ASTRONOMICAL NOTES.

Press, Volume LVII, Issue 17106, 30 March 1921, Page 11

ASTRONOMICAL NOTES.

FDR-APRIL,.",

(SESCIALLY YKITTHK TOE "THS TIIESS.")

(Br E. {J. Hogg, M.A., F.R.A.S.) ,

The planet Mercury rises on April Ist at 3.54 a.m., and on April lutli at 4.35 a.m. Venus seta on April Ist at G. 19 p.m.;' it is rapidly approaching the sun, and will be in interior conjunction with that body on April "3rd. Its distance from the- earth will then be about 32,150,000 miles. Mars sets on April Ist at 6.3G p.m., and on April 15th at 0.3 p.m. Jupiter rises on April Ist and April 15th at 4.48 p.m. and 3.51 p.m. respectively. Saturn rises on those dates at p.m. and 4.14 p.m. respectively. There will bo ail annular eclipse of tho sun on April Btli, but it will not bo visible in hew Zealand. An eclipse of thia type has but little interest for astronomers: the relative distances of the sun, moon, and earth are such that Itbje whole of the solar light is not at any time cut off. A fraction of the highly illuminated surface is always visible, and at no stago of the eclipse is it possible to see the corona or the prominences, which constitute at once the scientific value aud the spectacular imprcssiveness of a total eclipse of the sun.

A fortnight later, on April 22nd, there will /be a total eclipse of the moon, which' will enter the outermost fringe of the earth's shadow at 4.27 p.m. As the moon, however, does not rise on that day at Christchurch until 5.4 p.m. we shall miss the beginning of the eclipse here. The moon enters the umbra at 5.32 p.m., and will be totally obscured by 6.53 p.m. This stage will last for 42 minutes; illuminated edge of-the moon wdl reappear at 7.35 p.m., the' moon will leave the at 8.58 p.m., and tho eclipse will bo completed at 10.2 p.m. Though lunar eclipses have not the importance which attaches to total eclipses of the sun, they, always appeal • to the ordinary observer. This is no doubt partly due to the fact that* they can be viewed without any elaborate instrumental equipment. Though most of their essential features can be seen with the naked eye, the assistance of a small telescope or pair of field glasses is needed to detect satisfactorily the changes in colour which, take place as the dluminated port of the lunar disc dimmishes in size, and finally disappears. , The colour of the moon during the period of total obscuration in an eclipse was tho subject of an interesting paper, read before the Academy of (Science, Paris, on. December 6th lost, by M. A. Danjou. We know that the sun'a surface is liable to periodic disturbances of great violence; from a relatively quiescent state when "sunspots" are absent for months at a time, and the solar prominences are* comparatively small, the surface gradually passes into one of great activity. When the "spots" are numerous and of large area, and the prominences rise to a: great height aboVo the edge of the disc. These maxima of "sun-spots" follow each other'at regular intervals of about eleven years, and while the maximum lasts there is a diminution in the brightness of "the sun, but an- increase. uytbe heat emitted. At a minimum, the reverse of this' holds good—the Sun emits more light, but leas heat. - The last maximum of "sun-snots" occurred . in 1917, and the sun is now approach-. ins a minimum of activity. tfrom a study of lSu lunar ecllpse9 between the years 1827 and 1920, M. Danjou has drawn the following interesting conclusions. During tho two years following a minimum solar activity, the obscured moon during the total phase is very dark, grey, or coloured but slightly as we get f mtber away from a' minimum the colour during totality becomes brighter, and in tlw yeara just preceding the next minimum it is generally copper red ot orange in tint. When this minimum is reached the colour of the eclipsed moon reverts, suddenly to a grey sombre hue, sueh as it, had shown eleven years previously. Nothing particular characterises the moon's colouration at an eclipse Occurring: daring a maximum of solar activity M. Danjou oflers no explanation of the curious results to which he has been led by bis study of the records of these eclipses, and it does not seem easy to formulate a theory which will account for these changes in colour of ,the eclipsed moon. Wo are now apprjwehing a minimum of solar activity; if M. Daniou's deductions are correct, the moon on the night of April 22nd should be well, coloured, and present an even brighter appearance than it did on the occasion of ite last' ;eclipse—October 27th, 1920, visible here. ; \ Besides the "spots" which periodically dim the solar surface and the prominences which are always more or less visible at the edge of the solar disc, there are. certain peculiar formations called, iaculro (Latin, "a little torch ), which are nottoeable, especially near the darkened edge of the sun. These facola are irregular streaks of greater ■brightness than the general surface, leaking much like tho flecks of foam. wMdvWrk the surface of a stream below a waterfall; not infrequently they are from fire to twenty thousand miles in length, and cover areas immensely larger thim any terrestrial continent. In a paper recently reafc before-the B4£l Astronomical Society, Mr E. W. t Maunder. F.R.S., late of the Greenwich Observatory, &8 analysed the distribution <m tho 8un"« edge of the facuto observed at Greenwich during the period IbSth 1916 and deduces the remarkable result that the number of iacuto-to be seen on the eastern edge of the sun exceeds by three per cent, the number visible on the western limb. It may be said ihat thiß conclusion is not an altogether unexpected one, for in 190.'the fen AstronWier-Royal, Sir William Christie, communicated to the Society a paper by Mrs Walter Maunder, entiUocT"An Apparent Influence of the Earth on the Numbers and Areas of th* Sun-spots in tho Cycle 1889.1P01" which showed that there was' "a well-marked and steady pVeponfo"""* s # tIK Lu • bnlf of t8» sun's disc over the western half both as to the areas of tho spots Jnd'asto the numbers of the separate spots » M. Sykora, in a paper read m 1897 had shown that there was a sligjabut 'unraistakeable predominance of eastern ovor western prominences,, bnt later work on tho pronnnentes did riot altogether bear out his conchisions-fo though tbe eastern hemisphere was Jie more prolific from 1892 to 1900, inclusive, vet, with the year of minimum eolw activity, 1901, the tide turned and for four years the western limit had the advantage. In 1303-the eastern edge Biad recovered Us earlier predomi,°,FTom Mr Maunder's paper it would appear that tho faculao Lave, during thTperiod of 30 years, always been in cxcesVon the eastern limit, and though ! they have responded to changes in the I solar activity, it Suva not been so marked I ns in the ci» of tho prominences. It is, indeed, difficult to understand whytho number of faaOae seen from earth should be greater, on tho eastern edge which is «BWBaAu« U8 > * haa on tho limit which the sunjs rotation is carrying away from us. There does not eeeni to be any solar cause to which th» lack of symmetry in th« distribution of the faculae can be assigned, and it would appear, therefore, :n our present state of knowledge, 'that.the cause of the disproportion must-be terrestrial, and terrestrial only. We are at a loss to conceive any mechanism by which tffls apparent "earth effect" js, produced, vwk would jw>t be operative to ft simi-

lar, if notgreater, extent by the planets Mcrcurv and Venus. Cno mere problem is added to the number awaiting solution at the hands of the solar physicists. ■ If any reader of these notes, has access to a fair telescope, he will find that tho planet Saturn is presenting just now a rather unusual appearance: in fact, ho will find it « matter of considerable difficulty to catcli. a glimpse of thoso rings whidh constitute the ibief point oi interest in tho observation of this planet. Saturn we know takes about 2'JJ to revolve once about the sun, and'during this time the plane of the rings moves parallel to itself: hence at , intervals of nearly fifteen jcars ix, comes ] about that the plane of the rings will i pass tJlirc-uga tho earth the ritig* will then be edge on to tho taith, and will disappear entirely from viciv. In anticipation of such a passage, Saturn was examined with the 2Sin refractor of the Givenwich Observatory, on the inorni|i« of November 6th liiat, and the rings were, well seen. On the following morning, when tho conditions of seeing wero good, no trace of the rings was viiiule, and they wero not picked un again at Greenwich until .November l'ith. Professor Barnard also reports that wiien in the 40in refractor on November 7th, with his 12in refractor, lie cculd see the rings easily, but they were invisible iu the iOin retractor on November Ith, and it was not until November 21st that they came into view again. Other observers reoort the same sequence: the rings could be seen easily until just before November 7th, when the passage took place, but many days had to elapse after that date bctore tncy again became visible. The rings of Saturn get all their licht from the sun; hence when tho solar ravs are falling on one surface of the other, V underneath surface—if wo mav use such a phrasewill "vt no light directly from the sun. Now, prior to November 7th, observers . on tho earth were looking clown en the j bright surface of the rings, and though 'they wero very narrow, being seen almost edgewise, still they reflected sufficient light to be visible under good con- [ ditions. In November 7th trie passage of tho plane of tho rings through the earth took place, and fitter this date it was the under or unilluminated Bide which was presented to the earth, and, as this side shines only by irregular ravs which have struggled through the rin;rs, or bv light reflected from the bodv of the .planet, we can easily understand whv it was' such a difficult object to pick un. Tho piano of the rings will nass through tho sun about April 10th,'and after that date'the present unilluminated side will become the bright one, and Saturn will once again present a normal nppearanco, though It will be some considerable time before the rings open out sufficiently to bo oasily seen in a small telescope. It may' be added that, as result of-lus. recent observations. on Saturn, Professor Barnard concludes that the rings cannot be more thrrh 50 miles thick.