84 



KNOWLEDGE. 



[April 2, 1894. 



prevent the radiation of heat from the body of the moon. 

 Added to this, the poles of the moon never advance as far 

 into the sunht lunar hemisphere as the poles of the earth 

 advance into the earth's svm-lit hemisphere. 



It will be remembered that the earth's poles enjoy a 

 period of six months of continuous sunshine, during which 

 they advance into the sun-illuminated hemisphere to a 

 distance of more than twenty-three degrees from the zone 

 of sunset tints which continually encircle the earth, 

 dividing the sunlit half from the starHt half.''^ But the 

 moon's poles never advance as much as one and a half 

 degrees into the sun-lit hemisphere, for the lunar equator 

 makes an angle of only 1° 28' 45" with the ecliptic. The 

 north and south poles of the moon are consequently 

 never very far from the terminator or zone of sunrise and 

 simset on the moon. 



Our plate illustrates very well the sort of illumination 

 which objects near to the lunar terminator enjoy. The 

 steep sides of the lunar mountains are brightly lit up, but 

 the valleys and hollows and the lunar plains are left in 

 darkness, or are only faintly illuminated by very slanting 

 rays. A region constantly within a degree and a half of 

 the terminator would therefore receive very little heat, 

 though it would radiate as freely as any other part of the 

 moon, and we should expect to find it relatively colder as 

 compared with the lunar equator than the earth's polar 

 regions are compared with our equatorial regions. 

 Judging by analogy, therefore, if there is any water-vapour 

 in the lunar atmosphere, we should expect to find it con- 

 densed in greater quantities in the lunar polar regions 

 than in regions with a higher average temperature, but 

 there are no distinguishing white caps to mark the place 

 of the lunar poles, and we must conclude, either that the 

 lunar poles are not covered with snow, or that the polar 

 snow-caps extend as far as the lunar equator, and that the 

 only difference between the equatorial and polar regions is 

 that the layer of condensable vapour is thicker at the poles 

 than in the neighbourhood of the lunar equator. 



There is some evidence which tends to show that there 

 is some kind of condensable vapour in the lunar atmosphere, 

 for the mountainous regions and higher parts of the 

 moon's surface are always whiter than the low-lying land. 

 This is well illustrated by the beautiful lunar photograph 

 of the Brothers Henry, reproduced in our plate. The 

 limb or sharp edge of the moon is shown in it (as well f.s 

 in all other good photographs of the lunar crescent) as 

 very white, distinctly whiter than the lunar surface at a 

 little distance from the limb. At the limb we are looking 

 tangentially at the lunar surface, and only the tops of the 

 lunar mountains and high ground would be visible to us. 

 valleys and low-lying land being eclipsed by nearer regions 

 at a higher level. But the smooth limb of the moon is 

 always whiter than other parts of the disc, whether the 

 moon is gibbous or presents a thin crescent form. In the 

 crescent phase shown in our plate we are looking at the 

 shadow side of the mountains on the limb, and not at 

 the slope which lies in the full glare of sunlight. It might 

 be argued that the white band along the limb of the 

 gibbous moon is due to the fact that we are looking at the 

 sun-illuminated sides of the mountains on the limb, and 



* The zone of sunset tints glidei? over the earth always touching 

 two parallels of latitude. At midwinter aud niidsumiuer it lies 

 between two small circles, with a radius of about t\vent\ -four decrees 

 described about the northern and soutlieru pole. It is never a " great 

 circle," but always lies a little within the dark hemisphere, for the sun 

 in setting is raised by the refraction of the earth's atmosphere more 

 than half a degree. A little before the autumn equinox and a little 

 after the spring eqidnox it passes through the poles, and then as seen 

 from the outside would appear to spin round the parts of the zone 

 where tlie tints of inorning and evening meet. 



that these appear relatively bright as compared with less 

 vertically illuminated surfaces. The relative brightness 

 of objects near the terminator (or region of lunar sunrise) 

 evidently very materially depends on the slope which they 

 present to the slanting rays of the sim ; but as the sun rises 

 and the shadows grow shorter, the slope of the surface 

 makes less and less difference, until at the full moon, when 

 no hmar shadows can be seen from the earth, the differ- 

 ences of brightness of difl'erent parts of the lunar surface 

 must be due to actual differences in the albedo or light- 

 reflecting power of the lunar surface. In the case of the 

 mountains and cliffs seen along the limb of the crescent 

 moon, the slope of the surfaces seen from the earth is away 

 from the direction in which the sun's rays are falling, and 

 any appearance of greater whiteness must be entirely due 

 to the actual whiteness or light-reflecting power of the 

 mountains. 



We consequently have in this band of whiteness along the 

 lunar limb very forcible evidence that the lunar mountain 

 tops are white as compared with the average whiteness of 

 the lunar surface. The whiteness, and consequent bright- 

 ness, of this band along the limb is so considerable that 

 in lunar photographs it is the first part to become over- 

 exposed, and it then hides the irregularities of the limb, 

 which are well shown in the comparatively under-exposed 

 photograph of the Brothers Henry reproduced in our 

 plate. 



The white material which condenses on the lunar 

 mountain tops need not necessarily be snow. Any vapour 

 which condenses in small transparent crystals would look 

 snow-like and white, on account of the way in which such 

 crystals break up the light. Thus, the feathery crystals 

 formed from carbonic acid vapour, in the well-known 

 lecture experiment, look snow-like and white, and probably 

 crystals of frozen air would have a similarly snow-like 

 appearance. 



But if we accept the concurrent testimony of Profs. 

 Langley and Very, the Earl of Rosse, and Prof. Boys, 

 the temperature of the surface of the equatorial regions of 

 the moon during sunshine is not very far from 0° Centi- 

 grade, or the freezing point of water. At such a 

 temperatin-e, crystals of carbonic acid or of atmospheric 

 air would evaporate with extreme rapidity ; and it is not 

 conceivable that such rapid evaporation could go on during 

 the fortnight of the lunar day without giving rise to a 

 gaseous envelope about the moon, which would be easily 

 detectable by the distortion of the sun's image during a 

 solar eclipse, and by the phenomena observable during 

 the occupation of stars at the moon's limb. For there is 

 no recognizable change of whiteness from sunrise to sunset 

 at the lunar equator — that is, the white covering is not 

 entirely evaporated away, and the continuity of colouring 

 from the moon's poles to its equatorial regions shows us 

 that if there is any polar capping of condensable vapour, 

 the polar white caps extend from the pole to the equator, and 

 that they are not even materially diminished in extent in the 

 lunar equatorial regions during the fortnight of unclouded 

 sunshine passed through in every lunation. 



We have therefore to choose between two alternatives : 

 either the general whiteness of the moon is caused by an 

 extensive snow-like covering, which is not evaporated away 

 in the lunar equatorial regions during a fortnight of 

 temperature not far removed from the freezing point of 

 water, or the moon's whiteness is due to the natural colour 

 of its surface rocks, and we are then forced to assume that 

 all the lunar mountain tops are formed of white rocks, 

 while the plains and low-lying areas are formed of a darker 

 material. The latter assumption will not be seriously 

 maintained by anyone who considers the great disturbances 



