Aristarchus of Samos was an ancient Greek astronomer and mathematician who proposed the first known heliocentric model of the solar system. He suggested that the Earth rotates on its axis and revolves around the Sun, which is at the center of the universe. He also estimated the sizes and distances of the Sun and Moon relative to the Earth.

Aristarchus was born around 310 BCE on the island of Samos, and studied under Strato of Lampsacus, the third head of the Peripatetic School in Greece. He observed the summer solstice of 280 BCE in Alexandria, where he spent most of his life. He wrote several books on astronomy and mathematics, but only one of them has survived: On the Sizes and Distances of the Sun and Moon.

In this work, Aristarchus used geometric methods to derive the relative sizes and distances of the Sun and Moon from three assumptions: (1) the diameter of Earth’s shadow during a lunar eclipse is twice the diameter of the Moon; (2) the Moon and Sun have equal angular diameters as seen from Earth; and (3) at quarter Moon, the angle between the Sun and Moon is 87 degrees. Based on these assumptions, he concluded that the Sun is between 18 and 20 times farther away from Earth than the Moon is, and that the Sun is about 300 times larger than the Moon.

However, Aristarchus’s most revolutionary idea was his heliocentric theory, which he presented in a lost work that was mentioned by Archimedes in his The Sand Reckoner. According to Archimedes, Aristarchus assumed that “the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun on the circumference of a circle, the Sun lying in the middle of the orbit”. This theory implied that the universe is much larger than previously thought, since a moving Earth would not produce any noticeable parallax among the fixed stars unless they are very far away.

Aristarchus’s heliocentric theory was rejected by most ancient astronomers, who preferred the geocentric models of Aristotle and Ptolemy. His theory was considered impious by some philosophers, such as Cleanthes, who accused him of “putting into motion the hearth of the universe”. Aristarchus’s theory was also influenced by Philolaus of Croton, who proposed a central fire around which all celestial bodies revolve, but Aristarchus identified this fire with the Sun.

Aristarchus’s heliocentric theory was revived by Nicolaus Copernicus in the 16th century, who cited him as an ancient authority in his manuscript of On the Revolutions of the Heavenly Spheres, but later deleted this reference in his published book. Aristarchus is regarded as one of the greatest astronomers of antiquity, along with Hipparchus, and one of the greatest thinkers in human history.

Aristarchus also studied light and vision, and used correct geometry to calculate the angle of light reflected by the Moon. He found that the Moon reflects only 1/40,000 of the light it receives from the Sun, which is close to the modern value. He also attempted to measure the apparent diameter of the Sun at different times of the year, and concluded that it varies slightly due to the elliptical orbit of Earth. However, he did not realize that this variation is actually caused by the eccentricity of Earth’s orbit, not by the Sun’s.

Aristarchus’s works had a significant impact on later astronomers and mathematicians, such as Archimedes, Apollonius of Perga, and Pappus of Alexandria. He was also admired by Galileo Galilei, Johannes Kepler, and Isaac Newton, who recognized his contributions to heliocentrism and celestial mechanics. Aristarchus’s name was given to a prominent lunar crater and a Martian crater, as well as to an asteroid and a lunar mountain range.