Johann Bayer produced the most comprehensive pre-telescopic star catalog and introduced the nomenclature still in use for designating stars visible to the naked eye. His was also the first celestial atlas to represent the stars around the South Pole and to cover the entire sky.
Bayer was born in Rhain, Bavaria, in 1572. In 1592 he matriculated at Ingolstadt University as a philosophy student and later obtained a law degree. Though a lawyer by profession, he maintained a keen interest in astronomy and after moving to Augsburg published his comprehensive celestial atlas Uranometria on September 1, 1603. He dedicated the volume to two of Augsburg's leading citizens. The city council responded with an honorarium of 150 gulden. Bayer was later appointed legal advisor to the city counsel of Augsburg with an annual salary of 500 gulden.
The significance of Bayer's work lies in his innovative method for naming stars within each constellation. Though traditional constellations continued to provide a convenient means of dividing the heavens, the profusion of names for individual stars that resulted from the translation of Greek into various languages proved most cumbersome and confusing. Bayer sought to reform this situation by systematically identifying each star precisely and succinctly. He assigned to each star in a constellation one of the 24 letters of the Greek alphabet. If a constellation had more than 24 stars then additional characters were provided by the Latin alphabet. Thus, Castor and Pollux, the two brightest stars in the constellation Gemini, became Alpha Gemini and Beta Gemini respectively. Many stars in the southern skies were only carefully observed in Bayer's day and are today known by his designation, as for example with Alpha Centauri. Bayer's system is still in use, and as more and fainter stars have been identified, Roman numerals, both alone and in combination with alphabetic characters, have been resorted to.
The Uranometria contains over 2,000 stars, of which around 1,200 were taken from Tycho Brahe's (1546-1601) catalog. These were sorted into 49 constellation maps and two hemispheric charts that were beautifully engraved by Alexander Mair. Bayer retained the traditional 48 constellations of Ptolemy's (fl. second century A.D.) Syntaxis. He was also at great pains to cross-index the names of his stars with those in the catalogs of Ptolemy and others so as to facilitate their identification. In addition, the Uranometria's forty-ninth constellation map contained the 12 new southern constellations that had recently been defined by the Dutch navigator Pierter Dirckszoon Keyser, also known as Petrus Theodori (d. 1596). These are the constellations Apus, Chamaeleon, Dorado, Grus, Hydrus, Indus, Musca, Pavo, Phoenix, Triangulum Australe, Tucana, and Volans.
Despite the terminological convenience of his system, certain aspects of Bayer's celestial atlas created problems. First, Bayer's left-right labeling of constellations was the reverse of that used by all previous atlases. Second, he bracketed stars of the same magnitude in each constellation but failed to include the method whereby he assigned the letters within each bracket. It was initially assumed that he ordered them according to descending magnitude, but this created considerable confusion in later work on variable stars. An alternate hypothesis was that he employed spatial criteria. However, this interpretation has faced serious objections as well.
A devout Protestant and amateur theologian, Bayer was never comfortable with the traditional heathen names assigned to the constellation. In the Uranometria he therefore proposed alternate names from the Bible. Constellations in the Northern Hemisphere were named for figures from the New Testament while those in the Southern Hemisphere were given names from the Old Testament. Needless to say, this suggestion failed to gain wide acceptance.