Ancient Greek Astronomy: What the Greeks Got Right (and Wrong)

Long before telescopes, the astronomers of ancient Greece worked out the shape and size of the Earth, estimated the distance to the Moon, catalogued the stars and discovered a slow wobble in Earth’s axis. Some of their ideas were brilliant; others held science back for centuries. The video above tours the highlights. Here is a readable summary of who did what and why it still matters.

Building on older traditions

Greek astronomy did not start from scratch. Babylonian astronomers had kept careful records of the Moon and planets for centuries, and Egyptian calendars were tied to the rising of the star Sirius. What the Greeks added was a strong interest in geometry and in explaining the sky with physical models rather than only predicting it.

Aristotle: proof that Earth is round

In the 4th century BC, Aristotle gave observational arguments that Earth is a sphere. During a lunar eclipse, Earth’s shadow on the Moon is always curved. Travellers moving north or south see different stars, and some stars visible in Egypt cannot be seen from Greece. Only a round Earth explains both. (The popular idea that medieval Europeans believed in a flat Earth is largely a myth; educated people knew it was round thanks to these arguments.)

Aristotle was wrong about other things. He taught that Earth sits motionless at the centre of the universe, that the heavens are made of a perfect, unchanging fifth element, and that celestial bodies move in perfect circles. These ideas were hugely influential for nearly 2,000 years.

Aristarchus: a Sun-centred idea, 1,800 years early

Aristarchus of Samos, in the 3rd century BC, used geometry to compare the sizes and distances of the Sun and Moon. His measurements were far off because the angles involved were too small to measure accurately by eye, but his reasoning was sound and he concluded the Sun is much larger than Earth. That led him to suggest that Earth orbits the Sun, not the other way round. Few accepted it at the time, partly because nobody could detect the stellar parallax that a moving Earth should produce. (The stars are simply too far away; parallax was not measured until 1838.)

Eratosthenes: measuring the whole Earth

Around 240 BC, Eratosthenes of Cyrene, chief librarian at Alexandria, compared the angle of the noon Sun on the summer solstice at Syene (modern Aswan) and Alexandria. The difference was about 7 degrees, roughly one-fiftieth of a circle. Multiplying the distance between the cities by 50 gave him Earth’s circumference. Scholars still debate the exact length of the stadion he used. Depending on which value is right, his result was within a few percent of the true value, about 40,000 km, or at worst roughly 16 percent too large. Remarkable for a stick, a shadow and some geometry.

Hipparchus: the first great star catalogue

Hipparchus of Nicaea, working in the 2nd century BC, is often called the greatest astronomer of antiquity. He compiled a catalogue of about 850 stars with their positions and introduced a system for ranking stars by brightness, the ancestor of the magnitude scale astronomers still use.

Comparing his positions with older records, Hipparchus noticed that the points where the Sun crosses the celestial equator (the equinoxes) had shifted slightly. He had discovered the precession of the equinoxes: Earth’s axis slowly wobbles like a spinning top, tracing a circle about once every 26,000 years. This is why Polaris is our North Star today, but will not be in a few thousand years. Hipparchus also improved estimates of the Moon’s distance and developed early trigonometry.

Ptolemy and the Almagest

Claudius Ptolemy, working in Alexandria in the 2nd century AD, gathered Greek astronomy into a great work known today by its Arabic-derived name, the Almagest. It included a star catalogue of more than 1,000 stars and 48 constellations, most of which survive among the 88 constellations recognised by the IAU today.

Ptolemy’s model kept Earth at the centre. To explain why planets sometimes appear to move backwards (retrograde motion), he had each planet move on a small circle, called an epicycle, whose centre moved around a larger circle. With extra refinements this geocentric model predicted planetary positions well enough to be used for about 1,400 years, until Copernicus, Kepler and Galileo replaced it with a Sun-centred system in the 16th and 17th centuries.

Right and wrong at a glance

What they got right: a spherical Earth, a good estimate of its size, a reasonable Moon distance, a detailed star catalogue, a brightness scale, and the discovery of precession.

What they got wrong: Earth at the centre of everything, perfect circular orbits, and unchanging heavens. Even these mistakes were useful: they were testable models, and testing them is how better ideas eventually won.

See it for yourself

You can repeat some Greek observations at home. Watch Earth’s curved shadow during the next total lunar eclipse. Notice that the stars you can see change as you travel north or south. And with a long stick and a sunny day near the solstice, you can even attempt your own version of Eratosthenes’ experiment with a friend in another city.