Hipparchus of Rhodes | Biography

Hipparchus of Rhodes was a renowned Greek astrologer whose mathematical computations to chart the sun, moon, and stars led to his being named the founder of trigonometry. Hipparchus is also considered the founder of Greek astronomy for his systematic approach to the discipline, which based it on a solid scientific foundation. In his book, the Almagest, the famous Greek astronomer and mathematician Claudius Ptolemy holds Hipparchus in the highest regard, referring to him as the "man who loved work and truth."

Details of Hipparchus' life remain lost. What little is known about him can be found in the writings of Strabo and Ptolemy. According to several ancient sources, Hipparchus was born in the city of Nicaea in a part of northwestern Asia Minor that is now Turkey. The era in which he lived has been determined primarily from the dates of his astronomical observations. In the Almagest, Ptolemyrefers to Hipparchus making observations from Rhodes in Greece, indicating that he spent the latter part of his life there. According to some ancient references, Hipparchus also made astronomical observations from Alexandria, Egypt.

While Hipparchus the man remains a mystery, his reputation as a pioneer in astronomy and mathematics led to his image appearing on second- and third-century Nicaean coins showing a man contemplating a globe. The few surviving anecdotes concerning Hipparchus may be truth or legend. According to one anecdote, Hipparchus once caused a stir among fellow theater goers when he came in wearing a cloak to protect himself from a storm he had predicted. In another questionable anecdote handed down by Pliny, Hipparchus is credited with predicting eclipses of the sun and moon for 600 years.

Like many ancient Greeks, Hipparchus' interest in mathematics stemmed from his devotion to astronomical studies. Pliny reports that Hipparchus became the first Western astronomer to record the observation of a nova, or new star, probably around 133 or 134 B.C. Hipparchus discovered the star in the constellation Scorpio during one of his nightly observations of the skies. The sighting marked a phenomenal event in his time since the Greeks believed that all stars were fixed, with their position and number firmly established. The discovery led Hipparchus to create a catalogue of stars so future generations of astronomers could accurately determine the appearance of new stars and record changes in their positions and brightness.

Before Hipparchus, Greek astronomy was based primarily on observation and a few geometrical models. In Hipparchus' time, it was believed that the stars were located on a single sphere and revolved around the Earth. To create his catalogue of stars, Hipparchus developed the idea that their positions and distance could be determined by taking three stars at a time and forming a spherical triangle. Since Euclid's mathematical theorems focused on plane triangles, Hipparchus developed his own special theorems for working with spherical triangles. These efforts led him to create a table of chords, or straight lines in a circle. Using these chords, Hipparchus developed a method for finding general solutions for trigonometrical problems and, in effect, became the founder of trigonometry.

Hipparchus is also credited with introducing a new unit of measurement, the degree, which is still used to measure arcs and angles, and proposed the idea of dividing a circle into 360 degrees. He may also have been the first to use letters to indicate the points of a triangle. Substituting mathematical models for mechanical ones, Hipparchus extended the work of Apollonius of Perga on epicyclesand eccentrics.

In the realm of astronomy, Hipparchus' most noted achievement is the discovery of the precession of the equinoxes. By comparing early astronomers' measurement of the positions of stars with his own measurements, Hipparchus found that the stars seemed to have shifted systematically in the same direction. He went on to establish that this phenomenon was due to a shift in the position of the equinoxes.

Hipparchus' other accomplishments include accurately establishing the distance to the moon and calculating the length of the year to within 6.5 minutes of the modern year. He also improved upon or invented several observational instruments, including one for measuring the diameters of the sun and the moon. Hipparchus may also have invented the plane astrolabe and stereographic projection. In the area of geography, which was his other major field of study, Hipparchus advocated the use latitudes and longitudes.

Although Hipparchus was reported to have written several works on astronomy and geography, his only surviving work is the Commentary on the Phaenomena of Eudoxus and Aratus. This three-book treatise criticizes earlier astronomers' estimates of the position of stars and constellations. Other reported works by Hipparchus include Geography, On the Length of the Year, On the Displacement of the Solstitial and Equinoctial Points, and On Bodies Carried down by Their Weight. He also wrote Against the Geography of Eratosthenes, attacking the ancient Greek geographer's works, including Eratosthenes ' treatise On the Measurement of the Earth.

Fortunately, Hipparchus' ideas were saved and reported by those who followed. Ptolemy credits Hipparchus with providing much of the foundation for the Almagest, which was long considered the "Bible of Astronomy." Unquestionably, Hipparchus was an open-minded seeker of truth who helped establish astronomy and mathematics as evolving disciplines. Ironically, Ptolemy's Almagest was such a comprehensive achievement that it nearly obscured Hipparchus' achievements. If Ptolemy had not acknowledged his debt to this pioneering thinker, Hipparchus may have been relegated to footnote status in the history of Greek science and thought.