On his celebrated voyage round the Cape of Good Hope in 1497, Vasco da Gama was much impressed by what appeared to be a new group of southern stars which we today know as the Southern Cross. But Vespucci after his third voyage in 1501 insisted that he was the first European to see it and called it Mandorla in reference to the oblong glory surrounding the bodies of saints ascending to Heaven. Sixteenth century writers on navigation made frequent mention of the constellation. Curiously enough Dante has reference to it in his Divine Comedy, although it is certain that at the time he could not have seen it from Italy. It would appear, therefore, that he heard of it from travellers' reports. At the present time the Cross is always visible from all New Zealand latitudes and is a circumpolar constellation. In Australia, however, for latitudes north of the Tropic of Capricorn, it is below the horizon at lower culmination.
Owing to the 26,000–year precessional movement of the earth's axis, the Cross has in the past been visible from the Northern Hemisphere. Some 5,000 years ago it would have been seen from such countries as France, Spain, and Italy. The great Alexandrian astronomer Ptolemy in the second century concluded that the stars were part of the larger constellation of the Centaur, and celestial maps continued to depict them in this fashion until about the fifteenth century A.D. There is no knowledge as to who first depicted them as a Cross, but they are shown in this fashion on a globe made in 1592 by Mollineaux in England. Another interesting coincidence is that the Cross would have been visible on the horizon of Jerusalem during the period in which the Crucifixion took place.
In the course of 24 hours the Cross makes a complete circuit in the heavens around an invisible point known as the South Celestial Pole. This is the point where the projection of the earth's axis appears to pierce the heavens, and which would be in the zenith to an observer at the South Geographical Pole. The result of this diurnal motion is that the Cross may be seen upright and high in the southern sky at upper culmination, or inverted and very low on the southern horizon at lower culmination. Between these two positions the cross-upright makes all possible angles with the horizon. An approximation to the true south direction can be made by continuing an imaginary line along the upright right across the southern sky to the bright star Achernar. The half-way point on this line is not far from the South Celestial Pole which is of course the direction of true south.
Symbolic representations of the Cross, such as on national flags, show either four or five stars; and often the question is raised as to just how many stars are in the constellation. The four main stars give the cross effect, although the cross-member is somewhat askew. The fifth next brightest star is something of an interloper. Examination by the naked eye on a very clear night will reveal about 34 stars within the modern boundaries of the constellation, but with telescopes the number must run into many thousands. It is correct to say, however, that there are five principal stars. Two bright stars on one side of the Cross are often referred to as the Pointers, since the imaginary line joining them appears to point towards the constellation. They are, however, members of the constellation Centaurus, the brighter one (Alpha Centauri or Rigil Kent) being the closest star to our sun. Light travelling at 186,000 miles per second takes four and one-third years to reach us from it.
One of the very rich and highly interesting portions of the Milky Way passes through the Southern Cross, and examination by the naked eye shows many condensations which break up into magnificent clouds of stars when viewed through binoculars or a small telescope. Close by Beta Crucis is a magnificent telescopic star cluster known as the Jewel Casket, since it contains stars of varied brightness and colours. Adjacent to Alpha and Beta Crucis is a large, dark, cloud-like area, from which stars appear to be absent as seen by the naked eye. Popularly known as the Coal Sack, it is a cloud of gas and dust obscuring the light from the more distant stars of the Milky Way which silhouettes its outline.
The principal stars of the Southern Cross (scientifically called Crux) are given in the following table:
| (Epoch 1950.0) | ||||||||
| Star | Right Ascension | Declination | Magnitude | Spectral Type | Distance (Light Years) | Radial Velocity (Km/sec) | ||
| hr | min | degrees | min | |||||
| Alpha | 12 | 23.8 | –62 | 49 | 1.05 | B1 | 217 | –12 |
| Beta | 12 | 44.8 | –59 | 25 | 1.50 | B1 | 326 | +20 |
| Gamma | 12 | 28.4 | –56 | 50 | 1.61 | Mb | .. | +21 |
| Delta | 12 | 11.5 | –58 | 28 | 3.08 | B3 | 192 | +26 |
| Epsilon | 12 | 18.7 | –60 | 08 | 3.57 | K2 | 171 | –5 |
Alpha Crucis (sometimes called Acrux) is a triple star, the two principal components having magnitudes of 1.58 and 2.09. Gamma Crucis is a double star, and noticeably of an orange colour to the naked eye. Positive radial velocity values indicate that a star is receding from us, and negative values indicate that it is approaching us. Such values have been determined spectroscopically.
by Ivan Leslie Thomsen, F.R.A.S.(LOND.), Director, Carter Observatory, Wellington.