ASTRONOMICAL NOTES.

Press, Volume LXIII, Issue 19171, 30 November 1927, Page 14

ASTRONOMICAL NOTES.

FOR DECEMBER, 1927. • (SPECIALLY -WBrrTEM POE TEE PRESS.) [■Br B. O. HOGG, M.A., F.R.A.S.) The sun will enter the zodiacal sign Capricomus on December 23rd at 8.48 a.m., when the solstice occurs. This is the longest day in the Southern Hemisphere, and the one on which the sun attains its greatest meridian altitude, which for the latitude of Christchurch is 69 degrees 56 minutes. The distance of the earth from the sun on this day will be about 91,470 miles. The planet Mercury rises on December Ist at 4.30 a.m., and on December 15th at 4.29 a.m.; it will be seen well to the south of east; during the month its apparent magnitude will increase from —0.4 to —0.7 Venus rises on December Ist at 3.20 a.m., and on December 15th, at 3.30 a.m.; Mars rises on these dates at 4.38 a.m. and 4.13 a.m. respectively. Jupiter set 3on December Ist at 2.15 a.m., and on December loth at 1.24 a.m. Saturn is in conjunction with the sun on December 3rd; it will be seen in the eastern sky towards the close of the month well to the south of east. There will be an occultation of the planet "Venus by the moon on December 20th, the earth, moon, and planet being in a line at 4.27 p.m.; the phenomenon will be visible as a close graze in the latitude of Christchurch. There will be a total eclipse of the moon partly visible in New Zealand on December 9th; the moon will enter the penumbra at 3.23 a.m., and the umbra at 4.22 a.m. but the moon will set that day at 5.20 a.m., about five minutes before the total stage of the eclipse begins. There will be a partial eclipse of the sun visible in New Zealand on December 24th; the eclipse begins at 2.10 p.m., and the stage of maximum obscuration will occur at 4.29 p.m., when 0.549 of the sun's disc will be hidden by the moon; the eclipse will end at 6.18 p.m. Tie Rotation of the Galaxy. The telescope and photographic plate show that great numbers of spiral nebulae are dispersed through the space around us; some are seen full face, some edge-wise, and others in intermediate positions, but under whatever aspect we view them the forms they present j suggest strongly that they are rotating bodies. From a comparison of photographs of spirals taken after considerable intervals of time have elapsed Van Maanen and others some years ago obtained apparently such clear evidence of the motion of the outer parts of certain spirals that they had no hesitation in ascribing -rotation to these bodies and even ventured to calculate what their periods of rotation were. Though it cannot be said that all the conclusions to which .these astrondmers were led have been generally accepted, it seems fairly clear that they demonstrated the existence of rotation in these' eases. Such further support has been supplied by the spectroscope that there can now be little doubt that rotation is a characteristic feature of spiral nebulae. At present, however, we have little exact knowledge of the period of time required for a spiral to make a complete rotation; we can only infer from the facts at our disposal that this period is of very considerable duration. There is a growing belief that the galaxy, of which our sun is a diminutive member, is a spiral; the stars of our universe are massed in overwhelming proportion in the Milky Way, which girdles the heavens like a ring, and they have a much greater extension in this direction than in the one at right angles thereto, so that our system has 'more or less the shape of a lens similar to that of so many spirals when seen edgewise, and if the stars are actually distributed in this fashion it is difficult to see how our galaxy can maintain its form and not collapse upon its centre of gravity unless it is rotating about an axis perpendicular to the direction of the Milky Way. This rotatory motion, as a whole, must be regarded as superimposed on the individual motions of the stars much in the same way as when a wind is blowing the particles of gas in the atmosphere have the general drift added to their separate molecular motions. .

It is a matter of extreme difficulty to detect in the individual movements of the stars any systematic effect arising from the rotation of our galaxy; these proper motions as they are called are for the most part exceedingly small, often only a few seconds of arc in a century, and from these minute measures it is required to deduce what part of the observed displacement is a rotational effect. It is, therefore, of great interest to learn from a recent bulletin of the ■ Astronomical Institute of the Netherlands that the problem has been attacked with apparent success by Mr J. H. Dort. Without entering into a discussion of his methods or of the assumptions underlying them wc may state that Mr Dort finds that in the neighbourhood of our sun the linear speed of rotation is about 160 miles a second and that our galaxy as a whole moves through an angle of about half-a-second of arc in a century or, in other words, makes one complete revolution in about 250 million years. This period is of a somewhat higher order of magnitude than, that of certain spiral nebula for which the computation has been made, but it is not in such marked contrast at to make Mr Oort's conclusion improbable. It must be borne in mind that the problem in question is a most intricate one, and that even if only an approximation to' the truth has been obtained, a great step forward has been made, and we must be content to hope that as more accurate knowledge of the proper motions of the stars accumulates, the limits - within which the | speed of rotation of our galaxy lies may be drawn closer and closer. The Magellanic Clouds. Two of the most noteworthy objects to be seen in the southern skies are tho Magellanic Clouds, which at a, first glance may easily be mistaken for wisps of vapour. Under close examination they show many affinities with the Milky Way, as in them we find stars, clusters, nebula;, and nova;. No transverse motion of the clouds across the sky has been detected, but from measurements of the displaced spectre of seventeen of the gaseous nebula; in the larger cloud it has been deduced that the cloud is moving away from the earth at a speed of about 178 miles a second, while the I velocity in the line of sight of the only nebula measured in the smaller cloud is | consistent with the supposition that the two clouds share the same motion. Mr Oort now finds that the radial velocity which the clouds have can be accounted for by the same motion of rotation which he has deduced for the galaxv as a whole, thereby giving additional support to the view that these objects may be regarded as outlying portions of our stellar universe. The Nebula in Orion, Few objects in the heavens are of such interest as the bright nebulae, both from their striking appearance and the many problems they present for solution. As a typical case we may cite the nebula in Orion —seen by the naked eye as the misty gtar in the sword of Orion

nearest to the belt. In only a small telescope it shows up as a ragged sea of lightj and with increased magnification its beauty is enhanced. It is a glowing mass "of- gas of extremely lowdensity, situate about 700 light-years away from the earth, and some idea of its enormous magnitude may be formed when we know that its diameter is about 6J light-years, or 40 million-mil-lion miles. How is its light maintained from age to age? This is a question to which no clear answer cau be given, but right at the heart of the nebula is a group of stars, and it is thought by some that these stars may by their radiations excite the nebula to luminescence. When this and other bright nebulas are examined spectroscopically, in addition to the lines belonging to known chemical elements certain other lines are seen which do not correspond to anything with which we are acquainted on the earth. To the unknown substance whose radiations provide these lines the name of nebulium has been given, and just as helium was found on the eartli long after its existence in the sun had been shown spectroscopically, so it was hoped that nebulium would be traced terrestrially. Nebulium. The origin of the Nebulium spectrum has been the subject of two important communications in recent issues of "Nature"; they are highly technical in character, and we shall only attempt here to give the conclusions provisionally arrived at. On October Ist, Mr I. S. Bowen, of the California Institute of Technology, points out that there is much evidence that the lines in question are emitted by an element of low atomic weight, and as the spectra of the light elements, as excited in terrestrial sources, are well known, there must be some other condition existing in the nebula which causes additional lines to be emitted, and this extra condition he takes to-be the low density of the radiating medium. Bright nebulas are known to emit the spectra of oxygen and nitrogen, and Mr Bowen examines the possibility of the lines attributed to nebulium being really due to these elements under the peculiar condition! of density obtaining in the nebula, and after calculating, in the light of the latest theory of the atom, where bright lines should occur in their spectra, he succeeds in identifying eight of the nebulium lines with lines due in six cases to oxygen and in two to nitrogen. In "Nature" for October 22nd, Professor Fowler, F.E.S, reviews the whole question. He writes: "It is, of course, no longer permissible to suppose the existence of hypothetical elements to account for the long-standing mysteries of nebular spectra, and we must 'accordingly regard the nebular lines as being produced by known elements under conditions of excitation which have not yet been imitated in the laboratory." The general tenor of his letter is in strong confirmation of Mr Bowen's views, and it would seem that a line of investigation has been opened up which holds out good hopes of providing a full solution of the "nebulium" problem. Coronium —the hypothetical "element" to which certain lines seen in the spectrum of the sun's corona —yet remains to be traced, but it will doubtless be found that the lines in question owe their origin to the excitation in some pecular manner of the atoms of a known element.