Filling the remaining gaps

As the accuracy of astronomical observations improved and fainter stars were charted, the opportunities grew for innovators to introduce new constellations even among the area of sky known to the ancient Greeks. Any pretext that constellations should resemble the objects they represented had by now been abandoned; they had become simply named areas of sky.

Ten more constellations were introduced in the late 17th century by the Polish astronomer Johannes Hevelius (1611–87), filling the remaining gaps in the northern sky. These were listed in his star catalogue dated 1687 and depicted on his accompanying star atlas called Firmamentum Sobiescianum, both published posthumously in 1690. Hevelius insisted on measuring star positions with the naked eye even though he possessed telescopes for observing the Moon and planets; many of his constellations were deliberately faint as though he was boasting of the power of his eyesight.

Of Hevelius’s inventions, seven are still accepted by astronomers (Table 3). The rejected three were Cerberus, Mons Maenalus, and Triangulum Minus (see Chapter 4).

Lacaille’s southern survey

Although the northern constellations were now complete, there were still gaps in the southern sky. These were filled by the French astronomer Nicolas Louis de Lacaille (1713–62) who sailed to South Africa in 1750. There he set up a small observatory at the Cape of Good Hope (not yet known as Cape Town) under the famous Table Mountain, which impressed him so much that he later named a constellation after it, Mensa. At the Cape from August 1751 to July 1752 Lacaille observed the positions of nearly 10,000 stars, an astounding total in the short time.

On his return to France in 1754, Lacaille presented a map of the southern skies to the French Royal Academy of Sciences which included 14 new constellations of his own invention (see Table 4). An engraved version of the map was published in the Academy’s Mémoires in 1756 and Lacaille’s new constellations were rapidly accepted by other astronomers. Whereas Keyser and de Houtman had mostly named their constellations after exotic animals, Lacaille, being a child of the Enlightenment, commemorated instruments of science and the arts, with the exception of Mensa, named after the Table Mountain under which he had carried out his observations.

His final catalogue, and a revised map with the names of his new constellations in Latin, was published posthumously in 1763 under the title Coelum Australe Stelliferum. In the catalogue, Lacaille divided up the unwieldy constellation Argo Navis, the ship, into the subsections Carina, Puppis, and Vela that astronomers still use as separate constellations. As well as creating 14 new constellations, Lacaille eliminated a pre-existing one – Robur Carolinum, Charles’s Oak, introduced by the Englishman Edmond Halley in 1678 to honour his monarch King Charles II.

All those from Lacaille’s time onwards who gerrymandered the constellations did so without lasting success, but there were plenty of astronomers who tried to leave their mark on the sky. Constellation mania had reached its height by 1801 when the German astronomer Johann Elert Bode (1747–1826) published his immense star atlas, Uranographia, containing over 100 different constellations; but by then astronomers realized that things had gone too far, and during the ensuing century this number was eroded by a process of natural wastage. In 1899 the American historian R. H. Allen summed up the prevailing situation in his book Star Names and Their Meaning: ‘From 80 to 90 constellations may be considered as now more or less acknowledged’.

The final 88

The matter was settled once and for all by astronomy’s newly founded governing body, the International Astronomical Union (IAU). At its first General Assembly in 1922, the IAU officially adopted the list of 88 constellations covering the entire sky that we use today. (For more information on all 88 constellations and how the final choice came to be made see here.)

One serious deficiency remained, though: there were still no generally agreed boundaries to the constellations. Since Bode’s time cartographers had drawn freehand lines snaking between constellation figures, but these were arbitrary and varied from atlas to atlas. On behalf of the IAU a Belgian astronomer, Eugène Delporte (1882–1955), drew up a definitive list of boundaries for the 88 constellations.

For consistency with the earlier work of the American astronomer Benjamin Apthorp Gould (1824–96), who in 1877 had published boundaries for the southern constellations in his atlas called Uranometria Argentina, Delporte drew his boundaries along lines of right ascension and declination for the year 1875. The boundaries zig-zagged to ensure that all named variable stars remained within the constellations to which they were already assigned. Delporte also modified some of Gould’s boundaries, particularly in places where he had used diagonal lines and curves instead of perpendiculars.

Delporte’s work, approved by the IAU at its meeting in 1928 and published in 1930 in a book called Délimitation Scientifique des Constellations, amounts to an international treaty on the demarcation of the sky, to which astronomers throughout the world have conformed ever since. Constellations are now regarded not as star patterns but as precisely defined areas of sky, rather like countries on Earth. Unlike the map of the Earth, though, the map of the sky is unlikely to change.

© Ian Ridpath. All rights reserved