Medieval French Jewish astronomer, mathematician and philospher, considered one of the greatest of all medieval astronomers. Also known as Rabbi Levi ben Gershon, or Ralbag. He rediscovered the law of sines and published a sine table, correct to the 5th decimal. In his study of eclipses, he became the first astromer to rely on his own observations, rather than on Ptolemy’s. In his work,Sefer Tekunah, completed in 1328, Gersonides exposed the defects of Ptolemy’s system. Gersonides is the first person kown to have used a Camera Oscura for astronomical observations. Gersonides invented Jacob’s staff, a navigational instrument used for measuring angular separations, that was widely used for three centuries.