Astronomers’ Moon

CLASSICAL PHILOSOPHERS

Greek astronomy began with Thales, who was born shortly before 600 BC and lived in Miletus, a city of Ionia, which was a state on the western coast of what is now Turkey. As a philosopher he is regarded as one of the Seven Sages of Greece, and is considered to be the ‘father of science’. He set the seasons of the year and divided the year into 365 days. He also predicted a solar eclipse that occurred in 585 BC. It had been believed that the Moon was self-luminous, but he suggested that it shone by reflecting sunlight. Anaximander, a student of Thales, went to Italy in 518 BC. He opined that Earth floated in space – the prevailing view was that it was in some way supported on pillars through with the Sun passed during the night.

Pythagoras was born about 575 BC on Samos, an island off the coast of Ionia that was a crossroads between Asia, Africa and Europe. In his youth he reputedly visited Thales. Pythagoras considered the Moon to mark a fundamental boundary, in that it and everything ‘above’ was ‘perfect’, while Earth was subject to change and thus to decay. When critics argued that the markings on the face of the Moon indicated that it, too, was imperfect, it was suggested that the Moon was a mirror and the markings it displayed were really on Earth.

Around 450 BC Anaxagoras of Athens decided that Thales was correct in saying the Moon shone by reflecting sunlight. He realised that the Moon was spherical, and used this to explain its monthly cycle of ‘phases’. A generation later, Democritus, who travelled widely in ancient Greece, reasoned that the Moon was a world in its own right with a rugged surface, and he speculated that it might be an abode of life.

In the early fourth century BC, Plato, a student of Socrates, founded the Academy in Athens as the first institution of higher learning. Eudoxus briefly studied under Plato. After learning astronomy, he devised an explanation for the manner in which the constellations on view change with the seasons. He imagined the stars to be on a sphere that was centred on Earth, and the Sun to be on a slightly smaller concentric sphere made of transparent crystal which allowed the stars to be seen through it. The solar sphere turned around Earth on a daily basis, as did that with

the stars, but there was a slight differential in their rates that took a year to complete. Aristotle, another student of Plato, seized on this idea of ‘crystal spheres’ by proposing that there was one for each object that had an independent motion in the sky, and that their rotation was due to the action of angels. Although Eudoxus had envisaged crystal spheres only as a means of exposition, Aristotle believed them to be real and his views would come to dominate natural philosophy.

The points of light in the sky which move against the background of stars were called ‘planets’, meaning ‘wanderers’. In the third century BC Aristarchus of Samos suggested that the Sun might be located at the centre of the ‘planetary system’, with Earth being a sphere, rotating daily on its axis, and travelling around the Sun on an annual basis; but the idea attracted little support and was soon forgotten. Aristarchus also reasoned that because the Moon occults the Sun at a solar eclipse, the Sun must be further away – in fact, much further away. He also inferred that the stars must be considerably further away than the Sun, because they show no parallax when viewed from opposite sides of Earth’s path around the Sun. However, his reasoning on these matters was ignored. He interpreted a lunar eclipse as the Moon’s passage through the shadow cast by Earth, and made a fair estimate of the distance between the Moon and Earth in relation to the diameter of Earth. His contemporary, Eratosthenes of Cyrene, made the first realistic estimate of the Earth’s true diameter, thereby providing a scale to Aristarchus’s calculations.

At the end of the third century BC, Apollonius of Perga on the southern coast of modern Turkey was a Greek geometer with an interest in conic sections, and it was he who introduced the names to the ellipse, parabola and hyperbola. Although it was inconceivable that celestial objects should be less than perfect, detailed observations had shown their motions to be anomalous. Apollonius devised a geometrical scheme in which a celestial body would trace a small circle whose central point travelled in a circle around Earth; the small circle was termed the ‘epicycle’, and its centre was the ‘deferent’. This allowed the Moon to appear at times to lead and at other times to trail its perfect position. Furthermore, this accounted for why the size of the Moon appeared to vary in a cyclical manner. And of course, because the scheme involved only circles it restored purity.

Hipparchus, a Greek living in Alexandria, Egypt, in the second century BC, was the greatest of the classical Greek astronomers. His legacy was a star catalogue, but he also used a solar eclipse to estimate the relative distances of the Sun and Moon to a greater accuracy than had Aristarchus. He reasoned that although the Moon must orbit the Earth’s centre, the location of observers on the Earth’s surface provided the basis for parallax. On scrutinising records of eclipses that had been observed from both Alexandria and Nicaea, which lie on the same meridian but are some distance apart, he used the extents to which the Moon had masked the Sun’s disk to calculate the distance to the Moon relative to the Earth’s diameter. In fact, he calculated the distance of the Moon to within a few thousand kilometres and its diameter to within several hundred kilometres – although obviously he didn’t use kilometres as a unit of measure. Hipparchus also used measurements of the Moon’s orbit to assess Apollonius’s suggestion of deferents and epicycles, found it satisfactory, and provided measurements of the sizes of the epicycles.

In 80 AD the Greek historian Plutarch, who became a citizen of Rome, wrote the philosophical treatise Faces in Orbe Lunare in which he discussed the motion of the Moon across the sky, and how it maintained one face towards Earth as it turned on its axis. He thought that it was a world similar to Earth, and suggested it might be inhabited. A generation later, this latter point led the Greek storyteller Lucien of Samosata to write Vera Historia describing how a whirlwind lifted a ship from the sea and deposited it onto the Moon, where there was a battle in progress between the local inhabitants and invaders from the Sun. The story was a satire on the wars raged by the Greeks.

Claudius Ptolemaeus was born around 85 AD, probably in Alexandria, which was at that time under Hellenistic control. The Royal Library of Alexandria was founded at the start of the third century BC. Over the centuries it had built up an unrivalled catalogue, because whenever a ship docked in the harbour the authorities ordered copies made of any books that were on board. Ptolemy (as he is known in English) used his own observations of the stars and the resources of the library to refine the work of Hipparchus, and wrote up his findings in a book of his own. The library was sacked several times and eventually destroyed, but when this occurred is disputed. Although Ptolemy’s book was lost, an Arabic translation survived as the Almagest. He accepted Earth to be centrally located, celestial objects to be travelling in circles, Aristotle’s belief in the reality of concentric celestial spheres, and also Hipparchus’s endorsement of the deferents and epicycles as the reason for the anomalous motions. The Church of Rome accepted Aristotle’s philosophy, and so, despite its contrived nature, the ‘Ptolemaic system’ – as it became known, even although Ptolemy had not invented it – survived for over 1,000 years.