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Cassini is thirty-six miles in diameter, Aristillus thirty-four, and Autolycus twenty-three. The first named is shallow, only 4,000 feet in depth from the highest point of its wall, while Aristillus carries some peaks on its girdle 11,000 feet high. Autolycus, like Cassini, is of no very great depth.
ARISTILLUS. A larger and much more elaborate ring-plain, 34 miles in diameter, N. of Autolycus. Its complex wall, with its terraces within, and its buttresses, radiating spurs, and gullies without, forms a grand telescopic object under a low sun on a good night. It rises on the east 11,000 feet above the Mare, and is about 2000 feet lower on the W., while the interior is depressed some 3000 feet.
Just beyond the N. glacis is a large irregular dusky enclosure with a central mound, and another smaller low ring adjoining it on the S.E. The visibility of these objects is very ephemeral, as they disappear soon after sunrise. Aristillus is also the centre of a bright ray system. THEAETETUS. A conspicuous ring-plain, about 16 miles in diameter, in the Palus Nebularum, N.W. of Aristillus.
On comparing his star positions with those of Timocharis and Aristillus he found no stars that had appeared or disappeared in the interval of 150 years; but he found that all the stars seemed to have changed their places with reference to that point in the heavens where the ecliptic is 90 degrees from the poles of the earth i.e., the equinox.
Westward from the middle of an imaginary line joining Aristillus and Cassini is the much smaller crater Theætetus. Outside the walls of this are a number of craterlets, and a French astronomer, Charbonneaux, of the Meudon Observatory, reported in December, 1900, that he had repeatedly observed white clouds appearing and disappearing over one of these small craters.
Aristillus and Timocharis set up instruments and fixed the positions of the zodiacal stars, near to which all the planets in their orbits pass, thus facilitating the determination of planetary motions. He also found the sun's diameter, correctly, to be half a degree. He deduced the diameter of the earth, 250,000 stadia. Unfortunately, we do not know the length of the stadium he used.
E. of Eudoxus are two short crossed clefts, and on the N. a long cleft of considerable delicacy running from N.E. to S.W. It was in connection with this formation that Trouvelot, on February 20, 1877, when the terminator passed through Aristillus and Alphonsus, saw a very narrow thread of light crossing the S. part of the interior and extending from border to border.
They have been compared to lava streams, which those round Aristillus, Aristoteles, and on the flank of Clavius a, certainly somewhat resemble, though, in the two former instances, they are rather comparable to immense ridges.
The principal ray-systems are those of Tycho, Copernicus, Kepler, Anaxagoras, Aristarchus, Olbers, Byrgius A, and Zuchius; while Autolycus, Aristillus, Proclus, Timocharis, Furnerius A, and Menelaus are grouped as constituting minor systems. Many additional centres exist, a list of which will be found in the appendix. The rays emanating from Tycho surpass in extent and interest any of the others.
Passing into the region of the Mare Imbrium, whose western end is divided into the Palus Putredinis on the south and the Palus Nebularum on the north, we notice three conspicuous ring mountains, Cassini near the Alps, and Aristillus and Autolycus, a beautiful pair, nearly opposite the strait connecting the two Maria.
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