ASTRONOMICAL NOTES.

Press, Volume LXVII, Issue 20250, 30 May 1931, Page 6

ASTRONOMICAL NOTES.

FOR JUNE, 1931. (bpeciaw/t warns* *o» rsz pbbss.) [By E. G. Hooa, M.A., F.R.A.S.] The sun will enter the zodiacal sign Cancer on June 22nd, which will be tho shortest day of the year in the Southern Hemisphere; the meridian altitude of the sun on that day' at Christchureh will be only 23 degrees 2 minutes; the distance of tho earth from the sun will bo about 94,600,000 miles. ' The planut Mercury rises on Juno Ist at 5.13 a.m., and on June loth at 6.13 a.m.; it will approach' the sun rapidly after the latter of these dates and will be in superior conjunction with it on June 29th. Venus will rise on June Ist at 5.9 a.m., and on June 15th at 5.40 a.m.; Mars will set on these dates at 10.18 p.m. and 10.2 p.m. respectively. Jupiter sets on June Ist at 7.20 p.m., and on June 15th <at 6.89 p.m.; Saturn rises m these dates at 7.36 p.m. and 6.37 p.m. lespectivoly. Minor Planets 1930. The past year has been one of unprecedented activity among those members of the astronomical world who devote' their time to the detection of these smaller members of the solar family, the number discovered being 162, as against 150 in the previous year and 142 in 1928. Herron Reinmuth and M. Wolf, both of the Konigstuhl Observatory, Hoidelberg, credited with 67 and 37 respectively, were. the most successful observers ijr this field of work. The apparent magnitudes .of the asteroids in last year's list rauigod between 11.8 and 16.5; 1152 of these bodies have now received either names or numbers and are thus definitely recognised as inembere of the solar family; as this figure is only 38 in excoss of that of 1929, it will be seen that in the majority of cases the body caught on the photographic plate was not a new minor planet, but one which had been fount! at an' earlier date.. We may say that of the first 400 of these bodies only four are missnig; in these cases there may have been errors in the tabulation of the observations which caused incorrect Orbits to bo , assigned to the ones lost. It is now 40 yeors since photography was first used in the search for asteroids and we get some idea .of the 'power of this method when we learn that 323 was the number given to the first one discovered by this means. We note that the minor planet 1134 has been named Kepler, in commemoration of the tercentenary of his death which occurred in 1631, and that throe have been named Probitas. Persevere antia and Hilaritas in commemoration of the sterling qualities of their discoverer, •'ho late Dr. Palisa, a veteran worker in this field, who discovered 125 of these bodies, mostly at a time when equipment and technique were far less perfect than they are now. It; is told of Dr. Palisa that, needing hadly some instruments for the Vienna Observatory, of which he was director, N* intimated that he would r l ' right of naming ft minor planet he had just discovered for the sum or £BO j the offer hum? fire'for some time, bul at lnsfc was taken"tin by a.mombor of the Rothschild family and "Bettina was added to the list of asteroids. Eros., The intensive observation of the path among the stars of the, . sdmll asteriod Bros, which has been engaging the telescopes of all the leading. observatories of the world during the past five months, has now come to an end, and we must reconcile, ourselves to a long'period of waiting before W® shall know the main results of all thw work —vis,, the distance of the earth from the sun, and the mass of the moon. We may remark that it was not until ten years after tho previous visit of Eros to the vicinity of the eartn that the final calculations of the astronomers were published.. Of all the planetary bodies revolving about the sun, Eros is the one which makes the nearest approach to tne earth: in January last it was only about 16.200,000 miles from us, and ■showed itself under conditions very favourable for observation. Here then was aflordbd l an opportunity more favourable than had ever occurred before of learning that fundamental length in the solar system—our dwj tance from the sun. During the past two or three years much time has been spent' on« the'preparation Of maps or that part * he , sky tr ® verse 2 .* Eros, when near the garth, and it is disconcerting to know that when the asteriod did come under close scrutiny its actual path differed considerably from the caloulated one, thereby reducingtp somo extent thfi value pf tno maps previously made. . Another curious fact revealed by tile observations is that Eros appeared to be nearly a magnitude fainter than had been anticipated. At? present, we do not know to what causes can bo ascribed either the deviation of Ifiros in ite fcatli Or the diminution of the intensity Of its illumination. As Eros drew nearer to the earth,.its brightness steadily increased, until, in: January last, it attained an apparent magnitude of 7.8; now. as it is leaving our neighbourhood, its light is falling off rapidly, and soon it will not be visible except in large telescopes, but above this general waxing and waning of intensity, - there is a peculiar variation in brightness, which has given rise to . much speculation, and must be recarded at present as an unsolvedi probr Jem. The light of Eros is subject to a regular fluctuation in brillianco, which occupies about 5 hours 16. tikes t during this period there ore two Maxima, and two minima, and .between the brighter* of the two maxima and the fainter of the two minima, there te a difference of 1.1 magnitudes,,i.e., at one- moment during this . periodic rthanee in luminosity Ercxn is nearly ?£? times as bright as at another are the explanations offered of these curious phenomena. undoubtedly rotntcs on its ftxiSr and in allproba&ilitv 5 hours 16 mins. fs the time occupied by the rotation. One theory supposes that certain re- <*■<■>«« of the asteroid's surface ha\e reflecting power than others, .fo that as the sun's light falls, on good or bad reflecting , parts, the intensity increases or falls off. Let us suppow for a moment that the axis i„ which MarS turns lay w the piano m which it revolves about the sun, instead of being (« it actually is) nearIv at right angles to that plane. As snow and ice are good, reflectors, would appear very bright if we weiQ innkinc straight at one of the polar ramons- gradually the planet's rofc»v tSn woull carry this region, out ot field of view, and areas free from snow or ice, and with a lower rfaflectfnn imwer would present themselves JJS liaht would fall off until a minimum was reached. Further riU- ?■'" -would bring the other 'Polar Jo* SS before 5. and twelve hours. .ft# the first maximum we should observe » second, which would, i» its turn, j„ a llv fade away to a second nuniK which would slowly give rise* pother maximum as' the polar region first seen ag*"» came into view. # ln this wav wo may picture how during i the Martinn day the surface of .the nlanet would present two maxima ftnd xl- minima. Pursuing thw idea further wo 569 that at the Martian pole whioh wm enjoying its winter season there would fie a greater accumulation of icft snow than at the other, ana

consequently this polar region would bo much brighter than the other and; we should understand why the two Maxima differ in their brilliance; the difference ((if any) between the two minima we should have to ascribe to local surface conditions—nature of rock exposed, etc. Similarly in the case of Eros as in that supposed of Mars differences of reflecting power of the parts on which the sun's light is incident may _ cause the observed periodic variations in brightness. It may, however, be said that this explanation does not find much favour with those who have studied the phenomenon jn detail. It has been suggested that Eros is double/ that we have two small bodies revolving about each other, and that the variations in brightness ate due to mutual eclipse of one body by- the other. This is a.'possible, though unlikely, solution of the mystery; if any bucli duplicity on the part of Eros occurs, it would almost certainly hard been exposed by the large American telescopes. „ There are again others who would find the clu j to the periodic: variations in brightness in the shape of Eros. It has been suggested by some that Eros has the lorn of a dumb-bell, while others are of opinion that it is a very oblate spheroid with an equatorial diameter 25' to 29 miles long, while the axis about which it rotate.*? has only a length of trom eight to 10 miles. We shall make no attempt to judge between these conflicting theories ns to the cause of tlje regular fluctuations in the light of Eros; the problejn is a difficult one, and it will probably be a long time before any explanation is reached which will commend itself to all who have studied 'it... The Age of the Earth. At tho close of the last oentury current opinion held that the age of the earth was about one hundred million years, and this estimate, based on the amount of salt in the oceans, satisfied'at that time the requirements of the geologists and the biologists. With tho discovery of the radio-aotive elements science became possessed of :i nevv method of attack on the problem; it was found that uranium arid thor* into disintegrate spontaneously at con' strait determinable rates yielding lead as.fheir last product of. decomposition, and from the amount of lead, derived from either of the above-mentioned elements, present in any specimen of rock it is possible ,to calculate tho age of the bed .from which the upecimeri was taken. A committee of "the National Research Council of U.S.A., which has been considering this question during the past four years, has arrived at the conclusion that a .period of at least two -thousand million years has elapsed since the outer rooky surfaco of the earth consolidated. The oldest rock, whose ago -has been determined from its radium-lead content, is a piece of urauito from Sinyaya Pain, Carolia, Russia; it is 1862 million years old, und as it occurs in rocks that were intruded into the surrounding beds, which therefore must be. older, it is inferred that an age of at least two thousand million years must be ascribed Jo the earliest -crustal portions of;the earth. It may be added that. while there are jio'kuown astronomical triothoda by which this estimate of theage of the earth mav be confirmed, the period in questioif is pot inconsistent with astronomical probabilities; it remains to be seen whether this time-scale can, be reconciled with that required by the'geologist formation of the eeVern! stratified deposits now expowd on the surface of the earth, and also whether f th n r with the claims