B00B7H7M2E EBOK, page 4
As for astronomers, what mattered most to them was that there was no observational evidence whatsoever to support the vast distances to the stars that Aristarchus’s scheme required, while there was observational and physical evidence that made his Sun-centred arrangement seem highly unlikely:
If the Earth moves round the Sun, then we on the Earth should see some variation in the positions of the stars, relative to one another, as we view them from different points along the Earth’s orbit. No such variation had been observed (nor could it be with the technology available at the time). Aristarchus saw that this objection wouldn’t be valid if the stars were far enough away. Hence his suggestion that they were very far away indeed, perhaps even at infinite distance. The fact that the position of the stars, relative to one another, does change as Earth orbits – that there is ‘stellar parallax motion’ – wasn’t confirmed by observation until the mid-19th century.
If the Earth rotates on its axis, in fact, if it moves at all, this should have some noticeable effect on the way objects move through the air. Ancient astronomers realized that if the Earth rotates on its axis once every twenty-four hours, then the speed at which any point on its surface is moving is very great indeed. So, how could clouds, or things thrown through the air, overcome this motion? How could they ever move east? Even if not only the Earth but also the surrounding air rotates on the Earth’s axis, solid bodies moving through the air should still in some way show the influence of the Earth’s rotation.
It’s plain to see that heavy objects travel towards the centre of the Earth. If this law applies to heavy objects everywhere, then the centre of the Earth must be the centre of gravity for all things in the universe that are heavy. Furthermore, once a heavy object reaches the place towards which its natural movement sends it, it comes to rest. Applying this idea to the Earth, the inevitable conclusion is that the Earth must be at rest in the centre of the universe and that it cannot be moved except by some force strong enough to overcome its natural tendency. This argument was based on Aristotle’s concept of ‘natural’ places and ‘natural’ movements. It is easier to see the validity of it if you realize that Aristotle thought of everything beyond the Moon being made up of something called aether, which was neither ‘heavy nor light’.
The Sun-centred model did nothing to solve a problem astronomers had long been grappling with: the inequality of the seasons measured by the solstices and the equinoxes.
It would be inaccurate, and unfair to Aristarchus’s contemporaries, to say that his Sun-centred model was suppressed because of their ignorance and closed-mindedness. The fact is, it was an inspired guess that we now have the means to know was right. But there actually was nothing coming from observation then to recommend it over what was the more orthodox view of the universe – the Earth-centred view that had been around for hundreds of years and that would be brought to its most sophisticated form in the work of the astronomer Claudius Ptolemy (not necessarily related to the Ptolemaic dynasty) four centuries after Aristarchus. Ptolemy’s model would brilliantly explain the movements of the planets if the Earth is the centre – and, indeed, it solved the problems of astronomy as they were perceived at the time better than Aristarchus’s model. Aristarchus’s idea was a seed sown far too early, in a season in which it could not possibly germinate and take root. Ptolemy’s Earth-centred astronomy would dominate thinking about the cosmos until the 16th century AD.
That is not to say that no further progress was made in ancient times towards understanding the heavens.
Hipparchus of Nicaea, who lived in the second century BC, was one of the most skilled astronomers the world has known, and he laid the foundation for much that was to follow. Hipparchus had at his disposal a priceless collection of Babylonian astronomical records – a legacy of Alexander’s conquests – which he put to splendid use in his own astronomy, meticulously comparing the positions and patterns of stars and planets over the centuries with those he observed. Like Aristarchus and Eratosthenes, Hipparchus tried to find a way to calculate the distances and dimensions of the Sun and Moon. Part of his inheritance from the Babylonians was eclipse records spanning many hundreds of years. He also used a new line of reasoning, focused on the fact that there is no discernible change in the Sun’s position against the background of stars when we move from one point to another on the surface of the Earth. To put that in more scientific language, there is no ‘solar parallax motion’. Hipparchus worked on the assumption that observers on the Earth only just miss seeing solar parallax – in other words, that solar parallax is just below the threshold of visibility – and took it from there, with little success in terms of matching modern calculations.
One of Hipparchus’s most impressive achievements – which came from comparing his own observations with observations made about 160 years earlier – was discovering the change in the relative positions of the equinoxes and the fixed stars. That is, if we look at the stars on the evening of the spring equinox, and then again on the evening of a spring equinox some years later, the stars will not be in the same position. In fact, they won’t be in the same position again for 26,000 years! This phenomenon is the ‘precession of the equinoxes’. Though Hipparchus couldn’t discover its cause, he gave an accurate estimate of the rate of this change.
Of all Hipparchus’s writings only one youthful, minor work survives. Next to nothing is known about his life or where he spent it, except that his name indicates that he must have hailed originally from Nicaea, in the northern part of what is now Turkey. Information about his accomplishments comes only from the references of others, mainly Ptolemy, but that evidence is sufficient to show that Hipparchus was an extremely fine astronomer and that he vastly improved observational techniques.
Where did Hipparchus stand in the competition between Aristarchus’s model of the universe and the more orthodox one? Definitely pro-orthodox. Hipparchus was among those who did not accept Aristarchus’s Sun-centred cosmos, and he influenced others to reject it. Hipparchus felt obliged to abide by the evidence of observation – observational astronomy was, after all, one of his fortes – and, as we have seen, observation didn’t support Aristarchus and couldn’t confirm the enormous distances required by the Sun-centred model. Hipparchus’s own work contributed significantly to Ptolemy’s later Earth-centred model of the cosmos. Some scholars even insist that Ptolemy’s astronomy was by and large a re-editing of Hipparchus’s, that Hipparchus was the genius and Ptolemy the textbook writer.
The Roman Pliny the Elder wrote of Hipparchus:
Hipparchus did a bold thing, that would be rash even for a god, namely to number the stars for his successors and to check off the constellations by name. For this he invented instruments by which to indicate their several positions and magnitudes so that it could easily be discovered not only whether stars perish and are born, but also whether any of them change their positions or are moved and also whether they increase or decrease in magnitude. He left the heavens as a legacy to all humankind, if anyone be found who could claim that inheritance.
‘If anyone be found . . .’?
CHAPTER 2
Heavenly Revolutions
AD 100–1600
Now authorities agree that Earth holds firm her place at the centre of the Universe, and they regard the contrary as unthinkable, nay as absurd. Yet if we examine more closely it will be seen that this question is not so settled, and needs wider consideration.
Nicolaus Copernicus, De revolutionibus orbium coelestium
A FEW YEARS ago, Harvard astrophysicist and science historian Owen Gingerich received a flyer in his mail offering a $1,000 prize for ‘scientific proof-positive that the Earth moves’. A Mr Elmendorf, who posed this challenge, wrote, ‘As an engineer, I am astounded that the question of the Earth’s motion is apparently not “all settled” after all these years. I mean, if we don’t know that, what do we know?’
Indeed, whether the Earth moves can hardly be classed as one of the great unsolved mysteries of science. Most of us have accepted since we were children that we live on a planet that revolves on its axis and orbits the Sun. We learned in school that Nicolaus Copernicus introduced this controversial idea in the 16th century and that some men were persecuted for believing it. But in the end . . . ‘all settled’ . . . case closed. That was 400 to 500 years ago. What, we want to ask Mr Elmendorf, is the fuss about? And why has no one won the $ 1,000?
History and science turn out to be far more subtle and ambiguous than we were taught at primary school.
Certainly no scientific knowledge has a better claim to being ‘Truth’, with a capital T, than the knowledge that the Sun is the centre of our planetary system and that the Earth orbits it like the other planets. Yet our own contemporary science backs away and tells us that when it comes to proving whether there is an unmoving ‘centre’ and, if so, where it is, no one can make an air-tight case that any choice is wrong. Pick what you will, the Moon, Mars, the Sun, the Earth, your great-aunt’s dining table – the options are infinite – and it’s possible to come up with a successful mathematical description of our planetary system with that as the ‘centre’. In fact we’re being parochial if we limit the exercise to our planetary system. It would be possible to describe the entire universe using any chosen point as the unmoving centre – and no one could prove ‘You’re mistaken, that thing moves!’
The issue here, we must remind Mr Elmendorf and ourselves, is one of relative motion only. We can measure the motion of an object only in relation to other objects in the universe. We do best to picture everything in motion and nothing as being the centre. But we could, if we set our stubborn minds to it, choose the Earth as centre as our ancestors did and describe everything else correctly in relation to that centre, making a case that the Earth is the centre and the only thing that isn’t moving. If our mathematics were good enough, it would be impossible for anyone to show our choice was wrong. Of course it would also be impossible for us to prove we were right, because we could transfer our allegiance to Venus, or the Sun, or Alpha Centauri, or a black hole at the heart of the Andromeda Galaxy, and make a case for any of those.
If we haven’t given much thought to the implications of 20th-century science, we may be as chagrined as Mr Elmendorf to realize that because of the concept of relative motion, no one can provide knock-down proof that the Earth moves. Did we learn anything from Copernicus? Postpone that question for a moment, for relative motion isn’t Mr Elmendorf’s only problem. One tenet of science is that while an explanation can be extremely convincing and useful, none should ever be considered ‘final’ or ‘proved’, or ‘Truth’. All scientific explanations are, in principle, open to revision and even complete rejection when better ones come along. Henri Poincaré, scientist and philosopher of science, was referring to this open-endedness of science when he wrote: ‘If a phenomenon admits of a complete mechanical explanation, it will admit of an infinity of other [mechanical explanations] which account equally well for all the peculiarities disclosed by experiment.’ Does this apply even to the motion of the solar system? Indeed, that was the example Poincaré used.
This deeper 20th-century insight (or, some may prefer, mere technicality) notwithstanding, every generation tends to believe devoutly in the finality of its own science, always for the same excellent reasons: what we experience presents us with puzzles. We put our trust in plausible solutions, of which we can say, ‘Of course, that explains it!’ We choose the explanations that seem to make the best sense of things as we know them, and of things as we believe future generations will probably know them (to the best of our ability to predict). After all, that is the most anyone can ask of science in any era. But it isn’t final, unassailable truth. We criticize our ancestors for not bearing that in mind, while we continue to err in the selfsame manner.
Carrying our modern world-view along on a visit to the past is notoriously inadvisable, but, in this chapter and the next, doing so selectively would not be a bad idea. Leave behind scientism, the popular belief that current science is final Truth. Instead, bring along a less naive scientific world-view that recognizes the open-endedness of science. Bring along the concept of relative motion. With those things in mind, you’ll be prepared to appreciate and sympathize as two of the most brilliant intellects in history account for the observed movement of pinpoints of light in the heavens.
Almost no information whatsoever exists about the life or personality of Claudius Ptolemaeus, known to us as Ptolemy, except that he worked at Alexandria during the second century AD and died in about 180. There is no record of where he was born, and the name Ptolemy doesn’t indicate he was a member of the ruling family. Like Eratosthenes, Ptolemy was interested in a wide range of subjects including acoustics, music theory, optics, descriptive geography and mapmaking. Some of his maps were still in use as late as the 16th century. More significantly, Ptolemy drew together, from previous ideas and knowledge and out of his own mathematical genius, an astronomy that would dominate Western thinking about the universe for 1,400 years.
Ptolemy inherited an intellectual tradition that placed an unmoving Earth at the centre of the universe and that insisted that all heavenly movement occurred in perfect circles and spheres. Among his contemporaries who thought about such matters, most had come to assume that the physical appearance of things must be taken into serious account when one tried to figure out the structure of the universe. To be believed, an explanation must ‘save the appearances’. That may seem so obvious that it is hardly worth mentioning, but it isn’t an assumption present in all cultures nor was it supported by all schools of thought in the Greek and Hellenistic worlds.
The ‘appearance of things’, for Ptolemy, included what Hipparchus and others had recorded in star catalogues. Ptolemy also brought to his task an in-depth knowledge of previous attempts to explain and predict planetary movement as it is seen from the Earth. The origins of the longing to understand that movement are lost in pre-history, but Plato, in the fourth century BC, focused it in the question: ‘What are the uniform and ordered movements by the assumption of which the apparent movements of the planets can be accounted for?’ The ancient attempts to answer him, beginning with his pupil, the mathematician Eudoxus of Cnidus, were ingenious. Science historians disagree about how much Ptolemy’s work was a synthesis of some of these earlier ideas and how much it owed to his personal genius. Either way it’s clear he was a brilliant mathematician, and his achievement is almost unparalleled in the history of science.
When trying to understand Ptolemy it helps to pay an imaginary visit to a fair or amusement park.
Look first at a carousel designed for very young children. The horses do nothing else but move in a large circle. We’ll put a light on the head of one horse, switch off all other lights, situate ourselves at the centre of the carousel in such a way that we don’t turn with the carousel, and set it going The light circles us steadily, never varying in speed or brightness, never changing direction.
If the Earth were the centre and were not moving or rotating, and if all the planets were orbiting it in circular orbits, we could expect to see each planet as we see the light on this carousel. That is, roughly, the way we see the Moon and the Sun, though their movements include irregularities that foil any attempt to describe them quite that simply. But we definitely do not see the planets moving in this manner, and neither did ancient astronomers and stargazers. Even as early as Plato and Eudoxus, those who studied the heavens knew that a model with simple circular orbits centred on the Earth couldn’t adequately explain what was going on up there.
To illustrate one particularly mysterious problem: on the darkened carousel, suppose we see the light move ahead for a while, pause, reverse, then move forward again. The pattern continues to repeat itself. How to account for this movement? Someone might suggest that the light isn’t attached to a horse’s head at all. Instead it’s on the cap of the ticket-taker who is moving around among the horses. But the movement looks too regular for that. Not quite random enough. Try again. Suppose the light is at the end of a rope, and someone riding one of the horses is swinging the light around his or her head as one would a stone in a long slingshot prior to launching it. Putting that movement together with the overall circular movement of the carousel might explain the apparent reversing, as the light circles toward the back of the rider’s head. Or, if we want to stay with the notion that the light is on a horse’s head, perhaps the horse isn’t fixed directly to the floor of the carousel but instead is part of a mini-carousel attached near the edge of the large carousel. In other words, in addition to being carried around in the big circle of the carousel, the horse is also moving around in a smaller circle, chasing its own tail. This last scheme would be something like Figure 2.1. We could still accurately say that we are at the centre of the carousel and everything on it is moving around us. Also, all movement is in perfect circles, no matter how complicated and uncircular it may appear to our eyes.
Figure 2.1
The carousel is rotating, and on its periphery, the smaller disc is rotating on its own axis so that the horse with the light on its head (asterisk) is chasing its tail. From the centre of the carousel it seems to us that the light moves forward, stops, reverses its motion a little while, stops again and moves forward once more.
Figure 2.2 is an idealized picture of the pattern such a light might trace in time-lapse photography taken from a helicopter hovering over the carousel. From our position at the centre (E) we wouldn’t see the loops. The light would appear to go forward, then pause, then back up, then pause, then repeat the pattern. With the fair fully illuminated, it’s easy to account for movement like that in terms of perfect circles, but it would require considerable mathematical insight to do so if we could see nothing but a few moving lights.
