GLORY OF THE STARS

New Zealand Herald, Volume LXIX, Issue 21249, 1 August 1932, Page 6

GLORY OF THE STARS

HEAVENS IN AUGUST THE NATURE OF COMETS POSITIONS OF PLANETS BY CRUX ACSTRAtIS Among all the wonders ol the celestial vault there is nons which oxcites siicb public attention as the apparition of & comet, and the presence of one in our southern skies, discovered in our own Dominion, has drawn more than the usual amount of interest to these bodies. At such times many questions are put to astronomers regarding the nature of comets, so that an outline of our knowledge of these wandering bodies may not be out of place at the present time. A 1 - though comets are visible every year in fair numbers, it is not often that they attain sufficient brightness to be visible to the unaided eye, so that to the man in the street they are comparatively rare phenomena. What may be termed the principal characteristic of a comet is the coma, because this is present in every comet observed. It consists of a hazy cloud of faintly luminous matter, roughly oval or circular in ishape. The coma generally has a considerable volume, "he average diameter being given as 80,030 miles, but this volume is continually changing and often irregularly. - Near the centre of the coma is generally found a star-like point of light, known as the nucleun. Comctary nuclei are gener ally very small, one of the largest on record, that of the comet of 1882, being 1800 miles in a comet visible in June, 1927, had a nucleus of only one or two miles diameter. Both coma and nucleus ire visible in Comet Geddes. Varkrty In Cometary Tails The third characteristic, and the one which undoubtedly makes a comet in the popular mind, is the tail. Strangely enough, however, only a few comets pos- q sess tails, and they display these only when close to the Sun. Among the comets of this century, the only ones possessing prominent tails appeared in 1901, 1903 1910 (2), iind 1929. There is a great variety the taib which do occur. Some are like long, white roads through the constellations, others are spread out in a delicate fan-like struc ture, and some comets have as manv as seven separate tails The most striking fact about these tails 13 that they are ejected from the comet in a direction away from the Sun, and it is believed that the action of the Sun's light radiation has something to do with this repulsion. An average tail visible to the naked eye is seldom less than five or ten million miles in length, while in some cases tails have been known to exceed 100,000,000 miles. While opinions differ as to the masses of comets, there can be no doubt that these are very small, perhaps not one-millionth of the Earth's mass. With small masses and comparatively large dimensions the densities of comets are very small. It has been calculated that in 2000 cubic miles of the tail of Halley's Comet there was not more matter, and probably much less, than is contained in one cubic inch of ordinary air. Eclipse of the Sun The Sun will be in the constellation Cancer until August 10, when it will enter Leo. Its greatest altitude at Auckland increases daring the month from 35deg. to 44deg. On August 31 the Sun will be totally eclipsed as seen from portion of Canada and the United States, and from all portions of North America the Sun will be seen partially eclipsed on that day. Parties of astronomers, who are visiting America for the meeting of the International Astronomical Union, will view the eclipse from selected localities. During August the Moon will be in conjunction with the various planets as follows: —Jupiter, 4th, 5.30 a.m.; Mercury, 4th, 8.30 a.m.; Saturn, 14th, 11.30 p.m.; Mars, 28th, 9.30 a.m.; Venus, 28th, 2.30 p.m. ; Mercury, 30th, 9.30 p.m. Faint stars will be occulted by the Moon on the nights of August 9, 10, 12 and 23. Mercury and Jupiter are lying close to the Sun, visible shortly after sunset in the western evening sky. Mercury will be in conjunction with the Sun on August 17, 3nd Jupiter will be similarly placed on August 26. In the morning sky Venus and Mars lie close together, the former now shining with its greatest brilliancy, the latter still an inconspicuous object. Saturn is the only planet at present favourably placed for observation, being situated in Capricomns, j*ist part its opposition with the Sun. Its unique system of rings encircling the globe provides never-failing interest to tbe'tslescopisl.