**Aristarchus**(320 BC - 250 BC) was born in Samos, Greece. Perhaps as an astronomer, he was not as prominent as he deserved in the history of mathematics until the present day. For example, *Thomas heath* began the second volume of its history of the Greek mathematicians with the following words:

*The history of mathematicians has as a rule to pay little attention to Aristarchus of Samos. The reason, no doubt, is that he was an astronomer, and so it can be assumed that his work would not be of sufficient interest to mathematics. The Greeks knew him better, and called him "Aristarchus the Mathematician."*

Certainly, Aristarchus was both a mathematician and an astronomer, being widely celebrated as the first to propose a sun-centered universe. He is also famous for his pioneering attempt to determine the sizes and distances of the sun and moon. * Lampsacus strato*, who led the Aristotelian High School. Considered by many to be the Copernicus of the Classic Era, this astronomer revolutionized astronomy so much that his name was attributed to a lunar crater.

His conclusions about the organization of the Solar System, however simple, are admired to this day for their consistency. Until then, the most advanced conceptions were those of Pythagoras and Heraclides. They said the stars were motionless; that the earth would be in the center of the universe but rotate; and that at least the planets of Mercury and Venus would revolve around the sun.

Aristarchus went further, claiming that the movements of all these bodies could be more easily described by assuming that all planets, including the earth, revolved around the sun. This heliocentric model of the universe was, however, considered too bold and its The author was even charged with religious insult. Even so, the reaction against him was not as aggressive as it would frighten, almost 2000 years later, Copernicus, Kepler, and Galileo.

Aristarchus's writings on this subject were lost and we could only know his ideas because they were mentioned by Archimedes. However, we had access to other works of his own. In his work on the sizes and distances of the Sun and the Moon, Aristarchus sought to determine the Earth-Moon distance from the Earth-Sun distance by considering the triangle formed by these three stars at the beginning of the fourth crescent.

Aristarchus concluded that the sun would be 20 times farther from the earth than the moon. Although the true proportion is about 400 times, the procedure used was correct. The angle measuring instruments then available did not allow more accurate values to be obtained.

Aristarchus also sought to calculate the diameter of the moon relative to that of the earth, based on the shadow cast by our planet during a lunar eclipse. He concluded that the moon had a diameter three times smaller than Earth's (the correct value is 3.7). With this data, he deduced that the solar diameter was 20 times larger than the moon and about 7 times larger than the earth.

Perfecting measurements over the last few centuries, we now know that the earth's diameter does not reach one hundredth of the solar. Although their results were errors of an order of magnitude, the problem lay more in the lack of precision of their instruments than in their adequate working method. Moreover Aristarchus also calculated, more accurately than that of the ancient sages, the duration of a solar year. Aristarchus's inaccuracies are of little importance to his common sense. For him, it would be more natural to suppose that the smaller star revolved around the larger, not the other way around.

AstrolabeAncient instrument for measuring the height of stars above the horizon, used in the Middle Ages for astrological and astronomical purposes. |

Sources: Bibliography: Dictionary of Scientific Biography; Biography in Encyclopaedia Britannica;

* Photo taken from MacTutor History of Mathematics