Sphaerics
Sphaerics () was a set of three volumes on spherical geometry written by Theodosius of Bithynia in the 2nd century BC. These proved essential in the restoration of Euclidean geometry to Western civilization, when brought back from the Islamic world during the crusades and translated back from Arabic into Latin.

History
Theodosius appears to have written Sphaerics as a supplement to Euclid's Elements, presenting the spherical geometry of the celestial sphere for use in astronomy. Sphaerics had its own shortcomings, for example Hipparchus had already introduced trigonometry, but Theodosius makes no use of it, perhaps because he was basing it on an earlier work by Eudoxus.

Theodosius' primary purpose was to compile existing information. In the first book of Sphaerics, he defines a sphere as a shape whose surface is an equal distance from a central point, and then goes on to examine the geometry of spheres, extensively.

The work was important to the restoration to Western Europe of Euclidean geometry and mathematical astronomy, which were preserved in Arabic translations during the Islamic Golden Age, then translated back into Latin in the 12th century.

References

Ancient Greek mathematical works