Planisphaerium

The title can be translated as "celestial plane" or "star chart".

In this work Ptolemy explored the mathematics of mapping figures inscribed in the celestial sphere onto a plane by what is now known as stereographic projection.

Originally written in Ancient Greek, Planisphaerium was one of many scientific works which survived from antiquity in Arabic translation.

One reason why Planisphaerium attracted interest was that stereographic projection was the mathematical basis of the plane astrolabe, an instrument which was widely used in the medieval Islamic world.

[1] The word planisphere (Latin planisphaerium) was originally used in the second century by Ptolemy to describe the representation of a spherical Earth by a map drawn in the plane.