TRANSIT OF VENUS.

Globe, Volume XXIV, Issue 2703, 6 December 1882, Page 3

TRANSIT OF VENUS.

To-morrow morning the Transit of Venus, for which such elaborate preparations have been made, takes place. The whole duration of the transit will not be visible in this colony. The external contact at ingress takes place at Ih. 46min. ISsecs. a.m.. New Zealand meantime, and consequently when the sun rises the transit of Venus across the sun’s disc will be in progress. What observers stationed in Australia and New Zealand have to direct their attention to is, the internal contact at egress, that is to say when the outer edges of both the sun and the planet appear to coincide. This takes place at about Vh. 31min. 46sec., and the external contact at 7h. 42min. 9sec. on the morning of Thursday. It may be stated that the beginning of the transit will be visible in Great Britain, Southern Europe, and Africa. The whole of the transit can be seen in North America

(with the exception of the North-west portion of the continent), in South America, and in the AVest Indies. It will bo invisible in Northern Europe and throughout the whole of Asia (except a strip on the East of the Red Sea), and the Western half of the continent of Australia. The end of the transit will be visible, as wo have already stated in New Zealand, in the eastern half of the continent of Australi , the Pacific, and the north-west part f North America. Our readers are no doubt familiar, in a general way, with the object astronomers have in making such elaborate preparations for observing the transit. It is to determine with as much accuracy as possible the distance of the sun from the earth. The solution of this problem is of importance both from a scientific and from a practical point of view. Various methods have been adopted for determining this question, but the most accurate results are derived from observing the Transit of Venus. The task which has to be performed is to measure the parallax of that planet; in other words, to ascertain the extent of the apparent displacement of Venus on the sun’s surface, as seen by observers stationed at different places on the earth’s surface. Different plans have been adopted for determining this problem. There is the photographic method, to indicate definitely the apparent path of Venus as seen at different stations ; the heliometric method, which proposes to measure exactly the distances of the edges of Venus from the opposite edges of the sun, and so determine what is required, the least distance between the centres of the sun and the planet. The method of duration is a modification of the one originally proposed by Halley, and requires that both the internal contact at ingress and egress should bo observed, and that Venus should pass nearly along the diameter of the sun. The method known as De I’lsle’s “takes advantage of the fact that the ingress will take place later when seen from some parts of the earth than from other parts ; so with the egress of the planet from the suii’s disc. Hence if the absolute time of contact of Venus with the sun’s edge at ingress or at egress be observed at two places suitably chosen, the difference in time will be a measure of Venus’s parallax.” In the case of every nation using the De I’lsle method, and in the case of every expedition where only one contact is observed, the longitude must be determined with very great accuracy. This is done by the aid of chronon* 'tors, the telegraph, and by observations ct „he moon’s position. The first prediction, of the ti'ansit of Venus was made by Kepler, who calculated that it would take place in 1631, but his calculations were not strictly accurate, the event taking place during the time the sun was below the horizon, and was consequently invisible to astronomers in Europe. The first person who appears to have actually observed this interesting phenomenon was a curate of the village of Hoole, near Liverpool. He discovered some

inaccuracies in previous tables, recalculated the problem anew, and arrived at the conclusion that a transit of Venus, visible in England, would take place on the afternoon of November 24th (old style), 1039. But to make perfectly sure he commenced his observations on the 23rd, and had the satisfaction of witnessing the transit on the 24th, as predicted. But it was not till the transit of 1704- that any attempt was made to measure the sun’s distance, and the attempt was not very successful. _ Bettor results were obtained from the one in 1/09, and hotter still from the observations made in 1874. The next transit will take place in 2001.