Which ancient Greek astronomer is credited with founding trigonometry and discovering the precession of the equinoxes?

• A. Ptolemy
• B. Copernicus
• C. Galileo Galilei
• D. Hipparchus

Answer: (D)

Hipparchus is credited with both founding trigonometry and discovering the precession of the equinoxes. He didn’t use the modern sine function, but he built the first systematic tables of chords in a circle, which is the essential idea behind trig calculations used in astronomy. This allowed him to compute angles and celestial positions with greater precision and laid the groundwork for trig as a tool for astronomy.

For the precession of the equinoxes, Hipparchus compared his own star catalog with earlier measurements and noticed a gradual shift in the positions of the equinoxes relative to the stars. He estimated that this drift was about one degree every 72 years, giving a full cycle of roughly 26,000 years. This slow wobble of Earth’s axis is what we now call precession.

The other astronomers listed made important contributions in different areas: Ptolemy refined geocentric models and compiled extensive data, Copernicus proposed a sun-centered system, and Galileo advanced observational astronomy. But Hipparchus is the one linked to the birth of trig in astronomical practice and to identifying the axial precession.