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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.