down the middle by the terminator. It is a large semiellipse just north of the center of the crescent, and its length is about equal to the width of the crescent there. Close examination shows that it is not quite so bright as the rest of the crescent, which at first appears to be uniformly brilliant. As the moon grows older the mare will darken, and the rough territory which borders it on both north and south will brighten, presenting a strong contrast in both shading and texture, but this evening we might miss it entirely if we did not know where to look.
Mare Crisium is unique in its lack of connection with the other maria. We view it far around the curved surface of the moon where circles are foreshortened into ellipses. Consequently, when it is revealed fully tomorrow evening we shall not be surprised at its oval outline. Yet if we compare it in shape with some of the craters located about the same distance from the limb, we shall see that it is not flattened as much as they are. From such an observation we infer that Mare Crisium is not circular but truly elliptical with its long axis running east-west rather than north-south as it appears to us. Its actual dimensions are 270 miles (north-south) by 350 miles (east-west), giving it an area equal to that of the state of Washington. Its apparent area is only half as great or equal to that of Maine. Since its isolation and low reflectivity make it an easy landmark any time between the lunar ages of three and 17 days I shall use its apparent dimensions as convenient yardsticks for indicating distances between features. Thus "one Crisium length" will represent 270 miles, and "one Crisium width" will denote about 180 miles.
A conspicuous crater this evening is langbenus, which is 85 miles in diameter (Fig. 47). It also is bisected by the terminator, and it is located south of the Mare Crisium south shore a distance a little greater than one Crisium length. The walls, which rise 6500 feet above the surrounding plain and drop 16,200 feet to the black floor, appear exceptionally steep and rugged in this light, and their outline is completed 011 the west by a bright crest extending out over the dark side. Perhaps later you may see the sun rise on the central mountain which is small for such a large crater and a difficult object through binoculars at this phase. Continuing south along the terminator we come to almost equally prominent, larger petavius with dimensions of 99 by 110 miles. Its distance from the south border of Mare Crisium is double that of Langre-
nus, and it also is cut in two by the terminator thj, evening. At its center Petavius has a whole erom>' of mountains which easily are seen rising froiJ the black floor some 8200 feet. Here we view on« of the older craters but one which nevertheless ' 13,800 feet deep. Its bright northwest crest et tends slightly beyond the terminator, but the rest of (he west wall is dark.
Between Langrenus and Petavius is the Ianjg old, lava-flooded crater vf.xdelinus, 92 by 100 miles across and 14,700 feet deep (Fig. 47). Its floor also is in black shadow at the terminator, but it ¡j completely bounded 011 the west by the bright wall crest upon which the sun already has risen. We note that the northeast wall has been moved a dozen miles inward by the younger, well-outlined crater lame, 55 miles in diameter and 11,800 feet deep (Fig. 47). Farther south, about % the Lan-grenus-Petavius distance beyond the latter, the terminator cuts through furnerius, another major crater 81 miles in diameter and 11.000 feet deep. Like Vendelinus, it lias no central mountain or peak, but both have many intruding craters 00 floor and walls, some of which may be seen through binoculars with favorable lighting. In 1 few nights it may be noted that a spot just northwest of Furnerius is the center of a bright, broad ray system which extends outward more than 150 miles in many directions and bccomes so bright that crater outlines are obliterated in its glare.
Furnerius belongs to a subgroup of large craters known as mountain-walled plains. They are rather shallow for their size, and usually they have smooth floors, no central mountain, and low or no outside walls. They are often polygonal in outline rather than circular or elliptical, and they appear to have been formed simply by dropping their floors rather than having thrown up their walls by explosive action of some sort. Some authorities believe they originated in that manner.
If now we go north from Mare Crisium a distance a little less than that of I.angrenus to the south wc come to another large crater nearly bisected by the terminator. That is messala, an old mountain-walled plain the walls of which are deformed by smaller craters in several places. It 15 * weak feature compared to those we have just oo-served, but it is well outlined by a narrow |>l»<* cast wall and a bright west wall. The south h**j of the floor is still in darkness, but the north sectW is illuminated dimly. It is 72 miles across, and if north wall, which rises 8900 feet above the linear section 30 miles long. The curvature ¡/the south wall is reduced, and the crater has the apparent outline of an ellipse with the ends cut off-
It is a curious coincidence that all but one of the features we have observed this evening are aDDroximately bisected by the terminator and that ^ five major craters appear as scallops or black "bites" out of the thin crescent. Those black spots not typical lunar views. We are going to see a great deal more and see it much better as the aescent grows. However, since craters near the terminator are usually the easiest to locate, these black half views actually may be welcome as we orient ourselves on our first regular observing night. If the new moon of the current lunation had been "bom" in the early hours of the morning two clays ago, the terminator now might coincide with the west walls of the five craters, and they would show much lwtter. In any case, they will be lighted fully by the sun tomorrow evening.
Were the moon so turned that we could see these large craters, particularly Langrenus and Petavius, near the center of the disk they would be most impressive sights. Even from their outfield positions they show well. If you look closely at the crescent you are likely to see quite a few craters in addition to those I have pointed out, but most of them are smaller and likewise black. Should you be eager to press on you can identify most of them by reference to Chart I, but if you have located Mare Crisium and the six craters listed you have done well. Later, as you become accustomcd to binocular observation of the moon, there will be plenty of objects to find.
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