Striking full color example of Bayer's celestial chart of Hydra, with the stars heightened in gold.
Hydra (the water snake) is one of the 48 constellations named by Claudius Ptolemy. Its southern end reaches to Libra and Centaurus and its northern end borders Cancer.
Bayer's Uranometria, is one of the most important celestial atlases of the 17th Century and the forerunner of all star atlases which contained 51 star charts, of which 48 were Ptolomeic constellations.
Each plate has a carefully engraved grid, so that star positions can be read off to fractions of a degree. These positions were taken,from the catalog of Tycho Brahe. Brahe's catalog had circulated in manuscript in the 1590s, but was not published until 1602.
Another important feature of the Bayer's atlas was the introduction of a new system of stellar nomenclature. Bayer assigned Greek letters to the brighter stars, generally in the order of magnitude, so that the bright star in the Bull's eye became alpha Tauri (and the brightest star in the Centaur became our familiar alpha Centauri.) These letters were placed on the charts themselves, and also in a table that accompanied each chart. Bayer's charts are rarely offered seperately on the market.