Aristarchus of Samos (c. 270 BC): The First Sun-Centred Universe

Aristarchus of Samos, a Greek astronomer working around 270 BC, was the first person known to have proposed that the Earth orbits the Sun. He also invented a geometric method to estimate the sizes and distances of the Sun and Moon. His sun-centred, or heliocentric, model won almost no supporters in antiquity, but it anticipated by about 1,800 years the theory Nicolaus Copernicus published in 1543.

Life and background

Aristarchus was born on the island of Samos in the eastern Aegean around 310 BC and died around 230 BC. According to ancient tradition, he studied with Strato of Lampsacus, who headed Aristotle’s Lyceum in Athens and also worked in Alexandria. Little else is known about his life.

By his time, Greek astronomers already accepted that the Earth is a sphere. The usual view placed it motionless at the centre of the universe, with the Sun, Moon, planets and stars circling it.

Measuring the Sun and Moon

Aristarchus’ only surviving work is On the Sizes and Distances of the Sun and Moon. Curiously, it uses the traditional Earth-centred picture.

His key idea was simple. When the Moon is exactly half lit, the angle between the Sun, Moon and Earth is a right angle. If an observer measures the angle between the Moon and the Sun at that moment, geometry gives the ratio of their distances.

Aristarchus took this angle to be 87 degrees. From it he concluded that the Sun is between 18 and 20 times further from the Earth than the Moon. Using the sizes of the Earth’s shadow during lunar eclipses, he also estimated that the Sun’s diameter is between about 6 and 7 times the Earth’s.

His numbers were far too small, mainly because the exact moment of half Moon and the tiny difference from 90 degrees are very hard to observe. But his reasoning was correct, and his result that the Sun is far larger than the Earth may have led him to his more radical idea.

The heliocentric hypothesis

The book in which Aristarchus set out his heliocentric model has not survived. The main evidence comes from Archimedes, who in The Sand Reckoner reports that Aristarchus proposed the following:

• the fixed stars and the Sun do not move;
• the Earth revolves around the Sun in a circle, with the Sun at the centre of the orbit;
• the sphere of the fixed stars is so vast that the Earth’s orbit is tiny by comparison.

Plutarch and other writers add that he had the Earth turning daily on its axis. The last point, the vast distance to the stars, explained why no shift in the stars’ positions could be seen as the Earth moved.

Why the idea was rejected

Aristarchus’ theory found only one known follower in antiquity, Seleucus of Seleucia, a Babylonian astronomer of the 2nd century BC. Plutarch reports that the Stoic philosopher Cleanthes thought Aristarchus should be charged with impiety for moving the “hearth of the universe”, though it is unclear whether any action followed.

The main objections were scientific. Aristotle’s physics explained falling bodies by assuming the Earth was the centre of the universe. A moving Earth seemed to contradict experience, since people did not feel it move. Critics also argued that if the Earth spun on its axis, objects thrown upwards would land behind their starting point and clouds and birds would be left behind. These objections were only fully answered by the new physics of Galileo and Newton. Most importantly, if the Earth orbited the Sun, the nearer stars should appear to shift slightly during the year. This effect, stellar parallax, was too small to detect with the naked eye and was not measured until Friedrich Bessel did so in 1838.

Later astronomers such as Hipparchus and Ptolemy built highly accurate Earth-centred models instead, and these dominated astronomy until the 16th century.

Legacy

When Copernicus revived heliocentrism, he was aware of Aristarchus and mentioned him in his manuscript, although the passage was struck out before printing. Renaissance and later writers often called Aristarchus “the Copernicus of antiquity”. A bright lunar crater and an asteroid are named after him.

Timeline

• c. 310 BC: Aristarchus is born on Samos.
• c. 287 BC: Strato of Lampsacus becomes head of the Lyceum.
• c. 270 BC: Aristarchus proposes the heliocentric model.
• c. 230 BC: Approximate date of his death.
• Before 216 BC: Archimedes describes the theory in The Sand Reckoner, addressed to Gelon of Syracuse.
• 1543: Copernicus publishes De revolutionibus.
• 1838: Bessel measures stellar parallax for the first time.

Why it matters

Aristarchus showed how far careful geometry could take ancient astronomy and was the first known thinker to place the Sun at the centre. His idea failed for lack of evidence, not lack of imagination, a reminder that a correct theory also needs observations to support it.