FIGURE 4. The crater Copernicus and its surroundings in afternoon Illumination. Examples of almost every type of lunar surface feature arc shown on this plato taken with the 36-Inch Refracting Telescope. On November 19, 1969, Charles Conrad and Alan Bean touched down the Apollo 12 lunar module 3 crater diameters southeast of Lansberg within 65 yards of Surveyor 3 which had been soft-landed on April 17, 1967.
FIGURE 5. Three great craters at sunset—young Theophilus, old Cyrillus, and ancient Catharina. From » Plate taken with the 120-inch Reflecting Telescope.
FIGURE 6. The largo young crater Tycho and the h"Re old crater Maginus under afternoon sun. (120-Inch Reflector.)
In general, craters are best seen when near the terminator—the sunrise or sunset line that separates the bright and dark hemispheres of the moon. When the terminator lies within 200 miles of a crater, the sun as viewed at the crater is no more than 10 degrees above the horizon. Under such conditions even slight elevations cast shadows which are intensely black owing to the absence of a light scattering atmosphere. There is strong contrast between the black shadows and the bright areas in direct sunlight. Craters thus stand out sharply and give the impression of great height and steepness of wall and central mountain. That is an illusion which was not appreciated fully by all the stenographers of the past. Generally speaking, the craters are rather flat, with gentle to moderate slopes. For example, the prominent crater Theophilus was measured with high precision at the McMath-IIulbert Observatory some years ago, and a cross-sectional diagram was prepared from the measurements. It shows that if we let a dime represent the outline of the crater, the coin is about y3 too thick to show proportionately the elevation difference between the wall crest and the crater floor. Perhaps even more striking is the fact that if an observer should stand on the floor at the center of the great crater Clavius he would not be able to see any portion of the walls which rise 16,100 feet above that floor. All the walls would lie beyond and below the horizon owing to the curvature of the moon's surface, and the observer would have the impression of standing on a limitless, flat plain.
Quite a few of the more prominent craters and some of the small ones are known as ray craters because of their curious appendages. Rays are bright streaks that emanate more or less radially from certain craters. They show to the greatest advantage when the sun's altitude above the horizon is high at the crater, and they usually disappear entirely when the crater is near the terminator. Most of them extend from about 10 to 200 miles outward, and their widths range up to around 10 miles. They show best on the maria where their white material contrasts sharply with the dark background. They run across old craters and sometimes even up and down mountains with no apparent change in brightness or width. They must be very thin because none has ever been observed to cast any shadow whatsoever. By far the most outstanding ray system is associated with the crater Tvcho (Fig. 6). Tycho. easily seen in detail with binoculars, is located 200 miles west of a point about % the way from the south limb to the center of the disk. Although it is 56 miles in diameter, it might be overlooked easily when viewed near the terminator because of the abundance of craters in the region, some of which are considerably larger. Within a few days of full moon, however, no one could miss Tycho, which then lives up to Thomas Webb's appellation "the metropolitan crater of the moon" (Celestial Objects for Common Telescopes). Its broad, brilliant rays extend outward in all directions for many hundreds of miles, one ap. parently crossing the faraway Mare Serenitatis and running a total distance of 1500 miles. In fact, Tycho and its amazing rays give the full moon the general appearance of a peeled orange, the crater marking the point where the sections meet. The rays are important clues to the process of crater formation.
In addition to the craters, sometimes called "ring mountains," the moon has a considerable number of the more conventional mountains which resemble those we know here on earth. Most of them are found in ranges named after those of Europe and Asia Minor which, evidently, were the only ones known to the early lunar feature namers. The outstanding range is the Apennines (Fig. 7), which begins at a point midway between the center of the disk and the northern limb and stretches some 450 miles in a shallow arc southwestward toward Copernicus. These mountains, some of which have been measured to heights of nearly 20,000 feet, present a magnificent sight when near the terminator (moon's age about 7 or 21 days). Stripped of its contrasting shadows when the moon is near full, the range still is seen easily as the peaks of its crest shine out over the bright background like a string of sparkling jewels. The Apennines rise gradually through a series of foothills from the dark plain of Mare Vaporum on the southeast. The ascent continues northwesterly for 100 to 150 miles until the crest is reached near the boundary of Mare Imbrium. There the peaks appear to rise almost abruptly 2V2 to 3% miles above the plain, the straight edge of their base broken only near the center of the range by what looks like an enormous rectangular rubble pile 100 miles long, 30 miles wide, and about one mile high.
Two other fine ranges are associated with the Apennines in delineating Mare Imbrium. If the arc of the crest is extended northeastward across the 25-mile "inlet" that joins Mare Imbrium and Mare Serenitatis, the Caucasus Mountains are reached. They run northward 200 miles toward Aristotle (Fig. 8). Their foothills mingle with those of the
Alos which continue the elliptical curve 200 miles northwestward toward Plato (Fig. 9). West of Plato the rough boundary is characterized by two 150-mile scallops, the second and more prominent of which is formed by the Jura Mountains marking the periphery of Sinus Iridum. Most of the other ranges are less conspicuous, some consisting of only a few hills. Individual mountains that do not appear to be part of a range are rare. The best situated examples are found near the northeast edge of Mare Imbrium, south of Plato and south of the Alps. They are Pico and Piton, respectively, both of which are bright objects easily seen through binoculars (Fig. 9).
Where mountains are found valleys naturally are expected, and they occur in abundance. However, onlv those that are prominent because of unusual extent attract our attention as we view our satellite from a distance of nearly % of a million miles. The most striking is the one which cuts centrally through the Alps perpendicular to the line of the crest. The alpink valley (Fig. 9) is 1 to 13 miles wide and 110 miles long. It runs the entire width of the range, and it is perfectly straight with a smooth floor except where it cuts through the highest mountains and where landslides may have provided the rocky debris seen there. It may be observed through binoculars under good conditions when the moon is about seven to nine days old. The observer is strongly impressed by the artificial appearance of this feature. It is inconceivable that the Alps could have been formed in a way that would leave such a broad, straight highway through their center. One is tempted to speculate that the valley was carved out in a later epoch bv an enormous projectile that plowed through the range, its impact energy grinding to dust and heating to vapor the mountain masses before it and scattering the pulverized remains as an undetected blanket over a vast area. Several eminent authorities tell us that the Alpine Valley probably was so formed. Others maintain that such an explanation is improbable and that this and other valleys are but cracks in the lunar surface.
The rheita valley, near the southeast limb and about 150 miles southwest of the bright ray crater Stevinus, is a more readily noted example. A conspicuous feature of the three- to four-day moon, it appears as a black streak running south from the terminator almost to the limb, an apparent distance of 180 miles which stretches into an actual 300 miles when the effect of foreshortening is taken mto account. The valley proper is straight, 230
miles long, about 15 miles wide, and some 2000 feet deep. From its south end another rectilinear groove 100 miles long and 9 miles wide continues in a more southerly direction. Before first quarter it is almost completely washed out because of lack of shadow on its shallow floor. Two weeks later, when the sunset terminator falls across it, the valley again is seen easily through binoculars. It is usually black, but when the sun's rays strike it at the proper angle it appears definitely brighter than its surroundings. While the Alpine Valley is evidently a relatively young formation, the Rheita Valley must be very old since several craters cover it and the walls of several more encroach upon it.
Somewhat akin to valleys but generally much less conspicuous are the gaping cracks, known as clefts or rills, that split the lunar surface in many places. Near the center of the disk between Mare Tranquillitatis and Mare Vaporum run the aria-daeus rill (Fig. 10) and the hyclvus fill resembling parts of a great canal dug to conncct the two "seas." The former stretches 150 miles in a straight line composed of short linear segments hav ing only slight differences of heading and an occasional offset. It is uniformly three miles wide but of unknown depth throughout most of its course, which takes it through several hills. The latter, somewhat shorter and narrower, undergoes a 30-degree change in heading as it passes through the small crater Hyginus near the midpoint. It thus consists of two approximately linear segments. Rills show best when the terminator is near one end.
What might be termed the complement of a rill is a ridge or wrinkle. They occur in abundance around the edges of or across the maria. Although they attain widths of several miles, they have very little central height and usually go unnoticed on the plains. However, when near the terminator and approximately parallel to it they reveal themselves as one slope appears light and the other black. On such occasions they may be seen through binoculars meandering along the "twilight" zone of poor earthward reflectivity. Most of the ridges are unnamed, a prominent exception being the Serpentine Ridge which crosses eastern Mare Se-renitatis (Fig. 11).
Promontories and capes constitute another category of named lunar formations. As might be expected, they are the tips of the bright continental areas that jut out onto the dark maria. Easy examples are Laplace Promontory and Ileraclides Promontory that separate Sinus Iridum from Mare Imbrium on the east and southwest, respectively.
pIGUre 7. The Apennines, the moon's finest moun-tain range, In early evening. The Apollo 15 astronauts landed on Palus Putredlnls in the valley to the left of the h in "Hadley." (36-lnch Refractor.)
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