**Theodosius of Bithynia** (Greek: Θεοδόσιος *Theodosios*; 2nd–1st century BC) was a Hellenistic astronomer and mathematician from Bithynia who wrote the *Spherics*, a treatise about spherical geometry, as well as several other books on mathematics and astronomy, of which two survive, *On Habitations* and *On Days and Nights*.

Little is known about Theodosius' life. The *Suda* (10th-century Byzantine encyclopedia) mentioned him writing a commentary on Archimedes' *Method* (late 3rd century BC),^{[1]} and Strabo's *Geographica* mentioned mathematicians Hipparchus (c. 190 – c. 120 BC) and "Theodosius and his sons" as among the residents of Bithynia distinguished for their learning.^{[2]} Vitruvius (1st century BC) mentioned a sundial invented by Theodosius. Thus Theodosius lived sometime after Archimedes and before Vitruvius, likely contemporaneously with or after Hipparchus, probably sometime between 200–50 BC.^{[3]}

Historically he was called **Theodosius of Tripolis** due to a confusing paragraph in the *Suda* which probably fused the entries about separate people named Theodosius,^{[4]}^{[1]} and was interpreted to mean that he came either from the Tripolis in Phoenicia or the one in Africa.^{[5]} Some sources claim he moved from Bithynia to Tripolis,^{[6]} or came from a hypothetical city called Tripolis in Bithynia.^{[7]}

See also: Theodosius' Spherics |

Theodosius' chief work, the *Spherics* (Greek: τὰ σφαιρικά *tá sphairiká*), about spherical geometry, establishes a formal foundation for the mathematics of Greek spherical astronomy similar to the foundation Euclid's *Elements* provides for geometry in general. Euclid's *Phenomena* and Autolycus's *On the Moving Sphere*, both dating from two centuries prior, make use of geometric relationships proven in *Spherics*, so it has been speculated that they may have expected readers to be familiar with a treatise on elementary spherical geometry, perhaps by Eudoxus of Cnidus (4th century BC), on which the *Spherics* may have been based.^{[8]} However, no mention of this hypothetical earlier work or its author remains today, and it is also plausible that Theodosius was the first to formalize material which had been previously justified by informal physical demonstrations on a globe or armillary sphere.

In addition to the *Spherics*, two other works by Theodosius have survived: *On Habitations*, describing the appearances of the heavens at different climes and different times of the year, and *On Days and Nights*, a study of the apparent motion of the Sun. All three were transmitted together, as part of a collection of treatises now called the *Little Astronomy*, an assortment of shorter works on geometry and astronomy building on Euclid's *Elements*. During the Islamic Golden Age, the books in the collection were translated into Arabic, and with the addition of a few new works, were known as the *Middle Books*, intended to fit between the *Elements* and Ptolemy's *Almagest*.^{[9]} Theodosius' works were published in Latin in the 16th century.^{[10]}

Theodosius was cited by Vitruvius as having invented a sundial suitable for any place on Earth.^{[11]}