Hipparchus, better known as Hipparchos was a Greek mathematician born in 190 BC. Not much is known about Hipparchus’s life however it is deduced that his place of birth was Nicaea in Bithynia which is modern day Turkey. Though being one of the most influential mathematicians and astronomers, the details of his work are very scarce the most definite survived piece being his commentary on a poem by Aratus from the 3rd century the ‘Commentary on the Phainomena of Eudoxus and Aratus’.

Also in the list of his contributions are his books on optics and arithmetic, writings concerning geography and astrology and a treatise called ‘On Objects Carried Down by their Weight’.

Most of his astronomical work is known from ‘Almagest’ written by Ptolemy in the 2nd century AD where he used Hipparchus’s knowledge as a base for his own astronomical theories.

His contributions to astronomy are believed to be of significant use in modern day applications of the field. Being the first to calculate a heliocentric system he left his work as according to his calculations the orbits were not truly circular as was the belief of science of that time. Hipparchus had observed the stars from a time span of 147 to 127 BC using an instrument called ‘dioptra’. Some historians suggest that he was the inventor of ‘Planispheric Astrolabe’, an astronomical device. It was none other than Hipparchus who raised important questions such as what the length of a year was and what the lunar distances were. Curious to find an answer, Hipparchus extensively studied the solar and lunar motions and their orbits using several calculations and techniques. He also determined the distances and sizes of both the sun and moon.

##### The Discovery of the Equinoxes

Hipparchus is most famed for his discovery of the precession of the ‘Equinoxes’. An equinox is a term used to describe the time when the center of the sun is in the same plane as the earth’s equator. Using his own observations combined with those made by other astronomers particularly Aristarchus, Meton and Euctemon, he calculated the amount of precession and using this data also deliberated the length of the tropical year.

##### Other Works

Some mathematicians credit Hipparchus as being the founder of trigonometry. We know that he owned a trigonometric table which he used when deriving the solar and lunar orbits and their eccentricity. The text of Menelaus of Alexandria from the first century indicates that Hipparchus was familiar with spherical trigonometry and used it for calculating lunar parallax and rising and setting points of the ‘ecliptic’ (the path of the sun on the celestial sphere).

##### Death and Recognition

The works of Hipparchus are widely recognized today and commemorating his contributions the High Precision Parallax Collecting Satellite of the European Space Agency was given the acronym ‘HiPParCoS’. A lunar crater is also named after him and so is the asteroid ‘4000 Hipparchus’. An observatory in Los Angeles, California ranks him as one of the six greatest astronomers. Hipparchus is believed to have died in 120 BC.