Twentytwoday Moon

Tonight the moon has almost reached the last quar ter phase, and it is time for the midterm test of the second semester. To pass it, one must get up for the observation period between 3 and 5 a.m. each morning during the next week, weather permitting. Although the moon will rise around midnight at the beginning of the day on which it becomes 22 days old, it cannot be observed to advantage until a good three hours later. In fact, from now on it would be best to set your alarm clock to awaken you early each morning so you can get your period of observation in before daylight. You then will view the moon under the most favorable conditions.

Tonight the conspicuous Apennine Range meets the terminator at a sharp angle and makes a splendid appearance before bowing out for the next two weeks. The Caucasus are in darkness except for the peaks of their highest mountains. Those may be found on the dark side by extending the Apennine arc to the northeast. At the west end of the Apennines, Eratosthenes shows well with just the right amount of highlight and shadow. You may be able to resolve its central mountain. Beyond Eratosthenes, Copernicus still shines prominently, its multiple central peak lost against the bright floor background, but its once-brilliant ray pattern has faded considerably. The splash area around Kepler remains prominent, and the crater itself still is difficult to resolve because of the glare. Aristarchus continues to resemble a dazzling teardrop pendant. Tvcho in the south likewise stands out among its neighbors, but only short remnants of its once-stupendous r»y system can be found. The large maria, with the exception of Mare Nubium and Mare Humorum, have lost much of their contrasting pigmentation, but the dark-floored mountain-walled plains, Grimaldi and Plato, are seen easily. Grimaldi is a very strong feature and will continue so for sev eral nights.

North of the central Apennines, about V& the way across Mare Imbrium to Plato, a group of three conspicuous craters marks out a right triangle. The largest and least prominent of the three is Archimedes on the west. The two near the terminator are Aristillus on the north and Autolvcus on the south. Note that the interiors of the last two arc black except for the bright walls while the floor of Archimedes, only about 90 miles farther from the terminator, is illuminated completely with the exception of a narrow border in the shadow of the west wall-

Since the altitude of the sun at Archimedes is only a degrees higher than its altitude as viewed from Autolycus or Aristillus, we see here an illustration of the extreme shallowness of some of the great ring plains. In the case of Archimedes, the diameter is 40 times the depth and 50 times the height above the surrounding plain. The mountain arms that curve southwest from the crater wall look like a misplaced section of the Apennine foothills. North of the crater is the much smaller group of Spitzbergen Mountains. Southeast of the crater, against the Apennines. lies the small dark plain of Palus Putredi-Bis. West of the mountain arms and north of Eratosthenes, Timocharis shows well as a bright spot.

The individual mountain Piton shines brightly about two Plato lengths north of Aristillus, and its long, black shadow stretches out toward the terminator. Just beyond the end of that shadow, at the terminator, you may be able to resolve the thin elliptical rings that mark the weakly illuminated crests of the double crater Cassini, most of which is in darkness. Between Cassini and Plato curve the Alps, the sunlit faces of their mountains shining conspicuously and the tips of their extreme eastern peaks gleaming like stars out of the blackness of the dark side. Between Plato and Aristillus, and between Plato and Archimedes, look for the faint curved ridges that may indicate the edge of the great Imbrium impact crater. The strongest of the ridges includes the Spitzbergen Mountains, swings some 60 miles southwest of the group, and extends about 100 miles north of it, ending at a bright speck of a mountain. About halfway between that speck and the south wall of Plato is the prominent mountain Pico. Southwest of Plato and northwest of Pico lie the scattered members of the Teneriffe Mountains.

In the highlands north of narrow Mare Frigoris two of the smaller craters may be distinguished from their jumbled surroundings near the termina-tor' At the northwest outer wall of the huge, partly darkened enclosure William Bond stands the Class 5 Wkenes, 35 miles across and 8900 feet deep (Fig. 9). North of Epigenes is the younger, more prominent Anaxagoras, which is about the same size. It rises out of the west wall of the large excavation Coldschmidt, which is black except for a dimly-wnminatcd strip of floor along the southeast wall.

interior of Epigenes is % darkened, and that of Anaxagoras is completely lost in shadow. Their visi-'"ty is owing primarily to their brilliant inner east Wal|s. About two Plato lengths west of Anaxagoras the larger Philolaus stands out, its hilly floor framed between a black west wall and a bright east one. Perhaps a few more craters might be identified in the far north, but most of those in view tonight are extremely old, and their low walls present very little contrast beyond the impression of general roughness.

Near the midpoint of the terminator we find an isolated, square section of upland about one Sinus Iridum length on a side. It is bounded by Sinus Aestuum, Sinus Medii, and Mare Vaporum. Southwest of its center is the capsule-shaped double crater Nlurchison-Pallas outlined in white and black. Can you detect small Bode close to the northwest rim of the enclosure and slightly larger but weaker Ukert about three times as far from the northeast rim? Both show bright-bordered black interiors. On the southeast edge of the section look for slightly larger Triesnecker seen as a small bright ring. A little more than one Murchison-Pallas length south of Triesnecker and right at the terminator is the light-bordered black old bhaeticus, 32 miles long (north-south), 25 miles wide, and 5200 feet deep. That strangely distorted enclosure, shaped like a peach pit, evidently has been flooded with lava since its floor stands 3000 feet above the level of nearby Sinus Medii. As Webb long ago pointed out. it "marks exactly the moon's equator, and is one of the few spots to which the sun and earth may both be vertical." The reason why the earth can appear in the zenith as viewed from Rhaeticus is that the latter is located only five degrees (1% Copernicus lengths) cast of the average center of the lunar disk —within the range of places which the libration in longitude can bring momentarily to the exact center.

South of the Murchison-Pallas enclosure, a distance nearly equal to that between Archimedes and Eratosthenes, the great hexagonal mountain-walled plain Ptolemy shows well, its smooth floor fully illuminated and bounded by six linear wall sections of equal length that are black, gray, or white. Tangent on the north is much younger and smaller Herschel, about % black and V3 brilliant. Tangent to Herschel on the north is ancient, dilapidated, flooded sporf.b, 15 by 17 miles across and 2300 feet deep, the low, broken walls of which can be seen only when the terminator is nearby. We probably would not see it now were it not for a black valley at the foot of its outer east wall and the high, broad wall remnant of a former larger crater about 10 miles west of the west wall. Those markings give the general impression of a crater about 30 miles in

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