Aristarchus of Samos

Aristarchus of Samos (310BC-230BC) was a Greek astronomer who maintained that the Earth rotates on its axis and revolves around the sun. Aristarchus’s work on the motion of the Earth has not survived but his ideas are known from references by the Greek mathematician Archimedes, the Greek biographer Plutarch, and the Greek philosopher Sextus Empiricus. Archimedes said in his Sand-Reckoner that Aristarchus had proposed a new theory which, if true, would make the universe vastly larger than was believed (this is because a moving Earth should produce a parallex, or annual shift, in the apparent positions of the fixed stars, unless the stars are very far away indeed).

In the sixteenth century Aristarchus was an inspiration for the Polish astronomer Nicolas Copernicus’s work. In his manuscript of Six Books Concerning the Revolutions of the Heavenly Orbs (1543), Copernicus cited Aristarchus as an ancient authority who had espoused the motion of the Earth. However, Copernicus later crossed out this reference and Aristarchus’s theory was not mentioned in the published book. When the church was hostile towards Copernicus’s heliocentric theory (the view that the Earth orbits the sun), Copernicus was quoted to have said on his deathbed that he merely repeated what the ancient Greeks had said.

Aristarchus’s only extant work is On the Sizes and Distances of the Sun and Moon and is the oldest surviving geometric treatment of this problem. In a clever geometric construction based on lunar eclipses, he obtained values for the sizes of the sun and moon. He found the moon’s diameter to be between 0.32 and 0.40 times the diameter of the Earth. This was amazingly accurate, as the diameter of the moon compared with that of the Earth is 0.27 using today’s accurate measurements. Later, Greek astronomers, especially Hipparchus and Ptolemy, refined Aristarchus’s method and arrived at very accurate values for the size and distance of the moon.

Aristarchus’s mathematical and experimental astronomical methods enabled other Greek astronomers to build upon his methods to make other important discoveries. This included the phenomenon of precession, which is the rotation of the Earth around a common axis whilst rotating itself around its own axis (rather like a gyroscope), discovered by the Greek astronomer Hipparchus about 125BC.

It is now widely believed that without the astounding achievements of the great Greek astronomers and mathematicians, including Archimedes and Erastothenes, who accurately determined the circumference and diameter of the Earth, the discoveries by mathematicians and scientists during the Renaissance and Enlightenment, including the achievements of Galileo, Kepler and Newton, would unlikely to have been possible. Isaac Newton’s admission: ‘I stood on the shoulders of giants’ is an apt description of the unique contribution of the ancient Greeks in the development of modern science.