Johann Bayer (1572 – 1625) was a German lawyer and uranographer (celestial cartographer).
Bayer is most famous for his star atlas Uranometria Omnium Asterismorum ("Uraniometry of all the asterisms"), which was first published in 1603 in Augsburg and dedicated to two prominent local citizens. This was the first atlas to cover the entire celestial sphere. It was based upon the work of Tycho Brahe and may have borrowed from Alessandro Piccolomini's 1540 star atlas, De le stelle fisse ("Of the fixed stars"), although Bayer included an additional 1,000 stars. The Uranometria introduced a new system of star designation which has become known as the Bayer designation. Bayer's atlas added twelve new constellations to fill in the far south of the night sky, which was unknown to ancient Greece and Rome.
Uranometria : omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa
Uranometria is the short title of a star atlas produced by Johann Bayer. It was published in Augsburg in 1603 by Christoph Mang (Christophorus Mangus) under the full title Uranometria : omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa. This translates to "Uranometria, containing charts of all the constellations, drawn by a new method and engraved on copper plates". The word "Uranometria" derives from Urania, Muse of the heavens and "uranos" (oυρανός) the Greek word for sky / heavens. A literal translation of "Uranometria" is "Measuring the Sky".
Uranometria contained 51 star charts, engraved on copper plates by Alexander Mair. The first 48 charts illustrate each of the 48 Ptolemaic constellations. The 49th chart introduces 12 new constellations in the deep southern sky which were unknown to Ptolemy. The final two charts are planispheres labeled "Synopsis coeli superioris borea" and "Synopsis coeli inferioris austrina," or (roughly), "Overview of the northern hemisphere" and "Overview of the southern hemisphere."
Each plate includes a grid for accurately determining the position of each star to fractions of a degree. The positions used by Bayer to create the Uranometria were taken from the expanded 1,005 star catalog of Tycho Brahe. Brahe's expanded list had circulated in manuscript since 1598 and was available in graphic form on the celestial globes of Petrus Plancius, Hondius, and Willem Blaeu. It was first published in tabular form in Johannes Kepler's Tabulae Rudolphinae of 1627.
The use of Brahe's catalog allowed for considerably better accuracy than Ptolemy's somewhat limited star listing. The stars listed in Uranometria total over 1,200, indicating that Brahe's catalog was not the only source of information used. Bayer took the southern star positions and constellation names for the 49th plate from the catalog of Dutch navigator Pieter Dirkszoon Keyser, who corrected the older observations of Amerigo Vespucci and Andrea Corsali, as well as the report of Pedro de Medina. Uranometria contains many more stars than did any previous star atlas, though the exact number is disputed as not all stars on the charts are labeled.
Each of the constellations' stars are overlain on an engraved image of the subject of the constellation. For reasons unknown, many of the human constellations are engraved as figures seen from behind whereas they had traditionally been rendered as facing the Earth. This oddity led to some confusion in the literal meanings of certain star names.
Uranometria introduced the convention of labelling stars by Greek letters, known as Bayer star designations, a system which is still used today.