B00B7H7M2E EBOK (6 page)

Did Aristarchus also speculate that the other planets move round the Sun? It would seem a logical next step, but there is
no
historical evidence that he did. In any case, it’s unlikely that he understood the enormous significance of his model, that it provides, at a sweep, the basis for explaining the planets’ positions and movements far more simply than a model with the Earth as centre. It is impossible to tell from the surviving evidence whether Aristarchus really was personally disposed to thinking that the Earth moved around the Sun or whether he made the suggestion merely for the sake of argument, as in ‘Let’s just suppose for the moment that this is how things work.’ Why did this revolutionary suggestion came at this time and place in history? The simple answer may be that this was an intellectual environment that encouraged one to make suggestions and put forward hypotheses, even hypotheses based on assumptions that were known to be incorrect – in no way claiming they were true – as the starting point for an interesting line of enquiry.

With Aristarchus the question must also be turned on its head to ask not only why this idea emerged but why it died at birth. Seleucus of Seleucia, a Chaldaean or Babylonian astronomer (Seleucia was on the Tigris river) in the second century
BC,
took Aristarchus’s suggestion seriously – not merely as a hypothesis. Seleucus believed Aristarchus was right. However, no one else for the remainder of antiquity did, and the remainder of antiquity was by no means a dark age when it came to astronomy. It’s only partly correct to blame this resistance on an ideological attachment to having Earth and humanity the centre of everything.

If surviving information can be trusted, public opinion reacted almost not at all. Aristarchus’s idea must have been too far removed from common knowledge and common sense to draw much popular attention. The historian Plutarch reports one comment from the Stoic Cleanthes (the Stoics were reputedly weak in natural science and even ‘anti-scientific’) that Aristarchus of Samos ought to be indicted on a charge of impiety for putting the ‘Hearth of the Universe’ in motion.
There
is no record of anyone trying to take Cleanthes’s advice. Some philosophers scolded Aristarchus for trespassing in an area of knowledge that was
their
sole domain; and there were also complaints accusing him of undermining the art of divination.

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:

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.