Life Theodosius of Bithynia was a Greek astronomer and mathematician of the Hellenistic period, active in the 2nd or 1st century BCE. His ethnonym indicates an origin from Bithynia in northwest Asia Minor [1]. Beyond this geographical association and his profession, no biographical details are recorded.

Works He is known for three surviving treatises written in technical Koine Greek [1, 2]. His most influential work is the **Σφαιρικά (Sphaerica), a three-book compilation on spherical geometry essential for astronomy [1, 2, 3]. The other two are Περὶ οἰκήσεων (On Habitations), describing celestial phenomena visible from different latitudes, and Περὶ ἡμερῶν καὶ νυκτῶν (On Days and Nights)**, analyzing the varying length of day and night [1, 2].

Significance Theodosius's primary significance lies in his systematic exposition of spherical geometry. His Sphaerica became a standard textbook, replacing the earlier work by Autolycus of Pitane and was studied for centuries, with commentaries by Pappus and translations into Arabic and Latin [1, 2, 3]. His works collectively formed a core part of the later astronomical curriculum, securing his role as a key transmitter of Hellenistic mathematical astronomy.

Sources 1. Encyclopædia Britannica: https://www.britannica.com/biography/Theodosius-of-Bithynia 2. Stanford Encyclopedia of Philosophy: https://plato.stanford.edu/entries/aristotle-mathematics/supplement2.html 3. Oxford Research Encyclopedia of Classics: https://oxfordre.com/classics/view/10.1093/acrefore/9780199381135.001.0001/acrefore-9780199381135-e-6317