 Gersonides’ scientific works comprise mathematics and astronomy. His Sefer Ma’aseh Hoshev (The Work of a Counter, 1321) is concerned with arithmetical operations and uses of a symbolic notation for numerical variables. Gersonides’ major scientific contributions were in astronomy; his works were known by his contemporaries and influenced later astronomers. His astronomical writings are contained primarily in book 5, part 1 of Milhamot Ha-Shem. In 136 chapters Gersonides reviews and criticizes astronomical theories of the day, compiles astronomical tables, and describes one of his astronomical inventions. This instrument, which he called Megalle ‘amuqqot (Revealer of Profundities) and which was called Bacullus Jacobi (Jacob’s staff) by his Christian contemporaries, was used to measure the heights of stars above the horizon. The astronomical parts of Milhamot Ha-Shem were translated into Latin during Gersonides’ lifetime. One of the craters of the moon, Rabbi Levi, is named after him.