ASTRONOMICAL NOTES.

Press, Volume LVII, Issue 17058, 1 February 1921, Page 10

ASTRONOMICAL NOTES.

FOR FEBRUARY. | - (SPECIALLY WBITTEtT FOR "THB PBJtSS/') (BY E. G. HOGG, M.A., F.R.A.S.) The planet. Mercury sets on February Ist at 7;58 p.m., and on February 15th at 7.41 p.m., Venus at 9.9 p.m. and 8.41 p.m.., respectively, on these ■ \ dates, and Mars at 9 p.m. and 8.23 p.m. . The planet Jupiter rises on February Ist and February 15th at 8!53p.m. and 7.55 p.m. respectively, and! Saturn, on these dates at 9.13 p.m. and 8.15 p.m. respectively. Mercury, though setting so isoon after tihe sup, may possibly be seen owing to its brightness, which exceeds that' of any fixed star except'- Sirius. Venus will steadily increase in brilliance during the month. Mars and Venus, which were apparently very, near each other early in January—thougjh as a matter, of fact Mars was over 90,000,000 mile 3 further from, the earth than Venus was —-are now well separated.. The contrast in their brightness is very marked, there being a difference of near'.y 5} magnitudes .between them, or, in. other words, the. light reflected to us by ■ Venus is over 140 times as much sis that coming to lis from Mars. .

On the evening of February Bth. the planet Uranus will be about 9rain of arc from Mercury. Venus will be in conjunction with the moon on February 12th at 4»11 p.m.; the latter will p'as3 between Venus and the earth, but t&is interesting occultation will not be visible # in the southern hemisphere beyond latitude 16deg S. The two bodies will set" together on' that evening at 8.5S p.m., and their close approach will form an interesting sigfht in the western sky. "In these'notes for last month attention wai drawn to some of the more striking heavenly objects which may be seen north." of the zenith; if we examine the 6ky in the opposite direction .we do not see. sudh a bright array of stars, but in'the Magellanic clouds we l iave something which' is quite unique in the firmament. As their name imports, thej" are two nebulous or cloudy and might.easily

bo mistaken for two light clouds of water-vapour. They also produce at first sigjht the same impression on the unaided eye as might bo excited by two bright portions or the Milky Way. A line drawn from Sirius to Canopus. if produced, would pass through thu greater cloud and' near to the smaller. A clear moonless night is best suited for their observation, as in strong moonlight the smaller cloud disappears entirely, while the larger one loses mudh. of its brightness. They are, roughly speaking, oval in shape: the larger occupies a soace of about 'l2 square degrees in the heavens, and the. smaller about 10 square degrees. They are not connected with one another or with, the Millty "Way by any trace of nebulosity. Our earliest reference to the Magellanic clouds is in the writings of the Persian astror-omer. Abdurahman el Sufi, who lived in the tenth century, the "White Gx_" which he sa.w shining with a milky light far below- Canopus being undoubtedly the. larger of the two clouds. No mention is made of these celestial objects by European travellers until the beginning of the sixteenth century—a curious fact when we recall that the Normans (had, 200 years earlier, advanced as far along the west coast of Africa as Sierra Leone (lat.- Sdeg 30min N.), and that, in 1471 the Portuguese had passed beyond the equator. Eventually we find them referred to under the name of the "Cape ■ Clouds,'' a designation which they owe to the Portuguese, Danish, and Dutch pilots and travellers who commented on them 'as remarkable celestial phenomena seen during voyages to the Cape of Good Hope, discovered by Diaz, in May, 1487. It would 6eem, however, that the exceeding fame and the great duration (August, 1519, to September, 1522) of Magellan's circumnavigation of the globe, and the long sojourn of a numerous crew under the southern sky, obliterated the remembrance of all earlier observations, and spread, the name of the Magellanic clouds among the seafaring nations of the earth.

Coming to recent times we find that the first correct description of the ■'clouds'' was given in 1827, by Dunlop, at whose observatory, at Parramatta, N.S.W., so.much valuable pioneer work on the nefcuke of the southern heavens was done. Our principal source of information is, however. Sir John Berschell's'"Observations at the Cape." in which are.embodied the results of his four years' study of our skies. A copy of this valuable work may be seen in the reference department of the Public Library, and an examination of Herschell's chart of the larger, cloud will convey to.the mind a clearer idea of the complexity of this object than

any verbal description cati. Henschell determined the position in the larger cloud of 919 objects—6B2 of these were stars ranging from the sth to the 9th magnitude, 291 were nebulas, and 46 were star clusters—while in the smaller cloud he mapped. 200 stars, 37 nebulae, and 7 star clusters. Of these nebulae he remarks that they are of a peculiar character, and have no analogue in any other region of the heavens. The clouds combine, each within its own area, features which in the rest of the heavens are strikingly separated. Globular clusters are entirely absent from the Milky Way, and nebula of elliptic form are comparatively, rare there; .these objects—clusters and nebulas—? are congregated in the greatest abundance -in a part of the heavens most distant from the galactic whereas in the clouds they are indiscriminately mixed with the general starry ground. Though the Magellanic clouds cannot be regarded as two fragments of the Milky Way, disrupted from the main moss by some "Tremendous (Heaven defend' us[) ' 'Monstr'-infornl'-ingens-horrend-ous'' "partial impact," as has been suggested to the ©resent writer by an enthusiast for that theory, yet between the two there is a curious connecting link. From time to time temporary stars or n&vee appear in the sky: they are always observed within the boundaries of the Milky Way, and, after running through their various stages of brightness and decay, they apparently become what is known as Wolf-Rayet stars —a type characterised by a very peculiar spectrum —91 stars of this class are known to exist: 70 are situated in the Milky Way, and: the whole of the remaining 21 are found in the Magellanic clouds 1

Two other points of interest connected with the cfouds may be noted. About 18S8 Mr H. C. Russell, of the Sydney observatory, began a study of the Southern -Milky Way and t!he clouds with a &-inch portrait lens, and he states that an exposure of 4§ hours showed the structure of the greater cloud to be that of "a complex spiral nebula with two centres," while a structure somewhat similar to that of the dumb-bell nebula is visible in the smaller cloud. If a moving object is sufficiently bright it is possible to tell by means of the spectroscope what velocity is has in the line of sight, or determine what is called its°radial velocity, and during recent years part of the programme of work at the Chili branch of the Lick Observatory has been the study of the Magellanic clouds. It. has been found that seventeen of the gaseous, nebulae in the 'larger cloud have very large and approximately equal raajal velocities —

the average value of which is 178 miles per second awaj' from tko earth, and as these nebula) appear essentially a part of the cloud it seems reasonable to conclude that this mean velocity is the radial velocity of the cloud as a whole. Unfortunately the Wolf-Rayet stars are not bright enough to give satisfactory results. An examination of the individual radial velocities of the seventeen nebulas above referred to brings out the curious fact that the greater the distance of one of these nebulce from the South Pole, the greater is its radial velocity, and this lias led to the suggestion that the cloud as a whole may be in a state of rotation. This view is, however, opposed by Hertzsorung in a paper read at a. recent meeting of the Royal Astronomical Society. We may point out that a knowledge of the motion of a body to and fro in the line of sight throws no light on the velocity it may have, at right angles to that line. Tlio object of Hertzspnmgs paper is to show that the observed differences of radial velocities ot the nebulfe in the cloud can be accounted for on an hypothesis much simpler than that of rotation, viz., that all Darts of the cloud are moving m parallel lines, and with the same speed, and further, that the radial velocity .shown bv the only nebula measured m the ! smaller cloud is consistent with the i supposition that the two clouds share the same motion. This common velocity Hertzsprung finds to be t,la i about 38(1 miles a second That such huge structures as the clouds should be "moving through space with tins stupendous sneed is a result we mig find great difficulty in accepting did, wo not know that some of the. ( spiral nebulse appear, to be moving towards the solar svstem with velocities of this order of magnitude, 435 miles a being. the moan result obtained for those spirals examined by SlipJjer, which were bright enough to yiel satisfactory spectra. The scheme propounded by "the jnow Zealand Government for the future control of the observatory at Samoa has been accepted by the British Government, which has agreed to contribute its share in the cost of its upkeep: it is vorv unlikely that the other Govern- • ments concerned—Australia, and Canada. —will fail to agree with the proposed arrangements for t)he maintenance or the observatory, and so in the near future we may hope to see all tho parts of the British Empire which border oni tho Pacific Ocean cordially working together to support an institution where, work of tlhe greatest value to international science has been carried on. for the. past 18 years. Dr. Anganheieter, who has been director of the observatory since its foundation, is leaving Apia, shortly, «nd Professor Marsden and Mr Westland, of Wellington, have gone there to continue the observations, pending the appointment of a permanent director.

The selection of the site for the new observatory to bo founded in the South Island is going to prove a difficult task. Dr. Adams has visited certain of the suggested sites in Central Otago: though some of them are admirable Tor the purposes for which they are required, it would seem t!hat the great difference he'finds between, the day and night temperatures of the localities inspected is likely to prove a serious obstacle to their adoption. He has also examined the country inland from Oamaru, but no decision will be 'made* until definite tests with a telescope and othe/r .fastrumlsnts have been made. The authorities of the Yale University, .which is presenting the principal telescope to be used in the new observatory, are taking the greatest interest in tie work-.