Almagest

A date inferred, not recorded

Claudius Ptolemy probably published the work now called the Almagest around 150 CE, in Roman Egypt. No exact completion date or publication day survives; circa 150 is an inferred date, not one supplied by a colophon or a contemporary report.

One thing does anchor it from below. The Almagest contains parameters corrected from Ptolemy’s earlier Canobic Inscription, commonly dated to 146/147 CE, which establishes that the surviving form postdates that inscription — without proving completion in exactly 150.

Thirteen books of sky

The completed treatise ran to 13 books, treating the geocentric cosmos, spherical astronomy, the Sun, the Moon, eclipses, the fixed stars and the five visible planets. It offered mathematical models for predicting observed motions — considerably more than a declaration that Earth sat at the centre.

Its star catalogue is conventionally counted at 1,022 stars in 1,028 entries. Totals vary because the text contains duplicate entries and three additional stars associated with Coma Berenices.

The famous name is not Ptolemy’s

The original Greek title was Mathematikē Syntaxis — Mathematical Compilation. “Almagest” developed through the later Arabic title al-Majisṭī.

How much of the star catalogue derives from Hipparchus, whose work Ptolemy used and discussed, remains disputed.

Nothing resembling a launch marks the moment: no immediate change in astronomical practice can be tied to a particular day, or securely even to 150 itself.