ASTRONOMICAL NOTES.

Press, Volume LVII, Issue 17342, 31 December 1921, Page 10

ASTRONOMICAL NOTES.

FOR JANUARY. (SPECIALLY WRITTEN TOR "THE PBK3S." I (Bv E. G. HOGG, M.A., F.R.A.S.) The rare spectacle of five planets visible at once in the eastern sky is no longer with us: on December 28th the planet Mercury was in conjunction with the sun, and will soon be visible after sunset. On January loth it will set well to the south of west at 8.24 p.m., and, as its apparent magnitude is 0.9 or equal to that of Canopus, observers should have no difficulty in sighting it. Venus rises shortly before the sun, but the two are rapidly approaching each other, and the planet will soon be lost in the bright solar rays. Mars rises on January Ist at Ohr 40min, and on January 15th at Olir omin; Jupiter rises on these dates at midnight, and 11.7 p.m. respectively, and Saturn at 11.45 p.m. and 10.49 p.m. respectively. During 1922 there will be two eclipses of the sun, one of which—that of September 21st—will be partly visible in Chris'church, the sun being partially obscured just, before sunset. There will be no eclipse of the moon during the year. but. as some compensation for the absence of this popular astronomical event there will be no less than nine planetary eclipses by the moon—seven of Jup;j;er and two of Saturn; though these will not all be visible as occupations in the latitude of Christchurch, the close passage of the moon asid planet will be a striking sight in the sky. The first of the occuitations of Jupiter takes place on January 19th, conjunction of the moon and planet with the earth occurring at 11.2 p.m. As the moon does not rise that night until. 1045 p.m., it is doubtful if the beginning of the oocultation ■will be visible-in Christchurch, but the emergence of the planet from the bright limit of the moon should be looked for. When a star suffers occultation by the moon its disappearance, and reappearance are instantaneous, but in the case of a planet like Jupiter, whose disc on the night in question will have a diameter of about 33 seconds of arc, the processes of extinction ana emergence will occupy a very appreciable interval of time. A small telescope or field should be used in making the observation. ~ Our knowledge of. those strange members of our system —the spiral nebulce —is gradually extending. The velocities of a large number in the line of sight have been . measured' with the astonishing result that whenever this velocity attains a high value the nebula i 9 receding from the earth ; Y\ e now learn from a recent publication of the Astronomioal Society of the Pacific that Mr A. tan Maanen, by comparing photographs taken by the 60-inch telescope at Mt. .Wilson, with others, Becured some years ago with the Crossley reflector of the lack Observatory.. finds evidence of rotation in four spiral nebulae, which oorrespond to periods of rotation of 85,000, 160,000, 46,000, and 68)000 years respectively. It would probably be unwise to attach much importance to these results individually, but they are perhaps of value in indicating the order of magnitude of the time involved in the rotation; of a nebula. If, however, the average—--87,000 years—be at all correct, we cannot but regard it as surprisingly small. It is true that no accurate measurement has yet been made of the distance of a spiral nebula from the sun, ■but there seems good reason to. believe that these. .objects , occupy an immense volume of space, and are very far off 'indeed, and we should therefore expect the duration of a rotation to be correspondingly long. The relative motion of parts, or a nebula hafl been observed in a few cases, but there is great, difficulty in determining whether the/"displacements detected represent tbe movement of,masses of; nebulous, matter along the uncoiling arms of the spiral or rotation of its centred nucleus. " .Possibly, when we are in. possession of Mr van Maanen's paper we shaU see how he haa: been able to discriminate between these alternatives, and deduce the. results given above. >. In the issue of "The Press" for November 28th last appeared a cable from New York the discovery by Professor W. H. Pickering, of Harvard University, of. areas of vegetation on the surface of the moon, and the presence in that body of steam. For those acquainted with Pickering's lunar theories, this cable- contained nothing new; while other astronomers have wen unanimous in regarding our companion as a ■ dead planet without water or vegetation, but possessing possibly on extremely tenuous atmosphere, scorched by the sun's rayi during tne long lunar day, and subjected to a more than Arctic' temperature during its night, he has, he says, been convinced for more than twenty, years that there is vegetation on the moon, and, of course, when vegetation exists there ntust naturally be air and water for its support. The usual practice in lunar study.-is to examine those regions which, »at the time of observe tion, lie near the terminator or irregular boundary which separates the bright and dark parts of the surface, when striking, views which may be easily understood are obtained. If, however, we wish to learn something of the changes which, 'according to Pickering, are always taking place on the moon, we must watch tne brightly illuminated areas far removed from the terminator, and then,, he asserts, we may with quite a small telescope see within the walls of certain lunar craters a succession of changes of brightness and colouring which, can only be due to of snow, water vapour, and vegetation. As. many of our readers may possess a telescope of only Sin objector—and this was tne size of,the instrument used by Pickering in muon of his work—it may be of interest to give a brief account of what Pickering saw, though we may suspend our judgment as. to the interpretation of the observations. Hie two craters examined in greatest detail by Pickering are Theophiluß and Eratosthenes; it is with the latter that the cable previously referred to is concerned, and to which our description will mainly be confined. This crater, which lies about ten degrees to the north-west of the well-known Copernicus, is 37 miles in diameter, and its wails rise to a height of nearly three miles above an interior level floor, in the course of which is a conical peak about 10,000 ft high. The inner waUs of the crater have according to Pickering, a. gentle slope, which nowhere exceeds 20 degrees ; the cone itself appears to rise abruptly from the floor of the crfeter. As regards the vegetation supposed to exist here, Pickering writes: "The lunar vegetation is not green. It is grey like our sage brush, and some of our cacti, and black like our lichens—almost a purplish black in some places. . . The lunar vegetation is scattered, generally in rather small patches, over the surface. None .is sound near the poles. . . The vegetation when found, is often associated with minute craterlets. the craterlet occurring at or near tne centre of the 4ark area." The growth and decline of the vegetation are, we are told, very rapid. "If tbe lunar vegetation has leaves, we must consider that only the twigs and branches are visible near sunrise and sunset. Its form is, of course, quite I unknown to us, and a the phenomena I may oonsist merely in the darkening j and fading of .its exposed surface." I Professor Pickering has devoted much time to the study or the planet Mars, ' and he notes as a characteristic feature of the vegetation on that body the slow

movement of darkened areas (associated with 6anals) transversely over the surface, regions which were at first light becoming dark, and others which were dark becoming light. - lie finds that on the moon darkened areas similarly shift flheir Hpsition, and in addition are subject to a steady longitudinal growth or recession. As dark markings visible at full moon cannot be due to shadows, he inters that ti.e surlace changes observed must be'due to vegetation in the case of the moon as they are on the planet ilars- . v\ hen the light from the rising nun ! first strikes the central peak of Eratos- ; ; thenes it is faintly -illuminated; it . gradually brightens up, and soon dis- ; closes on its summit a small craterlet about 3500 feet in d ameter, t;:e interior of which, in the course of the lunation, gets brighter and brighter, readbee its maximum brilliancy, and then fades, i "This extreme brightness is doubtless due to snow rather than cloud.' ! Whilst the "snow'' is becoming m.ire ! and more visible, it is accompanied by la very curious change in position—the i interior snow begins to creep over the j wall of the craterlet, and has soon advanced a distance of half a mile beyond the wall. Changes of a similar nature occur on the inner and outer slopes of the crater itself, but these extensions of the "snow" are not simultaneous, but occur apparently in an orderly succession—first one region, then an adjacent one, t;iking up the pro ess of expansion, so that the "snow" appears to "shift and revolve w!ti in the crato-," this rhotion being counter-clockwise for the northern hemisphere of the moon.. ■ and clockwise 'for the southern._ 'furiously enough, the "clouds" whidh. are visible within many craters appear to revolve in.the opposite direction- "Why either of these aparent motions should • take place is a mystery to Professor Pickering. > Another difficulty which*, must • be faced is this: Whence comes the snow which creeps over the crater walls P It would appear from his other publication that Professor Pickering believes that in a remote past ice and snow were much more important features of tihe lunar surface than they are now. In dealing with the rxjntral peak of Theophilus, he writes: "Three peaks seem to be but tho vestiges of a once great central cone, whose sides were grooved' by glacial action, whidh scored them with a series of U.-shaped valleys into their present deeply-eroded, form. . . . The glaciers wliioh "onoe filled these vallevs have long since disappeared, but tho . nev6, or upper snowy area, whence they . sprang is still readily visible with even , a small telescope." We can imagine water vanishing from the surface of the moon either by escape in the form of vapour from the lunar atmosphere (when it would be entirely lost to the moon) or by sinking into the soil and interstices of the rocks forming the lunar crust, and it would appear that Pickering considers that in this sub-lunar water is a source from which can be drawn the aqueous vapour which becomes visible as the lunation pioceeds in the forms of cloud, steaiii, and snow, for when dealing with a small circular area distinctly brighter I>an the surrounding region he writes: i "This is thought to be freshly fa'ien stiow, precipitated from moisture i,iven out by a small invisible crater,'' and again of another bright area we read: "In order to withstand, the evaporation due to a tropical sun, unprotected by any appreciable atmosphere, this , snow must be, rather deep, even If it is being constantly renewed from the interior." There is small reason to Icheve that--the moon at any stage of its existence possessed a considerable supply of water; there itr little or no evidence that aqueous denudation lias played any part in-producing the pre.sfrfit surface features of the moon; there is great difficulty in- conceiving the physical, processes by which a 8 jlv crustal water-supply could produce The required amount. of moisture—Professor Pickering's observations appeir to him to be conclusive as to'the presence of snow oh the surface of the moon, i bus his treatment of the problem he i rain s hardly, seems adequate. We must now turn. our attention to i the vegetation whioh' Professo; Pirker- . ing tells us is observable on the surface of the moon; he discusses the crater of l Eratosthenes with a wealth of detail ' vhjth renders it no easy matter to get ■ a dear picture of what goes on ? but ap- ■ parently the bright areas which wet ■ the eye undergo changes of hue and I gradually become darker; patches of . deeper colour are connected up with ■ each other by "canals" or "runs" (as • he prefers to call them), and a shifting ■ dark area results whose origin can best i be accounted for by supposing thai some form of vegetation has spread . over the surface. Tn one caso we nru ) tojd that an area was first seen bright, - then it darkened, then became bright, , and then dark for a second time before f it disappeared in the evening shadows. On the strength of thiß we read: "We may say that the vegetation requires two and a half davs to appear,- it lasts for only two days, the land rests for these days, and then produces a second crop lasting for four days more. We i thus have evidence of a variety in the -lunar vegetation. . .- In quickness of development some of it even vies' with oui mushrooms and toadstools; the crop has to be brief in order to ripen at all." As illustrating the connexion between runs and Vegetation, the following extract must suffice:— "There is no more artificial looking formation apparent on Mors than the close double canal (on the moon) associated with the crater Aristillus. The two components of this canal stretch out straight, slighly divergent, and perfectly uniform, side by side, for mile after mile.. They originate on a small dark Area on the inner wall and, after ascending a slope over an elevation of 4000 fe«*t, descend.- still as two narrow uniform canals, to a large 'field on the outer mare. The darker portion of the field covers about 60 square milds and, as we watch it gradually darkening, it gives us irresistibly the impression of lieing in some way watered by them. How such a thing could be, however, it is impossible for us to conceive since we believe that no water can exist on the moon, except such as is retained in its soil by capillary attraction." '• ~ _ Here we must leave the matter. Professor Pickering stands in the front rank of American astronomers; his observations, carefullv made under conditions very favourable, to accurac have yielded him results denied to preceding selenographers, and even if we' are unable to accept the interpretation he puts on them, we cannot but be grateful to him for exciting our interest in a field of work which needs but a small telescope for its investigation.