Hipparchus of Nicea, also known as Hipparchus of Rhodes, was a Greek mathematician and astronomer, the most important of his time.

Hipparchus was born in Nicea, Bitinia (today Iznik, Turkey), around 190 BC. He is considered the first scientific astronomer. He was very precise in his research, of which we know a part because they were commented on in the scientific treatise *Almagest* of the famous Alexandrian astronomer **Claudio Ptolemy**, on whom he exerted great influence.

Their calculations of the tropical year, that is, the duration of the year determined by the seasons, had a margin of error of only 6 and a half minutes compared to modern measurements. He died in Rhodes, Greece in 120 BC.

Only one of his works, called *Commentary on Aratus and Eudoxus* which is not precisely his main duties. It was written in three books: in the first one it names and describes the constellations, in the second and third it publishes its calculations on the exit and entrance of the constellations, at the end of the third book it gives a list of shining stars. In none of the three books Hipparchus comments on astronomical mathematics. He did not use a single coordinate system but a mixed system of several types of them.

He made important contributions to both flat and spherical trigonometry, published the string table, an early example of a trigonometric table, whose purpose was to provide a method for solving triangles. He also introduced the division of the circle into 360 degrees in Greece.

In astronomy he discovered the precession of the equinoxes and described the apparent motion of fixed stars whose measurement was 46 ', very close to the current 50.26 ". He calculated an eclipse period of 126,007 days and one hour.

Hipparchus also calculated the distance to the Moon based on the observation of an eclipse on March 14, 190 BC. Its calculation was between 59 and 67 terrestrial radios, which is very close to the real one (60 radios). He developed a theoretical model of the movement of the moon based on epicycles.

Hipparchus of Nicea produced the first celestial catalog that contained approximately 850 stars, differentiating them by their brightness in six categories or magnitudes, classification that is still used today. This work was probably used by Ptolemy as the basis for his own celestial catalog.

On the latter, as has been said, he had a great influence and, by rejecting the heliocentric theory of Aristarchus of Samos, he was the forerunner of Ptolemy's geocentric works.

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