December, 1902.] 



KNOWLEDGE. 



275 



THE MOON'S SOUTHERN HORN. 



By E. Walter Maunder, f.r.a.s. 



The region of the moon shown iuthe present jilate is taken 

 from the fourth number of the "Atlas Photoj;raphique 

 lie la Liiue," pulilished by the Paris Observatory, antl 

 forms a continuation of the eiiuatorial region of the moon 

 described iu Knowledge for March, 1901, under the title 

 " Sunrise on the Sea of Plenty." 



Under the grazing illumination of the sun's rays, it is 

 seen that the moon's southern horn, like the major part of 

 the moon's southern hemisphere, is riddled with craters. 

 Here are a multitude of ringed and walled plains, craters 

 and craterlets, clefts and ridges, lying, some in disorder, 

 and some in an order so pronounced that it can scarcely be 

 accidental. As it was jiointed out in the earlier article, 

 the meridian of GO'-' has u|ion it a series of great walled 

 plains, Boussiugault f, Pontccoulant G, Fraunhofer, 

 Furnerius, Petavius, and beyond the southern horn the 

 sequence is carried on by Veudelinus, Langrenus, Webb, 

 Apollouius, Messala, and Schumacher, not to speak of 

 tlie Mare Crisium, which the meridian bisects. This 

 longitudinal arrangement of the great plains, taken iu 

 connection with the meridional ridges on the Mare Crisium, 

 seems to indicate that the cause is to be found in the 

 tidal attractiou of the earth on the once plastic erust of 

 the moon, which was then perhaps also exhibiting great 

 volcanic activity. 



Lying obliquely across the same meridian is another 

 series of formations, scarcely less remarkable. These are 

 four annuli composed of ring-plains and craters grouped 

 together like beads on a necklace. The diameters of the 

 annuli diminish in size, and their components also decrease 

 as they approach the south pole. The largest and most 

 northern imnulus lies immediately to the south of Furnerius, 

 and has for its southern edge the great Valley of Rheita 

 and the ring-plains which it seems to have broken down 

 iu crossing. On its western border lie Fraunhofer and 

 Vega and several other ring formations almost as large 

 and important as these. The south edge is closed by 

 several of the formations in the Vega region, but the 

 northern edge lies 0])en, save for the small crater Rheita B, 

 between which and Fraunhofer lies a ridge of high ground 

 which crosses the floor of the anuulus, bcmUug to the 

 west and traversing Vega. The southern edge of this great 

 aunulus breaks into the nortliern edge of the ne.\t, whose 

 southern apex is Hanno A, and, as a rule, the component 

 ring-plains of the second anuulus are much smaller than 

 those of the first. The two more southerly annuli arc 

 formed of smaller craters still, and their areas are much 

 more circumscrilx-d. Pontccoulant joins the southern 

 border of one to the northern border of the other, and 

 obliterates them in these regions. 



Three great valleys are seen stretching from the termi- 

 nator across the illuminated horn, half way to thii iU(jon's 

 limb, looking in the picture as if a hen had drawn her 

 triple claw across the rugged surface. The two narrower 

 shallower ones apjjear to spring from Fabricius, and skirt, 

 one to the west of twin plains Steinheil A and B, and the 

 other, farther to the west, to the borders of the second 

 anuulus. Tlie third and most northerly, is the colossal 

 Valley of Klieita, which, dwarfed by perspective, is yet 

 nearly two hundred miles in length. It is less regular 

 than the valley of the Alps, but it surpasses it visually in 

 importance. In April, 1901, when discussing the ridges 

 that cross the floors of the Maria Serenitatis and Imbrium, 

 we pointed out that where these ridges left the sea bed 

 and tr.J.versed the mountain ranges, there they seemed to 

 be transformed into valleys ; a notable instance being the 



prolongation of the great valley of the Alps into the Mare 

 Frigoris. as a ridge to Egede A. In the case of the Rheita 

 Valley and its companions, the only sea bed which is 

 visible on this plate, the Mare Australe, is too narrow and 

 foreshortened for it to be ascertained that the same coin- 

 cidence of ridge and valley takes place hero, but there are 

 faint indications which seem to render it probable, and 

 which might become more evident under a slightly different 

 angle of illumination. 



But as the sun rises higher on the moon's western limb, 

 all this complexity of detail seems to be blotted out as by 



Key Map of the Plate. 



a wet spomre, and the region has become blank, or presents 

 features that are unrecognisable when the sun's light 

 slants on it as on March 7tli, 1897. When the moon 

 appears to us about half full, then Boussiugault, Pontc- 

 coulaut, Steinheil, and all the lesser ring-plains disappear ; 

 the Mare Australe shows up more plainly as a thin streak ; 

 the riug of Petavius is gone, and only the centre crater is 

 to be made out ; and the two secondary plains, Petavius B 

 and Furn(;rius A, become the centres of brilliant 

 " aureoles " and the focus of bright rays, such as those 

 which emanate from Tycho. 



But there is one feature which is no less evident when 

 the moon is at the full than when it shows but a narrow 

 horn of light; this is the es]>ecial brightness of the 

 illuminated limb of the moon. On our plate it is visible 

 as a narrow, almost even cornice of light all along the 

 rounded edge. It is practically indeiwndent of the latitude 

 on the moon, or of its age. 



What is the cause of this ? There is, of course, on the 

 moon no atmosphere or dust-veil like tliat which renders 

 the limb of the sun so notably less brilliant than the centre 

 of the disk. But since the amount of light reflected by 

 the moon in any given direction should decrease with 

 increase of the angle made with the normal, we should 

 expect that — as we see to be the case with Jupiter and 

 Saturn — the limb should be inferior to the centre in 

 brightness. There seems but one explanation of the 



