Aristarchus of Samos

Aristarchus of Samos was an ancient Greek astronomer and mathematician who lived around 310 to 230 BC. He is famous for creating the first known model where the Sun is at the center of the universe, with the Earth moving around it. This idea was very different from what people believed back then, as most thought the Earth was the center of everything.

Born in Samos, Aristarchus probably studied in Alexandria and worked with Strato of Lampsacus. He made important observations, like noting the summer solstice in 280 BC, and built different sundials to tell time. Aristarchus also tried to measure the sizes of the Sun, Moon, and Earth, and how far apart they were. Even though his numbers were not perfect, his work showed that the Sun was much bigger and more important than the Earth.

Aristarchus's ideas were inspired by earlier thinkers such as Philolaus and Anaxagoras. He believed the stars were like the Sun but much farther away. Sadly, many of his ideas were not accepted in his time and were later replaced by theories from Aristotle and Ptolemy. Much later, Nicolaus Copernicus learned about Aristarchus's idea of a moving Earth, though it is unclear if he fully understood it was a model with the Sun at the center.

Heliocentrism

See also: Heliocentrism

A book by Archimedes, called The Sand Reckoner, talks 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 how far the Sun and Moon are from Earth. He thought 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. From this angle, he believed the Sun was about 20 times farther from Earth than the Moon. Today we know the Sun is about 400 times farther away than the Moon. Aristarchus saw that the Sun and Moon look almost the same size in the sky, so he thought they must be different sizes 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 found 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.