Size ranking: 88th
Origin: Petrus Plancius
The smallest of all the 88 constellations. Its stars were known to the ancient Greeks, and were catalogued by Ptolemy in the Almagest, but at that time were regarded as part of the hind legs of Centaurus, the centaur, rather than as a separate constellation. They subsequently became lost from view to Europeans because of the effect of precession, which causes a gradual drift in the position of the celestial pole against the stars, and were rediscovered during the 16th century by seafarers venturing south.
The Italian explorer Amerigo Vespucci (1454–1512) charted what seems to have been Alpha and Beta Centauri and the stars of Crux in 1501, but the most accurate early depiction was made by the Italian navigator Andrea Corsali (1487–??) in 1515. (Corsali’s diagram shows the stars as they would appear on a celestial globe, so their positions are a mirror image of the view as seen from Earth, as pointed out by the Dutch historian Elly Dekker.) Corsali described the pattern as ‘so fair and beautiful that no other heavenly sign may be compared to it’. Thereafter navigators began using the cross as a pointer to the south celestial pole, and it was adopted by astronomers as a separate constellation by the end of the 16th century.
Crux lies under the hind legs of Centaurus, and its stars were regarded by the Greeks as part of Centaurus. It contains a dark cloud of dust known to modern astronomers as the Coalsack, but named Macula Magellanica on this illustration from Chart XX of the Uranographia star atlas of Johann Bode (1801).
Crux first appears in its modern form on the celestial globes by the Dutch cartographers Petrus Plancius and Jodocus Hondius in 1598 and 1600; Plancius had earlier shown a stylized southern cross in a completely different part of the sky, south of Eridanus. Perhaps the first person to recognize the true identity of the stars of the southern cross was the English geographer and explorer Robert Hues (1553–1632), who was clearly familiar with Ptolemy’s catalogue. Hues had seen the stars of the southern cross for himself during a circumnavigation of the globe in 1591–2 and realized that ‘they are no other than the brighter stars which are in the Centaur’s feet’, as he pointed out in his book of 1594 called Tractatus de globis et eorum usu (Treatise on Globes and Their Use).
Hues’s book was translated into Dutch by Plancius’s colleague Jodocus Hondius in 1597. In that same year, Plancius received the first accurate observations of the southern stars made by his countryman Pieter Dirkszoon Keyser. There could then have been no doubt that Hues’s identification of the stars as part of Centaurus was correct.
Benefiting from this revelation, Johann Bayer drew the cross over the hind legs of Centaurus on his Uranometria atlas of 1603. The five main stars of Crux were listed as a separate constellation for the first time under the name De Cruzero in the southern star catalogue of another Dutch seafarer, Frederick de Houtman, published in 1603. The first printed chart to show Crux separately from Centaurus was that of the German astronomer Jacob Bartsch in 1624.
The stars of Crux
The constellation’s brightest star is called Acrux, a name originally applied by navigators from its scientific designation Alpha Crucis. At declination –63°.1, Acrux is the most southerly first-magnitude star. The names Becrux and Gacrux for Beta and Gamma Crucis have a similar modern origin, although Becrux is now only an unofficial alternative to its IAU-recognized name of Mimosa.
Crux contains a famous dark cloud of gas and dust called the Coalsack Nebula, which appears in silhouette against the bright Milky Way background. This was first described in an account by Amerigo Vespucci published in 1503 or 1504, where it was described as a ‘black canopus of immense bigness’.
Chinese astronomers worked at a similar latitude to Ptolemy, so they were able to see the same stars as he did, including those of Crux. However, the effect of precession gradually carried this part of the southern sky below their horizon about 1500 years ago, as it did for European astronomers.
The stars we know as Alpha, Beta, Gamma, and Delta Crucis were once part of the constellation Kulou, which represented a military depot. In their book The Chinese Sky During the Han, Sun and Kistemaker show these four stars forming a diamond-shaped tower at the southern end of the depot. Later, though, this feature was placed farther north among the stars of Centaurus. Probably Chinese astronomers gradually moved this part of Kulou northwards on their charts as Crux became lost from view. A similar transfer to more northerly stars over time affected other Chinese constellations in this region of sky, for the same reason.
© Ian Ridpath. All rights reserved