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, 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
catalogue and from 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.
 | ||
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. 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.
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 greater
accuracy. Accompanying Hevelius’s catalogue was an atlas,
Firmamentum Sobiescianum, engraved with great skill by Charles de la
Haye. 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.
 | ||
Johannes Hevelius’s influential star atlas Firmamentum Sobiescianum was published posthumously in 1690. Hevelius introduced 11 new constellations, of which seven are still accepted by astronomers today. His atlas portrayed the constellations 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. (Image © Tartu Observatory Virtual Museum.)
Firmamentum Sobiescianum suffers from the drawback that the constellation
figures are depicted back to front, as they would appear on a
celestial globe; 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.
Celestial mapping took another major stride
in the 18th century with the work of the first Astronomer Royal
of England, John Flamsteed, who catalogued nearly 3000 stars
with unprecedented precision from the newly founded Royal
Observatory at Greenwich. Flamsteed’s star catalogue, Historia Coelestis Britannica, was published posthumously in 1725, followed four
years later by Atlas Coelestis, a set of 25 maps based entirely on
Flamsteed’s own observations. The far southern skies,
below the horizon of Greenwich, are covered by one small chart
that depicts the 12 constellations of Keyser and de Houtman
plus Halley’s Robur Carolinum.
Flamsteed took particular care to depict
the constellation figures exactly as Ptolemy had described
them. The introduction of Atlas
Coelestis contains some disapproving
words about the way that Bayer had represented the
constellation figures in his Uranometria:
“Having drawn all his human figures,
except Boötes, Andromeda and Virgo, with their backs
towards us, those stars, which all before him place in the
right shoulders, sides, hands, legs or feet, fall in the left,
and the contrary ... whereby he renders the oldest observations
false or nonsense.”
Despite popular misconception, Flamsteed
did not introduce the so-called Flamsteed number system for
identifying the stars in each constellation; that was done in
1783 by the Frenchman J. J. Lalande. In a French edition of
Flamsteed’s catalogue Lalande added a column in which he
numbered the stars consecutively in each constellation in the
order that Flamsteed had listed them, and this is the system
that astronomers mean when they speak of Flamsteed numbers.
Stars are usually referred to by their Flamsteed numbers
– for example 61 Cygni or 70 Ophiuchi – only when
they are not already identified by a Greek letter.
One legacy of Flamsteed’s atlas which
is sometimes overlooked is the sequence of smaller popular
atlases that it inspired: Jean Fortin’s Atlas Céleste
in France (1776 and 1795), Johann Bode’s Vorstellung der Gestirne in Germany (1782 and 1805), and Alexander
Jamieson’s Celestial Atlas in England (1822), all of which in turn had
their own imitators. For illustrations of these, see my pages
on Old
star atlases.
Flamsteed’s catalogue and atlas set
new standards in astronomy, and I have used his atlas as one of
the sources for illustrations on these pages. The other main
source is the greatest of the old-style pictorial star atlases,
Uranographia, published in 1801 by the German astronomer Johann Elert
Bode, director of Berlin Observatory. (It actually appeared in
five parts from 1797 onwards, but 1801 was the final
publication date.)
Bode’s Uranographia was the first atlas to depict virtually all the
stars visible to the naked eye (i.e. down to sixth magnitude),
plus a fair selection of those down to six times fainter
(eighth magnitude). Over 17,000 stars are plotted, taken from
the observations of various astronomers including Flamsteed,
Lacaille, Lalande, and Bode himself. Bode intended the Uranographia to
be comprehensive – and he certainly succeeded, for in
addition to charting a greater number of stars than any
previous cartographer he also depicted more constellations,
over 100 of them. Among them were five constellations making
their debut on this atlas: Felis and Globus Aerostaticus were
both suggested by Lalande during the preparation of the atlas,
while Lochium Funis, Machina Electrica, and Officina
Typographica were invented by Bode himself. None of these five
survived the test of time, however.
Bode’s Uranographia marked the end of an era. Thereafter,
astronomers placed decreasing emphasis on the fanciful (and
physically meaningless) constellation figures of the Greeks,
concentrating instead on the exact measurement of position,
brightness, and physical properties of the stars.
An intermediate stage in the transition
from classical to scientific mapping was the Uranometria Nova of the
German astronomer F. W. A. Argelander published in 1843. In
this, the constellation figures, printed in red, were reduced
to shadowy insignificance by comparison with the stars. This
same two-colour style was followed by Argelander’s
countryman Eduard Heis in his Atlas
Coelestis Novus of 1872. These
atlases were the standard references for professional
astronomers of the day, and their choice of constellations
helped establish the eventual list of 88 adopted by the International Astronomical
Union.
By the end of the 19th century, 2000 years
of Greek tradition had finally given way to the
facts-and-figures approach of astronomical census-takers and
statisticians. Where the ancient Greeks imagined their gods and
heroes populating the sky, modern astronomers have discovered
the existence of an equally fantastic pantheon of objects with
names such as red giants, white dwarfs, Cepheid variables,
pulsars, quasars, and black holes.
© Ian Ridpath. All rights reserved
|
|
||||
 |
|
|||
|
|
||||
|
|
||||
|
|
|
|
|
|