Bayer’s Uranometria
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, 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 created
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.
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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 Alexander Mair. 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 1500 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.
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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. (Image © Tartu Observatory Virtual Museum.)
© Ian Ridpath. All rights reserved