Star Tales – Crux

This is the smallest of all the 88 constellations. Its stars were known to the ancient Greeks, but were regarded as part of the hind legs of Centaurus, the centaur. 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.

In 1501 the Italian explorer Amerigo Vespucci (1454–1512) charted what seems to have been Alpha and Beta Centauri and the stars of Crux, but the most accurate early depiction was made by the Italian navigator Andrea Corsali in 1515. Corsali described the pattern as ‘so fair and beautiful that no other heavenly sign may be compared to it’. (Incidentally, Corsali’s diagram seen in the link above may at first seem difficult to reconcile with the real sky, and has undoubtedly suffered from repeated copying; however, the Dutch historian Elly Dekker has pointed out that he drew the stars as they would appear on a celestial globe, so their positions are a mirror image of the view as seen from Earth.) 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. It contains a dark cloud of dust known to modern astronomers as the Coalsack, but named Macula Magellanica on this illustration from the Uranographia 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 respectively; Plancius had earlier shown a stylized southern cross in a completely different part of the sky, south of Eridanus. It seems that only after he received the first accurate observations of the southern stars made by the Dutch navigator Pieter Dirkszoon Keyser did Plancius realize that the stars of Crux had been listed in Ptolemy’s Almagest all along, as part of Centaurus.

The constellation’s brightest star is sometimes called Acrux, a name applied by navigators from its scientific designation Alpha Crucis. It is actually a double star, divisible through small telescopes into two sparkling blue-white points. The names Becrux and Gacrux for Beta and Gamma Crucis have a similar modern origin.

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 1,500 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.