50 



KNOWLEDGE 



[Makch 1, 1892. 



permanently associated with it. The assumptions made 

 use of by Professor Simon Newcomb are probably far too 

 liberal (see Old xml Xeir Axtrimuwi/, part XII. )• Our own 

 sun and even stars having a velocity of 15 miles a second 

 must probably be assumed not to belong to the Galactic 

 system. It is a curious fact that stars with the solar type 

 of spectrum have on the average large proper motions 

 compared with stars giving the Sirian type of spectrum. 

 and that Sirian stars cluster thickly towards the Milky 

 Way and the great star zone— while Solar stars seem to be 

 much more iiTegularly distributed. It points to the con- 

 clusion that the Galactic system is a cluster of Sirian stars 

 associated with nebula; through which the sun happens at 

 present to be passing.— A. C. Ranyard.] 



THE MOON'S ATMOSPHERE. 



By A. C. Eanyakd. 



IF the Moon had a copious atmosphere at all like our 

 own the parts of the Moon near its limb or outer 

 smooth edge woiild appear reddish and decidedly less 

 bright than the central portions, for the limb of the 

 Mo'on would be seen through a great depth of the 

 lunar atmosphere, and the hght of the limb would be i 

 reduced just as the light of our sun is reduced at rising or 

 setting as compared with the light of the mid-day sun. But 

 the limb of the Moon is not in the least dimmed or hazy. 

 In fact, it is always one of the brightest parts of the Moon's 

 disc, and is very sharply defined, as will be seen by examin- 

 ing the plate pubhshed with this number, which is made ; 

 from one of the beautiful photographs of the Moon taken 

 by the Brothers Henry, and if the Moon's atmosphere were 

 at all like our own we should expect to see a sensible zone 

 of twilight along the Moon's rough edge, but the shadows 

 between the illuminated mountain tops appear on the 

 photographs and to the eye perfectly black. There is no 

 recognizable refraction when the Moon comes between us ; 

 and a more distant object, such as the sun, during an 

 eclipse, when we should expect an atmosphere to cause a | 

 very evident distortion of the sun's limb where the Moon 

 cuts it, and there is no recognizable prolongation of the j 

 cups of the thin solar crescent into a narrow ring of light 

 such as was observable round Venus just before it entered 

 upon the sun's limb. 



But several observers of partial eclipses of the sun have 

 described a band or shade on the sun just outside the 

 Moon's limb. It cannot amount to a very notable 

 darkening of the sun's disc, for all observers do not notice 

 it ; some have looked for it and have not been able to 

 detect anything of the kind. On the other hand, many 

 photographs of the partial phases show a bright band on 

 the sun's surface running along the outside of the dark 

 Moon's disc. This brighter band on partial-phase eclipse 

 photographs was at first thought to be due to an eS'ect of 

 contrast, but its actual existence as a band of denser 

 photographic action on photographs taken during the 

 eclipse of August 7, 1869, in America, was demonstrated 

 by Dr. Edward Curtis, who showed that the photographic 

 action produced by the sun's light was denser close to the 

 Moon's limb than at a little distance from the limb ; so 

 that printed dots placed behind the negatives were hidden 

 by the brighter band adjacent to the Moon's limb, while 

 they could be distinctly seen through the photograph of 

 the sun's disc at a little greater distance from the Moon's 

 limb. 



Photograpliic plates often show differences of brightness 

 of different parts of the sun's disc which are not recog- 

 nizable by the eye ; thus a not too much exposed photo- 



graph of the whole sun's disc generally appears much 

 fainter towards the limb, while the eye hardly notices the 

 greater brightness of the sun's central parts as compared 

 with those near the edge of the disc. A band of incrfiiseil 

 density might, it is suggested, be caused in an over-exposed 

 photograph by a part of the sun's disc being dimmed 

 down^'by a lunar atmosphere, so that the well-known 

 reversing action (which after a certain limit of exposure 

 begins to take efl'ect) goes further on the brighter regions 

 of the sun's disc, away from the lunar limb, than on 

 the region close to the lunar limb. 



The earth's shadow through which the Moon passes 

 during a lunar eclipse is always found to have a slightly 

 greatei- diameter than the geometrical shadow calculated 

 from the known diameters of the sun and earth. The 

 excess of the diameter of the observed shadow region over 

 the calculated diameter of the g6ometrical_ shadow is 

 generally put down at about one-sixtieth, that is, the earth 

 casts a shadow as if its atmosphere to a height of about 

 66 miles above the sea-level acted as an opaque rind or 

 covering through which the sun's light cannot pass 

 without°having its intensity materially diminished. 



The Earlof Kosse and Dr. Boeddicker, in their account 

 of observations of the Moon's radiant heat* observed during 

 a total lunar eclipse, came to a somewhat similar conclusion 

 with regard to the heat-absorbing power of the earth's 

 atmosphere, which they believe is recognizable with their 

 great H-foot telescope and apparatus to a height of not less 

 than 190 miles above the earth's surface. 



The measures of the diameter of the shadow of the 

 earth through which the Moon passes during a lunar eclipse 

 are necessarily very indefinite, but we may probably 

 assume that the Moon has not an atmosphere which 

 corresponds in light-absorbing properties to the earth's 

 atmosphere at a height of 50 miles above the sea- 

 level, that is (according to the law of decrease of 

 density stated in the article on the Earth's Atmosphere in 

 Knowledge for November last), the Moon probably has not 

 an atmosphere which is ^hoi,^ P^i't as dense as oui- 

 own atmosphere at the sea-level. But our atmosphere, li 

 it were transferred to the Moon, would only be about one- 

 sixth as dense at the Moon's surface as it is at the 

 earth's surface, for gravity at the Moon's surface 

 is only about one sixth as intense as terrestrial gravity at 

 the earth's surface, and though the atmosphere would 

 continue to support about 30 inches of mercury at tlie 

 lunar surface, both the mercury and the atmosphere would 

 have their weight reduced to one sixth of their terrestrial 

 weights. Consequently the atmosphere at the lunar 

 surface being compressed by only one sixth of the weight 

 which it is compressed by here would occupy six times 

 the volume it occupies here, and it would be one sixth 

 as dense as the atmosphere at the earth's surface, 

 and neglecting dift'erences of temperature, the whole lunar 

 atmosphere would extend six times as high as the earth s 

 atmosphere-so that if, in pursuance of the reasoning of 

 1 the article m the November number, we assume that the 

 earth's atmosphere does not extend to a height ot 2oO 

 miles above the earth's surface, the lunar atmosphere 

 I would not extend to a height of 1500 miles above the 

 lunar surface. ^^ , , ^ . ,. 



Let us now follow the Moon backwards m time 

 i according to Prof. Geo. Darwin's _ assumption, until it 

 approached Roche's limit, where it would be torn to 

 pieces by the tide produced in it by the earth s attrac- 

 tion and broken up into a rmg, say (assummg the 



* Published in tlio Scientific Transactions of tlie Royal Dublin 

 SocietT, 1891. 



