Hipparchus is best known for founding trigonometry, creating the first comprehensive star catalog, and discovering the precession of the equinoxes, making significant contributions to the field of astronomy.
An equinox is an astronomical event that occurs twice a year when the Sun is directly above the Earth’s equator, resulting in nearly equal lengths of day and night. This happens around March 20 (spring equinox) and September 22 (autumn equinox) each year.

All the trigonometric identities can be derived from this image, using the pythagorean theorem.

All four triangles follow the pythagorean theorem. Starting with Sin(x) and Cos(x).

Remember that 2(θ) is the same as 2(θ+θ) and with the above image one can derive any proof.