**Hippark**, in Greek Hipparkhos, century astronomer and mathematician. II BC, was born in Nicaea, Bithynia. He lived in Alexandria, but worked mainly in Rhodes, from 161 to 126 BC.

**D**He stopped by the method and rigor of his observations. He created technically perfected instruments that allowed him to draw up a catalog of approximately eighty stars. He determined the celestial coordinates of each one and divided them into six quantities according to their luminosity. This research was inspired by the discovery (134 BC) of a new star.

**H**Icarco is one of the most representative scientists of the Alexandrian era. Invents a special diopter to measure variations in the apparent diameter of the sun and moon and introduces in Greece the division of the circle into 360 degrees, each divisible within 60 minutes by 60 seconds, a system invented by the Babylonians. Dividing the diameter of the circle into 120 parts determines, by calculation, and not simply by practical approximations, the value of the strings with respect to the various parts of the diameter.

**AND**entails a primitive formulation of trigonometry; establishes a string table to facilitate astronomical calculations that require recourse to their various values and develops a method for solving spherical triangles.

**N**The field of plane geometry elaborates the theorem known as Ptolemy's theorem.