JOST A MON

[Continued paraphrase of Allan Chapman’s Gods in the Sky, episode 2, from Channel 4.]

For all the brilliance of Hipparchus, the best of classical astronomy was yet to come. For this, we must turn to the greatest astronomer of antiquity, whose greatest achievement was also his greatest mistake. The crowning glory of ancient astronomy lay in the work of Claudius Ptolemy. He lived around 150 AD, and we know little about his life in detail, apart from that he spent most of his glittering career in the city of Alexandria. He wrote the book that was to change the subsequent course of astronomical history. He brought together the best of the ancient Greek astronomers into one work. We call it the Great Compilation; the ancients called it the Magna Syntaxis; but it has become famous under the title the Arabs gave it: the Almagest.

The Almagest contains hundreds of papers of astronomical observations, diagrams, theorems and hypotheses, all aimed at explaining the movements of the stars and planets. It demonstrates the eccentric orbits of Mars and Mercury, it charts the retrograde movements of Saturn, Jupiter and other planets, it describes the fixed rotations of the stars around the poles of the ecliptic. This is truly a scientific work of staggering complexity. But for all of Ptolemy’s genius, there was one fundamental flaw at the heart of his system: his working model of the Universe was geocentric.

Unfortunately, from our viewpoint, the planets do not seem to move at regular speeds or regular directions, as we might expect from simple planetary orbits around us. They speed up, slow down, go forwards, backwards, and generally misbehave. Today, we know that is because they are not revolving around the Earth at all, but rather the Sun. Nevertheless, without discarding his geocentric hypothesis, Ptolemy tried to construct a logical structural argument for the movement of the planets. The resulting system was ingenious, so much so that it persisted for centuries after him, becoming both religious and scientific dogma.

The geometric notion at the heart of Ptolemy’s theory is known as an epicycle. This can be demonstrated using a bicycle wheel, a few cogs, a small light bulb, and some sturdy knicker elastic (see figure: as you rotate the cycle wheel, the cogs move the bulb attached to the pedal in a circular motion of its own; this motion of a point moving in a circle around another point moving in a circle is an epicycle.) Imagine, therefore, that the centre of the Universe is the Earth, and every planet moves around a point that itself is moving around the Earth. Although the planet is moving at a constant speed about its orbital centre, from the Earth it would appear to move at a variety of different speeds, and indeed backwards and forwards over time. By building several such epicycles upon epicycles, Ptolemy was able to come up with a remarkably accurate picture of the Universe – accurate insofar as it was able to predict the appearance and location of the planets into the future.

But where did this leave the Greek gods in the sky? The explosion of knowledge from astronomers like Hipparchus and Ptolemy was unlike anything seen before. The older astronomical observations encoded in the form of myths had been developed over hundreds of years. But the astronomy of classical Greece was developed very quickly indeed, by philosophers rather than priests, and was far too complex to be encapsulated in simple tales. What is more, the same freedom that allowed the Greeks to speculate about the cosmos allowed them to question the existence of the gods. One philosopher – Alcmere - claimed that there weren’t twelve gods on Olympus, but only three, representing the Sun, the Moon, and one for the stars. Another – Parmenides - argued that there was only god, who lived in the Sun. And Strabo argued that there were no gods at all, but merely nature.

Greeks continued to worship their gods, but the deities began to be taken less and less seriously by scholars. Instead, they tended to explain ideas using poetry and philosophy, and at the centre of this understanding, we see morality and reason intermingled in a new and potent way. To the Greeks, reason and logic were inextricably linked to love and morality. Indeed, together, they formed the divine purpose of the cosmos. One of the most important themes in Greek philosophy was the relation between love and truth, and the Greeks who believed passionately in reason, declared that you couldn’t have one without the other. The connection between the love of truth and the truth of love was illustrated in the tale of Aphrodite and Hermes.

Hermes was the god of reason and truth. Like the Egyptian deity Thoth, Hermes was the giver of the word, the inventor of the alphabet, and language. He was the god of astronomy, of logic and mathematics, and of music, and time. And so he played the heavenly lyre with seven strings, corresponding to the sacred planetary week. We are told that Hermes was deeply in love with the beautiful Aphrodite, goddess of love. We are told that Aphrodite, flattered by the attentions of Hermes, agreed to be his lover, and from their union came the double-sexed being, sometimes called Hermaphroditus, and sometimes, the lovely Eros, the self-reproducing creator of the Universe. In this strange romantic relationship between the two divinities, we find a mythical poem which underlies the Greek love of reason.

For the Greeks, geometry was far more than a mere tool for making maps and charting the heavens. Their studies had revealed to them that there was an internal logic and absolute truth written into the very fabric of the cosmos. For them, there was nothing more beautiful than this truth, nothing more perfect than this, or more lovely. They called it the logos. The Christian astronomers who came afterwards called it God.