For a bit of context, the Phaenomena was a book by Eudoxus (c. 400 BC) explaining the then-current knowledge of astronomy; unfortunately there are no extant copies. A poem (also called Phaenomena) by Aratus (c. 300 BC) made the content more accessible, and was extremely popular. The only surviving work by Hipparchus (c. 150 BC) is a critical commentary on these two books, and it only survived because it was bundled together with several other commentaries on Aratus' poem which were copied as a group. Hipparchus synthesized Mesopotamian astronomical observations and measurement techniques with Greek spherical geometry, founding the subject we now call trigonometry. All of his other works are lost, but much of the content of Ptolemy's Syntaxis (a.k.a. Almagest, c. 150 AD) was taken from Hipparchus' astronomical and mathematical works.

Any additional fragments of Hipparchus' works is of great interest to the history of mathematics and astronomy.