Greek astronomy: from Thales to Ptolemy

Long before telescopes, the Greeks looked up at night and asked how the sky works. Their words for a star were ἀστήρ and ἄστρον; today Greeks say αστέρι or άστρο. Most stars keep their places as the sky turns, but a few move among the others. Xenophon calls them πλάνητες ἀστέρες, “wandering stars”, and from this idea comes the word planet, in Greek πλανήτης.

The sky was also useful. Hesiod’s farming poem, Works and Days, says that the star Arcturus rises about sixty days after the winter solstice, and tells farmers to prune their vines around that time.

The study of the stars had a name by the 5th century BC, ἀστρονομία. In Aristophanes’ comedy Clouds, a student points to some objects in his school and says that this is astronomy. The Greeks did not start from nothing: Babylonian observers had kept records before them, and later Greek astronomers such as Hipparchus used these records.

The first famous name is Thales of Miletus. Herodotus (1.74) says that he foretold to the Ionians the year in which day suddenly turned to night, during a battle between Lydians and Medes. Modern writers link this with the solar eclipse of 28 May 585 BC. How Thales could have known is another matter: Otto Neugebauer argued that there was no cycle for solar eclipses at a given place, and it may have been a guess. None of his writings survive.

Anaximander, also from Miletus, pictured the Earth as a cylinder hanging in the middle of the heavens. In the 5th century BC Anaxagoras, who lived in Athens, is said to have explained that the Moon shines with light from the Sun, and he called the Sun a red-hot stone. In Plato’s Apology (26d) Socrates says that the idea of the Sun as stone and the Moon as earth belongs to Anaxagoras. Ancient writers say that he was charged with impiety; proposed dates range from 450 to 430 BC.

In the 4th century BC Aristotle argued in On the Heavens (2.14) that the Earth is a sphere. Its shadow on the Moon during an eclipse has a curved edge, and a short journey south brings stars into view that cannot be seen further north. Later, Eratosthenes even calculated the Earth’s size.

Eudoxus of Cnidus, in the 4th century BC, explained the wandering of the planets with rotating spheres centred on the Earth. Whether he thought them real is unknown; Aristotle probably did, though this is disputed. Aristarchus of Samos (c. 310–230 BC) made a bolder suggestion. Archimedes, in the Sand-Reckoner, reports his hypothesis that the Earth revolves about the Sun. Few accepted it. Aristarchus’ one surviving book calculates that the Sun is about 20 times further away than the Moon; the real figure is about 400.

Hipparchus, born in Nicaea, was observing on Rhodes in 127 and 126 BC. He compared his figures with older ones, some of them Babylonian, and discovered the slow shift that we call precession. He probably made the first table of chords, an early form of trigonometry, and a star catalogue, perhaps of about 850 stars. Only one of his works survives, so we know him mostly through Ptolemy.

Ptolemy made observations at Alexandria between AD 127 and 141. His great book put the Earth at the centre and used epicycles, circles on circles, to predict the positions of the Sun, Moon and planets. It lists 1,022 stars in 48 constellations, and historians still debate how much he took from Hipparchus. Its Greek title meant “The Mathematical Compilation”, but through Arabic al-majisti it became the Almagest. Copernicus published his Sun-centred system in 1543, and the Almagest was replaced only about a century later.

Many of these words are alive today. An έκλειψη comes from ἔκλειψις, which also meant abandoning something. Κόσμος first meant order and ornament; today it means the world, and also simply people: «το ξέρει όλος ο κόσμος», everybody knows it. A κομήτης is a “long-haired” star, named after its tail. Ζώδιο, from ζῴδιον, “small figure”, gives the question «Τι ζώδιο είσαι;», “What’s your star sign?”. The Moon is σελήνη in science, but in everyday speech it is φεγγάρι, a different word, from φέγγος, “moonlight”. Ἀστρολογία once meant astronomy; today the two words are kept apart.

And the Milky Way? Aratus, a poet of the 3rd century BC, calls it Γάλα, “Milk” (Phaenomena 476). Aristotle reports that Anaxagoras and Democritus took it for the light of certain stars, and he rejected their view. In 1610 Galileo’s telescope showed that it is a mass of countless stars, and in the 1920s Edwin Hubble showed that the universe extends far beyond it.