(Chart II or III)
An outstanding feature tonight is the great hexagonal mountain-walled plain albatecnius, close to the terminator and about % the way from the south to the north limb. The black shadow of the east wall still covers the edge of the floor, and you may be able to see the small white central peak through binoculars as it stands out against the dark stained floor. The inner west wall is bright. Eighty-one miles in diameter and 14,400 feet deep, the crater reveals a noncircular outline which is unmistakable, and its broad, flat floor of dark material indicates that it has been filled partially with lava. Tonight it stands out in bold relief, but, like other excavations of its type, it will fade away as the sun rises higher and virtually disappear around full-moon time. Albategnius is an old formation that has acquired numerous wall craters, the largest of which is seen extending from the southwest wall almost to the central mountain. That is klein. It is 30 miles in diameter and 6600 feet deep, and it has a low wall on the northeast which indicates that it likewise has experienced the lava-filling process. In addition to its prominence when near the terminator, Albategnius has the distinction of being the first lunar object illuminated and detected by red laser radiation from the earth. That historic experiment was performed on May 9, 1962, by Louis Smullin and Giorgio Fiocco of the Massachusetts Institute of Technology.
Just north of Albategnius is the similar, larger, but far older and more dilapidated enclosure hipparchus. That vast hexagonal mountain-walled plain, which shows well only briefly as the terminator passes across it, is seen readily, but by tomorrow night it will have faded considerably. It is 83 by 89 miles across but only 7500 feet deep owing to partial flooding and other destructive forces that have been at work upon it. Numerous smaller craters have eroded the wall remnants and pitted the floor, and the ancient crater is well on its way to obliteration. The largest interior crater is seen easily near the north edge of the sunlit floor. It is horrocks, 17 by 19 miles across, of Class 1, and 9200 feet deep—more than 1500 feet deeper than Hipparchus. Its wall is bright, and its floor is hidden in black shadow. The slightly larger halley breaks the south wall of Hipparchus. A Class 2
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