CHAPTER TWO continued...
Star maps improved as astronomers surveyed the sky in more detail and with greater accuracy. The first great star atlas was produced in 1603 by Johann Bayer (1572–1625), a lawyer in Augsburg, Germany, who had a passion for astronomy. His Uranometria atlas devoted one large chart to each of the 48 Ptolemaic constellations, using star positions from Ptolemy’s Almagest and from the recently released catalogue by the great Danish observer Tycho Brahe; the southern skies not in Ptolemy’s catalogue were allocated one map, depicting the 12 new constellations based on stars observed by the Dutch navigator Pieter Dirkszoon Keyser. In all, over 2000 stars are plotted, twice as many as shown by Dürer. So popular was the Uranometria that it was reissued several times throughout the 17th century; its exquisitely engraved charts are true works of art.
Johann Bayer’s landmark star atlas of 1603 called Uranometria devoted individual charts to each of the 48 Greek constellations. The beautiful plates were engraved by the German painter and engraver Alexander Mair (c.1559–c.1620) of Augsburg. Here Hercules is seen holding a branch from the golden apple tree of the Hesperides. Bayer’s Uranometria was highly popular on account of its comprehensiveness, its artistic quality, and its introduction of the system of labelling stars with Greek letters. (Institute of Astronomy Library, University of Cambridge.)
Bayer’s atlas was notable for another reason: it introduced the convention of labelling bright stars by Greek letters, a system that astronomers still use. These are now commonly termed Bayer letters. For example, the bright star Betelgeuse is also known on this scheme as Alpha Orionis, meaning Alpha of Orion (the genitive, or possessive, case of the constellation name is always used). Since the measurement of star brightnesses was not a very precise art in those days, the sequence of Greek letters assigned by Bayer only approximately follows the sequence of stellar brightnesses in each constellation. In a number of cases, the star marked Alpha is not the brightest, as in Orion, where Beta Orionis (Rigel) is the brightest star. Gemini is another constellation in which the star Beta is brighter than Alpha.
Bayer did not assign Greek letters to the southern constellations of Keyser, perhaps reasoning that such a move would be premature. The Bayer lettering system was extended to the southernmost sky 160 years later by the French astronomer Nicolas Louis de Lacaille, on his map published in 1763. Constellations in the northern sky that were introduced subsequent to Bayer’s time were allocated Greek letters by the English astronomer Francis Baily in the British Association’s star catalogue of 1845.
Hevelius and the Firmamentum Sobiescianum
A few years after Bayer’s Uranometria appeared, astronomy was revolutionized by the invention of the telescope, which not only showed faint stars that had hitherto been invisible but also greatly improved the accuracy with which star positions could be measured. One man remained unmoved by this advance: Johannes Hevelius, an astronomer from Danzig (the modern Gdansk in Poland). Stubbornly, Hevelius continued to measure star positions with naked-eye sights throughout his life, worrying that lenses might introduce positional distortions.
Hevelius’s catalogue of over 1,500 star positions, Catalogus Stellarum Fixarum, was published posthumously in 1690 as part of a book called Prodromus Astronomiae. The catalogue contained 50% more stars than that compiled by the great Tycho Brahe a century earlier and the positional measurements were of comparable accuracy. Accompanying Hevelius’s catalogue was an atlas, Firmamentum Sobiescianum, engraved with great skill by Charles de la Haye. In this atlas and catalogue Hevelius introduced ten new constellations in the northern sky. For the southern stars Hevelius used the observations made by the English astronomer Edmond Halley from the island of St Helena, which were an improvement on those of the pioneering Dutchmen Pieter Dirkszoon Keyser and Frederick de Houtman.
Firmamentum Sobiescianum suffers from the drawback that the constellation figures are depicted back to front, as they would appear on a celestial globe (see the example below); this makes it difficult for an observer to match up the star patterns to the real sky. For this reason the Hevelius maps are not used for the constellation illustrations in this book.
Johannes Hevelius’s influential star atlas Firmamentum Sobiescianum was published posthumously in 1690. Hevelius introduced ten new constellations, of which seven are still accepted by astronomers today. His atlas portrayed the constellation figures from the rear, as they would appear on a celestial globe. This led to some awkward-looking representations, such as Auriga carrying the goat and kids on his back, shown here.
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