IN STARRY SKIES

Evening Post, Volume CXVII, Issue 71, 24 March 1934, Page 7

IN STARRY SKIES

THE "CRATERS" ON THE MOON

(By "Omega Centauri.")

The dark areas cm the moon's surface are characterised by immense' circular regions, more or less overlapping one another, with their margins broken in part by other forms. The roundness of these'regions is surprising, but its effect, sinks into insignificance before that of the spectacle revealed when tho moon is examined with a large telescope. The view is startling in its uiifamiliarity and in it's absolute difference from anything known on earth. The light areas, and to a much less extent the dark ones, are seen to be covered by tens of thousands of rings ranging in size from 150 miles in diameter to the smallest forms that caii be detected with high powers. Long ago some thirty thousand of these were counted on the hemisphere facing us, but the census can never be complete as the forms are in all stage 3of dilapidation or preservation. Many are almost completely obliterated and can be detected only during a brief phase of each monthly illumination. These rings are scattered apparently without order, but are particularly abundant in what is called the fourth quadrant, between the south and the west. They are scattered indiscriminately beside or on top of one another. Smaller craters appear anywhere on the-floor or mountain walls of larger ones, and few. of the great rings have rims-that are no', broken by others.1 The lunar scenery is thus completely dominated by these circular forms; Until recently it might have been said1 that; nothing like them can be found ■on earth: This is ho longer true, but the terrestrial forms, to -which -we shall have to refer later, though .large and' r striking, could only be seen with.high powers-if removed to

the moon. The lunar circles are astonishing not only on account of their multitude, but because of their gigantic size, and; though immersed in celestial silence, they may be said metapnorically to cry, .aloud for explanation." But before considering the question of origin it is well to have a clear perception of their most striking, eharae-. teristics. ■ -. -. '. ■'-.'•■.■

When the moon is, full thedark regions force themselves most insistently on our notice, but more careful observation shows that the light regions are not all equally bright. One of special brilliance is; near the North Pole, that is, on the margin below Mare Imbrium and Mare Serenitatis, as we see the moon from the Southern Hemisphere. The other is near the South Pole, which appears at the top of the disc. In the middle, of. the latter region is: a particularly bright spot, from which bril : liant streaks: radiate in most directions nearly half across the disc. At full moon this ring, which has been called "the metropolitan area of the moon,'? on the principle : that ,M all roads lead to Rome,'' looks like ( a brilliant round spot with' a dark\rin'g round-itl The dusky ring is darkest on its inner- edge and shades 'off gradually outwards. From its outer margin the bright rays start under more favourable conditions of illumination,1 that is, when 'it.is near the terminator and the sunligti't"' falls obliquely on it, the true form is revealed. The crater ;*which is called Tycho is about 54 miles in diameter. Its. central' floor is deeply depressed and little broken except by a central mountain, 6000 feet in height, whicK' has a much smaller one close beside it on the west. The mountain ring rises 17,000 ft above the central plain and has an unusually clear-cut and unbroken rim. The descent from . the rim to the central plain is rough and steep. On the outside the . slope, is much more gradual, and all : the immediate surroundings are at a much, higher level than the central , plain. Strange- to : say, when Tycho is near either- terminator, although its true form : is revealed, it'entirely loses its pre-eminence. Instead of standing out as .the most clearly-marked spot, it be-

comes just a single one of a multitude of similar forms. Some of its neighbours are much larger than itself. Within 100 or 150 miles of its margin are Maginus and Longomontanus, each about 100 miles in diameter, and Clavius, nearly 150 miles wide. But one distinction that Tycho has in this region is that it is the only large ring that is not sprinkled over with smaller similar forms. It appears to be the youngest of the group. If we were to classify lunar craters according to size "we should find the number to increase : very considerably with each decrease in size. There would have been no room for so many if all had been as large as Clavius: Each of the giant rings seems to have obliterated more or less completely a host of others. Traces of rings even larger than Clavius can be discerned amongst the ruins. Philip Fauth attempted to classify- the krown craters according to size. He gave a fairly detailed classification, but by grouping his results we may say that 63 craters have diameters exceeding 53 miles, 167 have diameters between 25 and 53 miles, 326 between 12} and 25 miles, and 1600 between three and 12$' miles"-■ If we go below three miles in diameter we find the craters are numbered by tens of thousands.

At the time of full moon several brilliant spots shine out within or on the margins of the dark areas. Three particularly noticeable ones are in Oceanus Procellarum, the Ocean of Storms. These are Copernicus, Kepler, and Aristarchus. These three have systems of bright rays spreading from them. These rays are neither so straight nor so long as those from Tycho, but are perhaps more beautiful, as they are much more numerous ■ and more feathery. Copernicus is a magnificent ring 56 miles in. diameter. Its mountain walls' rise 11,000 ft above, its central plain. Instead of a single central peak it !has a group of several, more than half a dozen of which are prominent atid attain elevations of 2000 or 3000 'feet.' The rim is not clear-cut like that of Tyeho, but has a wavy or

scolloped outline. The inner walls are extremely ; rugged, descending precipitously from the rim,' but rising again arid again into high > ridges and mountain masses for ten of • twelve miles, thus reducing the diameter of the central plain to a little over 30 miles. The region outside the rim appears to have been affected much -more profoundly than that about most other craters. It looks as if pre-existing forms had been wiped-out over an area. 50 to 100 miles •wide all round the rim. In this are great rings almost obliterated, and hundreds of small craters, only two or three miles in diameter; Many of "which are strung out in almost continuous lines. To the east and north of Copernicus are innumerable mountains, known as the Carpathians. The ones north of Copernicus form part of the" great mountain wall of Mare Imbrium. Those to the east appear to be the summits of similar mountains whose •lower spurs have- been overwhelmed by a subsequent catastrophe. Kepler is more like a small edition of Tycho. It is. only 22 miles in diameter, is very deep; and has a clear-cut edge and central peak; It'stands out very clearly, when near, the terminator. Aristarehus, north-east of Copernicus and two-thirds of.the.way towards-the edge of the disc, is in some lightings the most brilliant spot on the surface of the moon: With, its neighbour Heroditus, 24 miles in diameter, it lies on the edge of a rough and rather' . obscure, ■ approximate] v circular,- area, most of which ,is dark incomparison with the rest of Oceanus Proeellarum. It. contains. several almost obliterated .rings and confused mountain masses which look like the remains of mountain walls of some socalled Two of the' strangest rays on; the Iniiar surface reach this highly disturbed area. They appear to be true arcs. of. circles for. hundreds of miles, arid yet their origin' is obscure. The longer one seems to start much further south, perhaps in Cardanus or Krafft, and seems to wander aimlessly until it reaches'the rim of Seleucus, from which it proceeds along a definite arc for some hundreds of miles, almost touch;ing the edge-of the rough area just described. ■ ■*' ■-^ ■'■ •