Aristarchus of Samos

Aristarchus of Samos was an ancient Greek astronomer and mathematician who lived around 310 to 230 BC. He is best known for creating the first known heliocentric model, which placed the Sun at the center of the universe, with the Earth revolving around it. This idea was very different from the common belief at the time, which held that the Earth was the center of everything.

Born in Samos, Aristarchus likely studied in Alexandria and worked under Strato of Lampsacus. He made important observations, such as the summer solstice in 280 BC, and built different types of sundials to track time. Aristarchus also tried to measure the sizes of the Sun, Moon, and Earth, and the distances between them. Although his numbers were not exactly right, his work helped him understand that the Sun was much larger and more important than the Earth.

Aristarchus's ideas were influenced by earlier thinkers like Philolaus and Anaxagoras. He thought the stars were like the Sun but much farther away. Sadly, many of his ideas were not accepted during his time and were replaced by geocentric theories from Aristotle and Ptolemy. Much later, Nicolaus Copernicus learned about Aristarchus's idea of a moving Earth, though it is unsure if he fully understood it was a heliocentric theory.

Heliocentrism

See also: Heliocentrism

A book by Archimedes, called The Sand Reckoner, tells us about Aristarchus’s big idea. Aristarchus thought the Sun, not the Earth, was at the center of everything. He believed Earth moved around the Sun and spun once every day. He also thought stars were very far away, like distant suns, which is why we don’t see them moving as Earth travels.

Even though Aristarchus had this great idea, most people at the time still believed Earth was at the center of the universe. It wasn’t until much later that Copernicus, Johannes Kepler, and Isaac Newton brought this idea back and explained it better.

Distance to the Sun

Main article: On the Sizes and Distances (Aristarchus)

Aristarchus wrote a book about the distances to the Sun and Moon, but he believed the Earth was at the center of the universe. He tried to measure the angle between the Sun and Moon when the Moon was half full. Using this angle, he thought the Sun was about 20 times farther from Earth than the Moon. We now know the Sun is actually about 400 times farther away than the Moon. Aristarchus noticed that the Sun and Moon look almost the same size in the sky, so he believed their actual sizes must be different because they are at different distances from Earth.

Size of the Moon and Sun

In his work On the Sizes and Distances of the Sun and Moon, Aristarchus talked about the sizes of the Moon and Sun compared to Earth. He watched a lunar eclipse to help with his measurements. He looked at how long it took for Earth's shadow to cover the Moon and used this to guess the angle of the shadow. He figured out that the shadow's width was twice the Moon's width during a full eclipse.

Aristarchus also used the distances between the Earth, Moon, and Sun. He made a triangle and used geometry to find that the Moon's width is about one-third of Earth's width. He discovered that the Sun is about 19 times wider than the Moon and about six times wider than Earth.

Legacy

The lunar crater Aristarchus, the minor planet 3999 Aristarchus, and the telescope Aristarchos are named after him.