Hipparchus

Lore & Background

Hipparchus was born in Nicaea, Bithynia, and likely spent most of his later life on Rhodes, where he probably died. He was a working astronomer between 162 and 127 BC, with Ptolemy attributing observations to him from 147 to 127 BC, some made in Rhodes. He obtained information from Alexandria and Babylon, though it is unknown if he visited these places. Relatively little of his direct work survives; only his commentary on the astronomical poem by Aratus was preserved by later copyists. Most knowledge of him comes from Strabo's Geography, Pliny's Natural History, Ptolemy's Almagest, and commentaries by Pappus and Theon of Alexandria.

Hipparchus developed trigonometry and constructed trigonometric tables, solving several problems of spherical trigonometry. He compiled the first known comprehensive star catalog from the western world, which was later incorporated into Ptolemy's catalog. He is credited with possibly inventing the astrolabe and the armillary sphere, which he may have used for his star catalog. He also contributed to optics, developing an atomist theory of light. He systematically exploited Babylonian astronomical knowledge and techniques, using their units such as the cubit and finger, and adopted the division of the ecliptic into 360 degrees.

Reader's Guide

Hipparchus's significance lies in his foundational contributions to astronomy and mathematics. He is considered the founder of trigonometry, having compiled the first known trigonometric table, which he used to compute the eccentricity of the orbits of the Moon and Sun. His discovery of the precession of the equinoxes was incidental but profoundly important for understanding Earth's motion. He created the first comprehensive star catalog in the western world, which served as a standard for centuries. His quantitative models for the motion of the Sun and Moon were the first accurate ones to survive, and he made use of earlier Babylonian and Greek observations. The stereographic projection is ambiguously attributed to him, and he may have known spherical trigonometry. His work influenced later astronomers like Ptolemy, and he is often called the 'father of astronomy,' a title conferred by Jean Baptiste Joseph Delambre in 1817. Despite the loss of most of his writings, his legacy endures through the methods and discoveries that shaped subsequent science.

Did You Know?

• Hipparchus is considered the founder of trigonometry and compiled the first known trigonometric table.
• He discovered the precession of the equinoxes incidentally while studying the stars.
• His star catalog was incorporated into Ptolemy's Almagest and can be reconstructed by subtracting two and two-thirds degrees from Ptolemy's star longitudes.
• Coins were made in his honor in Bithynia in the second and third centuries, bearing his name and showing him with a globe.