Submitted by admin on April 22, 2009 - 22:32
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.