimes see the solar eclipses as total and other times
annular. This implies that the moon's distance can't be constant. When
the moon is close to us, iy can block off the sun entirely. If it's a
little farther away, it fails to do so and we see an annulus, a ring of
light. This means its orbit cannot be a simple circle as Ptolemy had
assumed. And finally we note also that the moon and the sun appear to
move at varying speeds across the starry background -- again
inconsistent with simple circular motion.
Here is a reminder of what we mean by retrograde loops. If you were to
go out night after night and look at the motion of Jupiter in this
case, over the winter of 2004-2005, you would have noticed it behaving
as shown here. It begins by drifting slowly from right to left, from
west to east, across the starry backdrop. But then as you get towards
the end of 2004 that motion slows, and it turns around and starts to
drift back to the right again, finally coming to a halt a second time
in June of 2005, and then picking up speed and moving off to the left.
This is not something we see on short time scales: it is over a period
of many months, but nonetheless a very dramatic and strange motion
inconsistent with simple Ptolemaic assumptions.
Ptolemy was determined to preserve circular motion and constant speed,
so he accommodated the retrograde motion by building a set of ‘circles
within circles’ as shown diagrammatically here. Look at the left hand
panel for example, and you see that Mars moves around a small circle
called an epicycle, and in the meantime the centre of that epicycle
moves around the earth itself. The net effect is that Mars seems to
make little loops back and forth as it passes across our field of view.
The situation is even more complex than that because in fact he decided
that he could afford to move the centre of these larger circles (called
deferents) away from the earth. So in fact the earth is no longer the
unique centre of the solar system, which would seem to violate one of
his fundamental assumptions. The situation became yet more complex in
ways that are described in the movie to which I’ve provided a link, but
please don't agonize over the details. It's very difficult to try and
describe all of this but I'd like you to at least watch that and get
some sense of just how complex the whole model became very quickly,
with the circles within circles to try and save the observations.
There's actually an astonishingly simple explanation for retrograde
motion, but that didn't become clear for 1500 years until the
Copernican revolution. In quick anticipation of that, though, here is
the modern correct explanation. The crucial ingredients are the
following: (a) All the planets orbit the sun: the sun is the centre of
the solar system. (b) The planets that are farther out move more
slowly. The earth therefore is overtaking some of these outer planets
on the inside track, so to speak. And that means that they appear to
drift backwards for a time.
This is shown in a diagram on the right, here, where we see the earth
moving from positions numbered one through seven, over about half of
its yearly orbit, in the same time as an outer planet moves a smaller
distance, the seven dots shown. Our perspective means, therefore, that
we see the planet seem to slow down and drift backwards for a time
because we are, as I say, overtaking it ‘on the inside track.’
If this static picture doesn't clarify that understanding for you, look
at the link provided to a small computer realization of the cause of
retrograde motion which, in a Copernican model with the sun at the
centre, is very straightforward to understand.
Ptolemy next had to explain why Venus and Mercury never seem to stray
very far from the sun from our point of view on the earth. He answered
this in the following way: he assigned them each an epicycle so that
Venus for example would be moving around its small epicycle at the same
time as that was moving around the earth in the larger circle, as shown
in the diagram here. However he required that the epicycles of Mercury
and Venus would always be aligned with the sun so that, for example, we
would see Venus sometimes a little ahead of or to the left of the sun
and visible in the evening sky; at other times it would be a little bit
behind or to the right of the sun, visible in the morning sky -- but
never very far away from the sun in the sky and likewise Mercury. No
such constraint was required of the outer planets, Mars, Jupiter,
Saturn. And no explanation was given why Mercury and Venus needed to be
treated in this very special way.
As you can see the Ptolemaic model became very complex and very
contrived! It's interesting to ask whether Ptolemy and those who
followed him really believed that the solar system had this complicated
structure, or whether they just treated this as a useful working tool
to predict where the planets would be into the future, perhaps for
astrological reasons. We may never be sure of the answer to that
question but it is certainly contrived!
King Alphonso X, in the thirteenth century, commissioned a set of
‘Alphonsine Tables’ to show where the planets would be for months,
years, and decades to come, and on seeing the work that this entailed,
he remarked that “if the Lord almighty had consulted me before
embarking on creation I should have recommended something simpler.” But
the bottom line is that it did indeed work, and the calculations gave
good, reliable prescriptions for working out where the planets would be
for many centuries.
The Antikythera Mechanism
Ptolemy’s model was in a sense a ‘paper clock’: one could use the
calculations to work out where the planets would be. Interestingly, in
1900 a real piece of clockwork was found that dates back to ancient
Greek times, and shows a remarkable level of technology which we had
not appreciated until recently. In 1900, pearl divers found a
shipwreck near Antikythera, and a mysterious mechanism, badly corroded
and collapsed was found on board. Pictures of it are shown here.
The Antikythera mechanism has been studied very deeply ever since then,
and it appears to have been a mechanical machine for predicting the
motions of the sun and the moon and the planets, phenomena like
eclipses, and so on: a very sophisticated mechanism for the times, and
beyond what we had originally thought the technology was capable of for
those days. There is a very nice realization of it here, as it may have
appeared in life. Please follow the link and enjoy the presentation.
The Antikythera shipwreck had more than just the mechanism on board.
There was also a bust found of the philosopher of Antikythera, shown
here in a picture on the left. This is found actually in the museum in
Athens, where I picked up this postcard. On the right there is a
picture of another person. I encourage you to think about whether or
not there is a resemblance.
As noted, Ptolemy's model works pretty well. Up to the time of the
Alphonsine Tables, it was still being used to predict where the planets
would be for many years into the future. But how would you ever prove
the model wrong, as contrived as it is? Well, there are two obvious
answers. First of all, if we could demonstrate that Venus and Mercury
go around the sun, that would clearly resolve the issue. How would we
do that? Well, we'll see. Secondly if the earth is moving around the
sun, than we should note parallax of nearby stars: a nearby star will
appear to drift around because of our changing perspective. That would
prove that the earth itself is moving through space. Both of these
discoveries needed the development of telescopes and that's something
we look forward to in the coming units.