Hipparchos-Hipparchus of Rhodes

Hipparchos of Rhodes (190-120 BC):

1) was the inventor of trigonometry;

2) is credited as the first person to suggest that the earth rotates on its axis;

3) was the first person to obtain a measurement of the earth’s diameter;

4) calculated the distance to the Moon (59 to 67 earth radii—60 is correct);

5) calculated the length of the year to within 6½ minutes;

6) discovered the precession of the equinoxes (46" compared to the modern
              50.26", and much better than the 36" Ptolemy obtained 300 years later);

7) was the first person to appreciate the vast distance to the stars;

8) created a star catalogue of about 850 stars;

9) introduced the division of the circle into 360° into Greece.

10) may have been the first person to reliably predict solar eclipses;

11) may have invented the astrolabe and the armillary sphere;

12) may have been the inventor of the Antikythera mechanism.

Probably born in Nicaea, Turley, little is known directly of Hipparchos' work; although he wrote over 13 books, none survive to this day. What we do know about Hipparchos comes from Strabo's (64 BC-AD 24) Geographia, from Pliny the Elder's  Natural History (1st century AD), and from Ptolemy's Almagest (c. 150 AD) and commentaries on it by later writers. Considered by many to have been the greatest astronomer of ancient times, he used a a heliocentric model to calculate and explain planetary orbits, but abandoned that work when he discovered that the planets did not move in perfect circles as the understanding of his day required. What is remarkable here is that he actually made observations and measurements to investigate the natural world: the underpinning of the modern scientific method. Almost 1500 years later in 1609 Johannes Kepler discovered that planets moved in ellipses—a conclusion Kepler strongly avoided until he was left with no alternative.

Hipparchos probably did most of his mature work on the island of Rhodes where he is also thought to have died. (This supports his authorship of the Antikythera mechanism, as Rhodes was the probable origin of the ship on which it was found.) Hipparchos' image—usually shown sitting and looking at a globe—has been discovered on many coins made between 138 and 253 AD.