Sixteenseventeenday Moon

(Chart IV)

You may wonder, from the title of this chapter, and with justification, how it would be possible to observe the moon at two different ages on the same night. The underlying idea here is similar to tW of a question which my students usually regard « a trick question: "Does the moon rise every day

After we have learned a little lunar theory our impulse is to reply confidently, "Yes, of course." However, if we recall that the moon rises, on the average, some % of an hour later each day, we readily can visualize it rising on, say, Monday night at 11:45 P M- rising again the following night at about 12:30 a.m. Thus it would rise each night as expected, but the second rising would occur on Wednesday morning, and Tuesday would have come and gone without the event having taken place. The 16- and 17-day moons rise several hours before midnight, but if we wish to observe under the best possible conditions, we have learned that we should select a time when our satellite is near the celestial meridian, or at least several hours' journey above the horizon. So it is likely to be around 11 p.m. when we find the 16-day moon well up in the sky, and, if our studies continue for more than one hour, that same moon will be 17 days old before we finish. So we can observe the moon at two different ages during the same night without any hocus-pocus at all, and we are likely to do so at about this phase.

Tonight we see Mare Crisium fully revealed for the last time as the sunset terminator reaches its eastern shore. Its surface is quite dark, and it appears to be bordered by a light ring, some 50 miles wide, of very rugged mountains except on the east where most of the ring is lost in darkness. These are mountains indeed since they rise to heights as great as 13,500 feet above the plain on the south and 14,400 feet on the north. Thus the mare resembles a huge crater when the sun is low, and tonight it presents to us strong evidence of its origin. Stretched out in a line running north from Mare Crisium toward the terminator Cleomedes, Ceminus, and Messala are conspicuous. Their interiors still are bright, but broad shadows have grown along their inner west walls to give them contrast again, and their east crests are brilliant. Between Cleomedes and Ceminus, and closer to 'he former, the smaller burckhardt shows well With bright and dark walls. A Class 1 irregular crater, 27 by 34 miles across and 15,900 feet deep, 'J appears to have been formed on top of two slightly smaller tangent craters. It has removed about half their walls, and it gives the impression ot a face with huge ears when examined under excellent conditions. We have noted many instances of smaller craters in the floors and walls of older ones, but intruders larger than their hosts are rela-e'y rare. Just east of Geminus, the small central mountain of which might be visible tonight, is the smaller Class 1 bernouilli, 31 miles in diameter and 11,500 feet deep. Its floor is far from level, being 1600 feet higher on the south than on the north.

North of Messala, east of weak Atlas, and close to the terminator, we see briefly conspicuous Class 2 mercury, 40 miles in diameter and 9800 feet deep. Its east wall has been damaged severely, but that is a detail a bit beyond our reach. We find only a black interior with a bright east rim. Northwest of Mercury, north of Atlas, and about one width inside the terminator, we could scarcely miss the ever-prominent, dark ellipse of Endymion which tonight has acquired a bright rim. Note the distance from Endymion back to the Geminus-Ber-nouilli pair, and lay it off in the opposite direction from Endymion northwest to the terminator. There you may see a pair of very narrow and rather irregular dark ellipses with bright north walls. The larger one on the west is fetermann, 48 miles in diameter and 12,100 feet deep. The east one is cusanus, 40 miles in diameter and 11,800 feet deep. A line down the backbone of the Caucasus Mountains through Aristotle passes over the pair which is close to the limb even under favorable libration.

The southeast shore line of Mare Crisium is broken by the bright acarum promontory which juts conspicuously northward for 40 miles over the dark plain and continues another 40 miles before disappearing beneath the lava floor. The ridge rises to heights of 12,000 feet above the plain. Wilkins and Moore write: "On several occasions, a mistlike appearance has been witnessed near Agaram, especially when the Mare is bisected by the terminator under sunrise illumination." At the terminator east of the point where the Agarum Promontory meets the normal shore line you may find the bright-rimmed dark ellipse of temporarily prominent Class 5 condorcet, 49 miles in diameter and 10,100 feet deep. Its hidden floor is said to be convex. If the present lunation began early, Condorcet will be lost completely in darkness tonight.

Proceeding southward to the east shore of Mare Fecunditatis, we view for the last time another feature which has been an outstanding landmark every night since it first appeared on the thin crescent of the two-day moon. Langrenus is magnificent with the terminator in the rough highlands just east of it. Its glare has faded, but highlights and shadows have returned to bring out its details and show it at its best. The double central mountain is seen, and some of the structure of the brilliant

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