Theodosius of Bithynia was a Greek astronomer and mathematician who wrote the Sphaerics, a book on the geometry of the sphere. Born in Tripolis, in Bithynia

Bithynia

Bithynia was an ancient region, kingdom and Roman province in the northwest of Asia Minor, adjoining the Propontis, the Thracian Bosporus and the Euxine .-Description:...

, Theodosius is cited by Vitruvius

Vitruvius

Marcus Vitruvius Pollio was a Roman writer, architect and engineer, active in the 1st century BC. He is best known as the author of the multi-volume work De Architectura ....

 as having invented a sundial

Sundial

A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style onto a surface marked with lines indicating the hours of the day. The style is the time-telling edge of the gnomon, often a thin rod or a...

 suitable for any place on Earth. His Sphaerics provided the mathematics for spherical astronomy

Spherical astronomy

Spherical astronomy or positional astronomy is the branch of astronomy that is used to determine the location of objects on the celestial sphere, as seen at a particular date, time, and location on the Earth. It relies on the mathematical methods of spherical geometry and the measurements of...

, and may have been based on a work by Eudoxus of Cnidus

Eudoxus of Cnidus

Eudoxus of Cnidus was a Greek astronomer, mathematician, scholar and student of Plato. Since all his own works are lost, our knowledge of him is obtained from secondary sources, such as Aratus's poem on astronomy...

. Francesco Maurolico

Francesco Maurolico

Francesco Maurolico was a Greek mathematician and astronomer of Sicily. Throughout his lifetime, he made contributions to the fields of geometry, optics, conics, mechanics, music, and astronomy...

 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.