and the clean-cut ditch walls pointed to the conclusion that the lunar soil has the physical properties of fine-grained, damp beach sand. Inspection of dents produced when the shovel was pressed against the surface answered a question of vital importance. The mare would support the Apollo space craft and its exploring astronauts.
About one Crisium width southwest of Bullialdus, along the shore of Mare Nubium, are two similarly sized craters connected by a ridge and foreshortened to resemble a pair of bright-rimmed dark eyeglasses. They are the Class 5 craters mercator, on the southeast, and campanus on the northwest. Their respective diameters are 29 and 31 miles, and their depths are 4300 and 6700 feet. Although not prominent, the two craters may be seen through binoculars, and they constitute one of many examples of pairs of similar craters found on the moon. Here is another clue to the story of our satellite that awaits interpretation. Long ago Webb called attention to such arrangements and noted that often the pair is aligned in the north-south direction, a "rule" which Mercator and Campanus violate. South of the two craters is a triangular dark patch that might be considered part of Mare Nubium except for a few intervening spots of light upland territory. That little plain goes by the fanciful name talus epidemiarum, which constitutes its most striking feature. The largest structure on Palus Epidemiarum, located near its south edge, is Class 5 capuanus, a fig-shaped crater 43 by 47 miles across and 6200 feet deep at the west wall. Its smooth, dark floor has been reported as elevated above the outside level, and at times it does give that impression on the northeast where the wall is extremely low. However, the Army Map Service has found the floor 1000 feet below the palus plain.
About one Clavius length southwest of Capuanus look for a curious bathtub-shaped enclosure with black interior and bright walls. It is a Class 2 multiple crater 44 by 58 miles across and 10,500 feet deep. The object is hainzel, and the long-accepted "double" description will suffice for our observations through binoculars. However, visual and photographic studies with good interior lighting and good conditions show that Hainzel actually is a triple crater, the larger southeast portion consisting of two smaller craters side by side. Tangent to Hainzel on the south, at the terminator, is the extremely old enclosure mee, 85 miles in diameter and 7500 feet deep. Hainzel and numerous smaller craters have broken its walls and encroached upon its floor. Most of its rough interior is illuminated to night, but it is dim and not easy to see. Under higher sun its dilapidated walls and pitted flog, merge into the general background, and the crater disappears. Neison's excellent map (1876) gives no indication of an enclosure there, but Ooodaere shows it on his 1910 chart, unnamed at the timet
Another fine crater deserves our attention. sciieiner is conspicuous near the terminator west of Clavius, south of Longomontanus, and at the northwest edge of Blancanus. It is a mountain-walled plain of Class 2, 71 miles across and 15,100 feet deep. About % of its bombarded floor is black but it is well outlined by bright walls. The inner west wall is considerably broader and brighter, but an appreciable outer east wall is certainly present despite the contention that mountain-walled plains lack such a feature.
Along the central part of the terminator we notice a large, smooth, darkened plain. It is the southeast section of the largest of all lunar plains, the oceanos procellarum, which stretches from Copernicus south to ancient Parry, from Copernicus northwest to the uplands west of Sinus Iridum, and from those areas westward almost to the crowded, crater-walled limb. Dollfus has found the light from the southeast part of the plain polarized to a greater degree than that from any other part of the moon. Baldwin has measured one of the lowest elev ations there, a depth exceeded only by the center of Mare Im-brium. Nikolai Barabashov has detected a greenish hue over most of the Oceanus with a brown tint in the far south.
As the full phase approaches, the maria continue to darken, or rather appear darker as the uplands reflect light more efficiently. The darkest tonight are the great eastern maria—Tranquillitatis, Fecun-ditatis, and Crisium. The least darkened are Oceanus Procellarum and the nearly linear Mare Frigoris. Far to the east a pair of small, patchy, a"« poorly defined dark areas may be noted. They are southeast of Mare Crisium and northeast of Mare Fecunditatis. The north one is mare undarum, and its near neighbor on the south is mare spumaNS-The area of each scarcely is larger than that of the bright crater Langrenus south of them on the shore of Mare Fecunditatis, and they seem out of place in jj,e maria class. I did not mention them during the nights of the crescent moon since they were then almost invisible because of the general brightness of the region. Near full, they show with considerable contrast.
Two additional maria sometimes may be seen close to the east limb if the libration is favorable, that is, if the east edge of Mare Crisium is more than ifo Crisium width from the limb. East of Mare Crisium is make marcinis, appearing as an irregular ¿ark line parallel to the limb and about 150 miles long. South of it, and east of Mare Spumans, is the more obscure mare SMYTiin, a similar line some 225 miles long. They would show much better if we could view them from directly above, because the east-west extent of Mare Smythii is nearly as great as its north-south dimension, and that of Mare Mar-ginis is half again as great. Cher the western edge of Marc Smythii our first Lunar Orbiter took its initial series of photographs on August 18, 1966, from an altitude of 133 miles. The pictures, which it later developed and televised, clearly showed craters less than 100 feet across in a region where the best previous observations had given only vague indications of features several miles in diameter. With favorable libration in latitude, or longitude, or both, mare iiumboldtianum may be seen as a weak, narrow, dark ellipse or streak close to the northeast limb. A line from Copernicus through the gap between Mare Serenitatis and Mare Imbrium touches the elliptical plain which is 170 miles across. Do not confuse it with the shorter, wider black ellipse in front of the mare which is the crater Endvmion. Under likewise favorable libration. a series of small dark spots may be noted spread over a considerable portion of the southeast limb. They are some of the bays on the edge of mare australe. a strong and really large dark plain, the major portion of which lies beyond the limb.
Tycho is a brilliant spot tonight. Look for the 'aint, shaded halo which separates it from the bright ray system. The strange halo will seem to darken in the next few nights as the rays grow stronger and longer. Copernicus likewise is brilliant, and its unique ray system is strong and fully developed. The Apennines are nearly washed out 'gainst the bright background but can be traced. Eratosthenes, near the west end of the range, has become a test object. Can you find it? Quite a few the craters we have observed previously appear as bright spots, but more about them at full moon.
West of Copernicus, % the way to the termina-tor, is a crater a little larger than Euler with bright walls and interior, except for the shadow covering its east quarter. It is surrounded by a bright-ray splash pattern that will grow much brighter and, a few nights hence, will extend outward fully 200 miles. You are looking at the important Class 1 kepler, 20 miles in diameter and 7500 feet deep. So interesting and varied are the surface features around that modest crater that the U. S. Geological Survey chose it as the first lunar region to be mapped geologically. The superb chart, I-355 (LAC-57), published in 1962, is the work of R. J. Hackman. The area also was one of the earlier sections to be charted for navigation by the Aeronautical Chart and Information Center, U. S. Air Force. The crater shows well tonight, but soon the ray pattern will become so brilliant that Kepler itself will be difficult to locate in the glare. In addition to the striking ray patterns of Kepler and Copernicus, the region is of particular interest because of the large number of domes found there. Ranging up to 18 miles across, they are scattered in the area between the two craters and between Kepler and the Carpathian Mountains They show only when near the terminator, and none is a good object for binoculars since all are evidently rather smooth and less than 1000 feet high. If you want to look for one, the largest in the region is located about halfway from Kepler to the middle of the Carpathian Range, its distance from Kepler being equal to the Copernicus-Reinhold separation About one Plato length south of Kepler and not quite so close to the terminator may be seen the much older, hexagonal, flooded crater encke, of Class 5, 18 miles in diameter, but only 2300 feet deep. It is located near the outer edge of the "solid" portion of the Kepler ray system, and so it is not an easy object to resolve.
"Twice on the night of November 1-2, 1963, a large region on the moon near the crater Kepler glowed red, as recorded on photographs made at Pic du Midi Observatory by the authors," wrote Zdenek Kopal and Thomas Rackham in Sky and Tclescope, March, 1964. They then went on to describe two of numerous photographs in deep red light, taken that night of full moon, on which the surface brightness around Kepler nearly doubled its normal value during brief periods. The two astronomers attribute the striking phenomenon, which affected some 23,000 square miles of lunar surface, to luminescence induced by waves of high-energy particles ejected from the sun during brief outbursts called solar flares.
The harbikcer mountains do not amount to much if compared with the great ranges that bound Mare Imbrium (Fig. 44). They are thinly scattered over an area some 40 by 100 miles. While they are fairly bright, the group is generally quite inconspicuous. Consequently, the 5800-foot elevation which the U. S. Air Force has found for the highest "peak" seems much more reasonable than the 8000 feet reported by earlier investigators. Tonight the sun has just risen over them, and their individual members shine like stars 011 the plain one Serenita-tis width north of Kepler.
The medium-sized Class 1 crater aristabchus is seen easily tonight at the terminator north of Kepler (Fig. 44). Kepler, Aristarchus, and the larger Copernicus mark out an Isosceles triangle with Kepler at the vertex. The triangle, which actually is delineated by bright rays near full moon, is almost a right triangle, and later you may notice two craters at its northwest corner. Aristarchus, 25 miles in diameter and 11.900 feet deep, is the east one and the more conspicuous by far. In the words of Wil-kins and Moore: "This is the highlight of the moon, being the most brilliant objcct on the lunar surface. At full it is so bright as to dazzle the eye and confuse the details." Webb states that it is "visible to the naked eye on the bright side, and with the telescope 011 the dark." Tonight we see principally the west inner wall as a bright crescent that is almost a scmiellipse. The black interior and the east wall seem to merge into the background. Later the unique feature of the ray system may be seen as a triangular light patch extending westward from the crater across the south rim of Herodotus which is Aristarchus' neighbor.
Not only is Aristarchus tlie moon's brightest spot, but in recent years it has become one of the more interesting spots because of well-observed outgas-sing that occurs there from time to time. Kozyrev reporting in Nature, June 8, 1963, mentions spectrograms which he had obtained in 1955 that "indicated the existence of a luminescent glow in the Aristarchus crater." He then describes a series of such observations made between November 26 and December 3, 1961, several of which showed the presence of a luminous cloud of gas covering a small area at the center of the crater. After photometric analysis of the spectrograms, Kozyrev concludes that the gas was molecular hydrogen.
On October 29, 1963, James Greenacre, of the Aeronautical Chart and Information Center, U. S. Air Force, was at work on the draft chart of the region. He was observing with Percival Lowell's splendid 24-inch refractor at Flagstaff, Arizona, adding fine detail to the chart which had been prepared from the world's best photographs. He had been engaged in such work for more than three years and had spent at least 50 hours in the study of Aristarchus and its env ironment. Rated as a very cautious observer, skeptical of reported lunar changes, Greenacre was astonished to see "a reddish orange color over the domelike structure" just north of Herodotus. Instantly he noticed another such spot a little farther north, and a few minutes later he saw "an elongated streaked pink along the southwest interior rim of Aristarchus," The changes, which he describes in detail in Sky and Telescope, December, 1963, were confirmed by his observing assistant Edward Barr, but they could not be seen with the six-inch finder on the Lowell telescope. Within 20 minutes of the first observation the colors had faded away and the area had regained its familiar appearance. A close watch for the next two weeks failed to reveal anything unusual, and the sunset terminator passed over the crater, leaving it in darkness for another two weeks. As soon as sunlight reached Aristarchus again, the observations were resumed, and on November 27 the reddish streak on the rim of Aristarchus reappeared and persisted for one hour and 15 minutes. It was seen by John Hall, Ixiwell Observatory director, and several other observers, including students using the 72-inch reflector of the Perkins Observatory. Hall suggests that the outgassing is caused by the return of the sun's heating rays to the crater after two weeks of cold darkness. That suggestion fits Greenacre's observations perfectly, but it seems to be somewhat inadequate when applied to those of Kozyrev which were made between 7 and 14 days after the sun had risen over Aristarchus.
Whatever the final interpretation may be, the remarkable events at Aristarchus again dramatically confirm in a general way the long-discounted claims of a host of astronomers, both professional and amateur, who for two centuries have asserted that changes do take place on the moon. However, before we accept with posthumous apologies all the claims of the past, we should weigh these significant facts. First, the photographic observations of Alter (Alphonsus) and Kozyrev were made with reflectors of 60 and 50 inches aperture, respectively. Second, the visual observations of Greenacre and his associates were made with one of the world's larger refractors located in an unexcelled atmospheric environment. Third, Greenacre was unable to see any of the October changes wit»
a professional-quality six-inch telescope, and the November event could not be detected at Flagstaff with a 12-inch telescope! So don't expect to see lunar changes through your binoculars or small telescope. You might do so, but the probability of success is exceedingly low.
hebodotus, of Class 5, 20 by 22 miles across and 4700 feet deep, suffers from a most unfortunate location (Fig. 44). At best, it is noted as the dim crater just west of dazzling Aristarchus. Near full moon, when the Aristarchus rays are brightest, Herodotus is overwhelmed by them and virtually disappears as far as the binoculars astronomer is concerned. Tonight it is only a bright speck beyond the terminator, but tomorrow night it should show fairly well. Its smooth floor, fully as dark as the surrounding plain, will contrast sharply with the illuminated portion of Aristarchus' interior, as you will see tonight if you wait a few hours.
Just beyond the north shore of Sinus Iridum, and near the middle of the Jura Range, the Class 1 crater bianchini shows well with about % of its interior in sunlight. Its 25-mile diameter makes it almost identical in size with Aristarchus, and its distorted walls rise 10,000 feet above the floor. About two Plato lengths southwest of Bianchini and a little farther removed from the sinus than is Bianchini, we find the similar crater sharp, of Class 1, 24 miles across, and 10,500 feet deep. Continuing in about the same direction for the same distance, we come, near the terminator, to Class 1 mairax, 26 miles in diameter and 11,200 feet deep. To this linear array of three almost identical craters may be added a fourth on the plain northeast of Sharp and northwest of Bianchini. The last is Class 1 iiarpalus, 26 miles in diameter and 9800 feet deep. Of the four inconspicuous craters, Harpalus is the most prominent, partially because it rises from a smooth plain and partially because it has higher reflectivity. If successful with the last four, you might look for two smaller new craters on the north edge of the Jura uplands, foucault is south of Harpalus, and boucuer is east of Harpalus. Their respective dimensions are 15 miles across and 7200 feet deep; 15 miles in diameter and 8200 feet deep. Both are difficult objects. The plain on which Harpalus stands looks like part of Mare Frigoris, but it is known as sinus roris.
Two Crisium lengths south of Kepler the bright mountain-walled plain cassendi awaits our inspec-tion (Fig. 20). It may be located also by running a line from Plato to Copernicus and extending it '4 its length. Gassendi, of Class 5, 70 miles in
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