ASTRONOMICAL NOTES.

Press, Volume LVIII, Issue 17625, 30 November 1922, Page 5

ASTRONOMICAL NOTES.

NOTES FOR DECEMBER. (SPECIALLY WEITTEN TOB "THE PREr?.") E. G. Hogg, M.A., F.R.A.S.) j The sun enters the zodiacal sign on December 23rd, at 2.27 m 'l . Wh . en tlle solstice occurs: the sun • ,, 1S rea clies its highest altitude PhriTif I." 1 " 0 ' 11 for the latitude of Chnstchurch is 6 9deg 56m.n. The earths distance f rom the sun on this day is about 91,330,000 miles. j Mercury will be in con-' Ji the SUII on Member 7th, bodv WiU be to ° Close t0 that mouth ° | C S6en Uatll tiie entl of tlie month, when it may be sighted sethour *l 6 m . the soudMvei t; about an on n atei i t ' le SUll ' Vellus rises Oil December Ist at 3.38 a.m., and on ! Sh l ? th at ba-JSttton ww niuui briir'ir. . — LJiI S Ilia.Mlast December Loth ft if 9" & nU > a , lld on nses on th, « ,i ° p " m - am J? d , aUis al a -°° a -'»- and a.m' and i C if- V^ iy ' and Satui ' u at An Zl I, - 10 respectively, the i)' iMpt | Ut opportunity for seeing ti'e iltmtr Tn* U 'V J oecur evening 0 f December 20 th a*' it 7min P 'of " a " may , be tound about „f ro—-or less tliau one-quarter »ft i nonh and Mar* f f< of Iran us wiil be about m& nni 011 tllat day 136»illion by the nioiifvn-H ?. f i. the , planet Saturn ber 14th tlip n take place on Decerning in a ~ |-' Planet plienomenon will 4 out Aew Zealand, and life f®. V U '? Ughappearance and, later on f a al dls " ence of the planet and ' M cmer S" should be vpi-v ring-system, ers. y Sweating to observthat the by many of Mars wo"dd iea TtT a ° PP ,° siti , (J , n Sce tion r°- our kn^vledgeTthe el 'sS ° ™porr»ilo7h°,ve tl f **"'*■ to the present Published up not been seen before J lttle fiat has usual conditions appear tol'f Ul f" of thp „|, ln „ t > Lno,v n features gives a detailed Sunt of^one^lal-ge n ¥ our. Theie was no trace of it on July Bth it was very brilliant on July on July loth it was larger but fainter' on July SS ?ltli t U u°a fc ' r< T ylsh streaks': on July nth it had split into three separate parts, of which only one re°n t^ 16 day. On July ipSi h . e „S E,< "' la,i '™ d «* »"»»i

It is a common experience for Martian observers to see white patches near the planet s limb, but these generally £PP ear + , ne t ar + , the central meridian, indicating that they are morning or even- \ n £ or hoar frost. The large spot described by Shpher persisted, however, in lull strength during the entire Martian day, and it is probable that it was m great measure due to an accumulation , of dust, in the higher parts of the atmosphere; if this be the case it Would show that winds of high velocity occasionally pressing on the planet s surface, and these when they blow over the' "desert" regions, produce severe dust storms, and" thus transfer large amounts of loose material from one district to another. There is but little trace of water in the equatorial regions of the planet, and henoe any permanent changes of aspect which are taking place there must be due for the most part to the action of wind-borne matter which by accumulating in existing hollows tends to bring about the general uniformity to the surface level, which is so marked a feature of the planet 1 s topography. Our acquaintance with the Martian atmosphere is very slight; it seems to be fairly well established that water vapour exists in it, and hence we may contend that oxygen and hydrogen enter into its composition, but there our knowledge ends. The idea that the polar caps are condensed carbon-dioxide and that -this gas therefore is an important constituent of the atmosphere reets on a very insecure basis, while the belief that the gases—nitrogen and carbondioxide—are present depends on hypothesis that certain seasonal changes which are observed on the planet's surface are due to the growth and decay of some form of vegetation. When the planet is observed in a telescope the prevailing tint seen is red •and henoe it is inferred that "desert" conditions exist over vast regions We learn from a brief notice in "Nature"—September 30th—that Dr. Fountain has suggested in a recent issue of the B.A.A. Journal that the prevailing colour of these parts is due to the existence there of accumulations of red ferric oxide or, more popularly speaking, rusted iron. During l<Jng periods of time in the past the lighter gases haive, he thinks, been escaping from the planet's atmosphere, so that it is now relatively rich in oxygen. He quite reasonably assumes that Mars, like the earth, suffers from an incessant bombardmentof meteoric matter, but while our atmosphere forms a very effective protecting mantle, so that the major part of the matter entering it from outside is consumed there, that of Mars is so attenuated that considerable quantities of meteoric matter are able to penetrate it and reach the planet's surface, where the iron constituents are speedily oxidised —and thus impart to tlie landscape the observed hue. It is easy to underestimate the protection given to Mars by its atmosphere, tenuous as it is generally held to be. When discussing its propertied it, is usually assumed that the atmospheric pressure at the surface of the planet is about equal to that which .vouil be found m our own atmosphoie at a height of nine miles above sea-level. Now it has been pointed out previously in these notes that as a general rule shooting stars become visible at an average height of about 80 miles from the earth's surface and are finally extinguished at a height of about 50 miles. Prior to becoming visible the shooting-star has traversed a long path in the outer parts of our atmosphere, where in all probability hydrogen gas is the main constituent, and during this stage of its flight it has bad its temperature raised with little or no destruction of its material taking rtlace: when at length it reaches the parts where oxygen begins to form a considerable percentage of the atmosphere chemical combinations occur which lead to its rapid dissipation. It should be added that there s good •eason to believe that the larger bodies, which reach the earth as meteorites, although they are moving initially at the rate of 30 or 40 miles a second, lose all this velocity through the retarding friction of the atmosphere and finally reach the earth's surface with a comparatively small speed acquired under the 1 gravitating influence of the earth. When we try to apply these considerations to Mara we see that any body falling towards it will be moving for long distances thro.i<rh an atmosphere where the pressure and density

are far in excess of what obtains in our own atmosphere at the height of ou miles where shooting-stars are volatilised, and therefore we may confidently expect that the Martian atmosphere will destroy a high percentage of tne meteors entrapped in it. We h, aT ® further to bear in mind that as -".ars is at a much greater distance from the sun than the earth it will encounter in all probability far fewer meteors than the earth does and also that the average velocity with which these bodies will be moving will be far less tlian in the case of the earth. On the whole, then, it seems reasonable to conclude that the surface of Mars dees not suffer a more serious bombardment of shoot-ing-stars the earth. V.'hat amount of meteoric matter reaches the earth's surface is largely a matter of co ije tnre. Accord ng to one authority, Prtfcs or Younj, if we accept ono grain as the average weight of a shooting-star, the amount received dai.y would bo only about one ton, after ive have made a reasonable addition for metioritrs. If we multiply this estimate by one hundred—which is certainly liberal—then the annual amount received would reach the quite respectable figure of 36,500 tons, and yet, even at this rate, if we assumed the average meteor to be tiiree times as dense as water, i-fc-wou'd take about 1 ,GOQ.O X).~>jO years to acc;:nui ate a layer one inch thick over the carta's surface! If we apply those assumptions to the case of Mars we find a difficulty in.arriving at tho same conclusion as Dr. Fountain that Mars owes the colour of the large '•desert" areas of its surface to accumulation of oxitiised meteoric matter and prefer tc think that there is some simpler explanation of the phenomenon. to_ the case of the earth and Prolessor loung's computation, we and that he thinks that the meteoric matter so added to the earth's mass would not shorten the year by more than one-thousandth part of a second 111 one million years, nor would it decrease tr.e earth's distance from the sun part of an inch in a year. "We further I team that if we assume that the hun- ! ared tons of matter received daily were ! movill fi. with a speed of 20 miles a i second just before the impact with the eaith took place, the amount of heat , ese.oped per square yard of the earth's j s V r ,'?? e ln a year by the transformation i 0 ~n s euer py would be about as much as ie sun imparts to the same square yard in one-tenth of a second. While iLnfc unwise, to attach much weight to any 0 f Professor loung s calculated results, they are not without fhcir value in giving us some : nriwT 1 li® €xtr emelysmall effects +1 G + C l . le me tcors wliich reach the earth and m reminding us of some leads COnsequences t0 which their fall