Levi Ben Gershom
1288-1344
French-Jewish Mathematician and Astronomer
One of the few Jewish scholars of any discipline who came to prominence in Christian Europe, Levi ben Gershom dealt with problems in arithmetical operations and trigonometry. As an astronomer, he observed several eclipses and developed Jacob's staff, a mechanism for measuring the angular distance between heavenly bodies.
Known variously as Gersonides, Leo de Bagnolas, Leo Hebraeus, and Ralbag, Levi ben Gershom was born in the French town of Bagnols in 1288. He first came to prominence in 1321, with the publication of Sefer ha Mispar, or Book of Numbers. Among the subjects involving arithmetical operations discussed in the book was that of root extractions.
In 1342, Levi produced a work translated as On Sines, Chords, and Arcs. An investigation of trigonometry, On Sines presented five sine tables and examined the sine theorem for plane triangles. It is evident that Levi, while maintaining his faith (he wrote several commentaries on the Jewish scriptures), enjoyed a level of support from the Christian community unusual for a Jew in medieval Europe. Thus in 1343, the Bishop of Meaux commissioned him to write The Harmony of Numbers, a commentary on the first five books of Euclid's (c. 325-c. 250 B.C.) Elements.
On October 3, 1335, Levi observed a lunar eclipse, as well as an eclipse of the sun in 1337. He created a geometrical model to describe lunar movement and made a number of celestial observations using the camera obscura. The latter, a windowless room in which small quantities of light were allowed to enter, was a precursor to the modern camera and Levi was one of the first scientists to make use of it.
For his astronomical studies, Levi developed the Jacob's staff, which was about 4.5 feet (1.37 m) long and 1 inch (2.5 cm) wide, with a number of perforated tablets made to be slid along the staff. Each of these tablets represented a fraction of the staff's length and by using varying placements he was able to measure the distance between celestial objects, as well as their altitude and diameter.
In his Sefer ha-hekkesh ha-yashar (1319), Levi challenged cosmological ideas handed down by the Greek astronomer Ptolemy (c. 100-170). Many of the latter's assertions—for instance, his insistence that the earth is the center of the universe—were indeed spurious. Levi, however, made a few mistakes of his own, maintaining for instance that the glow of the Milky Way is a reflection of the light produced by the sun.
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