Hipparchus (CA. 190-CA. 120 BC)

Hipparchus (born in Nicaea) was a Greek geographer and cartographer who was also considered to be one of the greatest astronomers of ancient times. Even though almost all his writings have been lost, his theories have been handed down by Ptolemy and Pliny the Elder.

Hipparchus thought that astronomical observations and methods were fundamental for geographic studies. He was also critical of his predecessor Eratosthenes and argued that the geographer of Cyrene developed his theories without the support of mathematical calculations. His attention also focused on the use of instruments such as the diopter (which measures the angles of light), which enabled him to estimate the size of the sun and the moon and their distance from Earth. Through the use of a gnomon (part of the sundial that casts the sun's shadow), Hipparchus could also determine the latitude of a place, and he estimated its longitude by observing the beginning and the ending of solar eclipses from different places. He calculated the length of the equator to be 39,960 kilometers. Hipparchus was said to have accurately estimated the inclination of the plane of the ecliptic (the intersection of the celestial sphere with the plane of the mean orbit of Earth), and he discovered the precession of the equinoxes (the fact that they move westward along the ecliptic). Finally, he is remembered for having almost exactly calculated the duration of the solar year.

In the field of cartography, Hipparchus was interested in finding a solution to the representation of a spherical surface on a plane. He dedicated himself to the study of the stereographic projection, a particular way of mapping that was probably already known to the Egyptians and that allows the depiction of the curved surface of Earth on a flat map. In cartography, it is used to map the area of the poles, with the result that meridians look like rays originating from a point and parallels look like circles centered at the origin. This type of projection has three essential properties: It is smooth, and conformal, but it does not preserve areas. The fact that it respects angles was extremely useful for navigators in ancient times.

On the basis of sailors’ descriptions of the tides along the Gibraltarian, Spanish, and French Atlantic coasts and of those occurring along the Eastern Asian coastline, and taking into account the observations of the Babylonian astronomer Seleucus of Seleucia, Hipparchus deduced the existence of a vast continent that separated the Indian Ocean from the Atlantic.