Theodosius of Bithynia (ca. 160 BC – ca. 100 BC) was a Greek astronomer and mathematician who wrote the Sphaerics, a book on the geometry of the sphere. Born in Tripolis, in Bithynia, Theodosius is cited by Vitruvius as having invented a sundial suitable for any place on Earth. His Sphaerics provided the mathematics for spherical astronomy, and may have been based on a work by Eudoxus of Cnidus. Francesco Maurolico translated his works in the 16th century. In addition to the Sphaerics, two other works by Theodosius have survived: On Habitations, describing the appearances of the heavens at different climes, and On Days and Nights, a study of the apparent motion of the Sun.