(b. Rain, Germany, 1572; d. Augsburg, Germany, 7 March 1625)
Bayer, a lawyer who was an amateur astronomer, established the modern nomenclature of stars visible to the naked eye. He enrolled at Ingolstadt University on 19 October 1592 as a student of philosophy, but later moved to Augsburg. There, on 1 September 1603, he dedicated his Uranometria, a popular guide to the starry heavens, to two leading citizens and the city council, which promptly rewarded him with an honorarium of 150 gulden. On 13 December 1612, Bayer was appointed legal adviser to the city council of Augsburg, at an annual salary of 500 gulden. He died a bachelor.
The oldest surviving star catalogue, contined in Ptolemy’s Syntaxis, lists forty-eight constellations, with each star located in reference to a constellation by means of a verbal description. This was often cumbersome, and did not always direct every observer to the same star, particularly after the original Greek expression had been translated into one or more languages. This lack of compactness and of precision had long prevailed when Bayer undertook his reform by unambiguously and succinctly identifying every star visible to the naked eye.
In essence, the novelty of Bayer’s method consisted of assigning to each star in a constellation one of the twenty-four letters of the Greek alphabet. For constellations with more than two dozen stars, he resorted to the Latin alphabet after exhausting the Greek letters. He placed these Greek and Latin letters on his star charts, beautiful copper-plate engravings by Alexander Mair. In addition, Bayer reproduced the traditional numeration of the stars in the constellations, as well as the many and very different names used by Ptolemy and his successors, whose works Bayer studied with painstaking care. In this way he sought to facilitate the identification of any star in his Uranometria with the same star as it had been recorded by his various predecessors. Thus, just a few years before the invention of the telescope enormously increased the number of stars visible with the aid of an optical instrument, Bayer produced a stellar nomenclature that astronomers still use for most stars visible to the naked eye.
In Bayer’s own time the popularity of his work was further enhanced by his forty-ninth plate, which displayed twelve new southern constellations. These had recently been defined by the Dutch navigator Pieter Dirckszoon Keyzer (Petrus Theodori) of Emden, who corrected the older observations of Amerigo Vespucci and Andrea Corsali, as well as the report of Pedro de Medina.
On the other side of the ledger, two aspects of Bayer’s Uranometria created difficulties. In the first place, what his predecessors had called the right side of a constellation’s figure, he labeled the left side (Table 4, verso). Second, in bracketing stars of the same magnitude in each constellation (Table 1, recto), Bayer failed to indicate on what basis he assigned the letters within each bracket. The widespread assumption that he used the order of decreasing magnitude led to considerable confusion in the study of variable stars. The alternative assumption that he used some spatial arrangement is likewise open to serious objection.
I. Original Works. Bayer’s main work is Uranometria, omnium asterismorum continens schemata, nova methodo delineata, aereis laminis expressa (Augsburg, 1603). Explicatio characterum aeneis Uranometrias imaginum tabulis insculptorum reprints the text of Bayer’s Uranometria but omits its plates. Bayer also collaborated on Julius Schiller’s Coelum stellatum Christianum (Augsburg, 1627).
II. Secondary Literature. There is no biography of Bayer. Some information about his life is provided in Franz Babinger, “Johannes Bayer, der Begründer der neuzeitlichen Sternbenennung,” in Archiv für die Geschichte derNaturwissenschaften und der Technik. 5 (1915). 108–113. Bayer’s reliability is analyzed in F. W. A. Argelander, De fide Uranometriae Bayeri (Bonn, 1842). Bayer’s collaboration with Schiller is discussed by Ernst Zinner in Vierteljahrsschrift der astronomischen Gesellschaft, 72 (1937), 64–68.