Hipparchus

Hipparchus (c.190-c.120 BCE) was a Greek Astronomer who is known as the ‘Father of Trigonometry’.
He is thought to have been born in Nicaea, Bithynia, and lived and worked in Rhodes where he probably died. His influences came from Babylonian Astronomy.

Hipparchus is known as ‘The Father of Trigonometry’ because he compiled the first known Trigonometric Table. He used this to calculate the eccentricity of the Sun and Moon’s orbits.
Trigonometry allowed Greek Astronomers to calculate any triangle, and so build astronomical models, where they could predict the movements of the stars.
He recorded values which gave the length of the chord for every angle, and compiled a simple Table of Chords, now lost. He used Pythagoras' Theorem. However, Hipparchus only gave chords for arcs that were multiples of 7.5 degrees.
Ptolemy then developed this and produced his own more complex Table of Chords.
The accepted Rule today is:
- ‘The chord of an angle equals twice the sine of half the angle’

‘Commentary on the Phaenomena (appearances) of Eudoxus and Aratus’:
- This was an Astronomical Poem by Aratus, which introduced the Constellations and the Milky Way, and showed how to calculate the times of their rising and setting.
He recorded writing a total of fourteen Works, but only the ‘Commentary’ has survived.

He produced an accurate map of the stars which appears to be the first map using Stereographic Projection, where circles and angles in the sky are transferred onto a flat map.
This forms the basic design for an Astrolabe, although he is not considered to have built one.

Stars positions in the Night Sky were listed according to their position relative to the Sun’s Path in the Sky, the Ecliptic, not the Celestial Equator.
The Ecliptic divides the Earth’s Sphere into two Hemispheres, as does the Equator.
The reason for using the Ecliptic Coordinate System was that most of the orbits of the planets have a small inclination relative to the Ecliptic, and relative to each other.
He did use Right Ascension, but only for a few stars.

He produced possibly the first design for an Astrolabe, although he is not known to have built one. Previously astronomers used a Gnomon.

He may possibly have made the first Armillary Sphere, from which he produced his predictions for rising and setting stars.

Ptolemy states that Hipparchus was his source for calculating the Mean Synodic Month.

The Precession of the Equinoxes was discovered by Hipparchus.
The Precession explained why the Tropical Year is 20 minutes shorter than the Sidereal Year.