Lying exactly on the lunar meridian, but trailing south from just below the center of the disc, one will find three impressive craters named Ptolemaeus, Alphonsus, and Arzachel (Figure 10.27). The trio is so distinctive that it is almost impossible, even for a beginner, not to recognize these three huge craters. Ptolemaeus is the
largest, with a diameter of 153 kilometers. Being so close to the center of the lunar disc, it is arguably the largest crater that is perfectly circular to the telescopic observer. Although the dark floor is smooth in appearance, a careful inspection under low illumination reveals that it is covered with various shallow depressions and craterlets. Resolving tiny craterlets on the floors of Ptolemaeus was a challenge for the legendary U.K. telescope maker and photographer Horace Dall (1901-1986) as there are myriads of them between one and two kilometers across: the practical resolution limit in the photographic era. There is one, larger, flooded crater on Ptolemaeus floor, too. This feature is known as Ptolemaeus A or Ammonius and is nine kilometers in diameter. A ghost crater (i.e., submerged under a lava flow) labeled "B" lies immediately north of Ammonius. The giant crater Albategnius lies just to the east of the region and is a spectacular formation in its own right.
The second crater in the adjoining trio is the crater Alphonsus. Although Alphonsus is much smaller, at 119 kilometers in diameter, it is no less interesting and has some controversial history associated with it. In 1955 the astronomer
Dinsmore Alter took some photographs of Alphonsus with the 1.5-meter (60-inch) reflector at Mt. Wilson Observatory in California. He took filtered photographs in blue-violet and in infrared light and part of the floor of Alphonsus appeared blurred in the blue-violet pictures but sharp in the infrared. This is not a surprise to any webcam imager as seeing is always more stable and less scattered in the infrared. However, the results were interpreted as there being a lunar atmosphere on the floor of Alphonsus! Worse madness was to follow. On November 3, 1958, the Soviet astronomer Kozyrev claimed he had obtained a spectrograph proving that hot carbon gas was being emitted from Alphonsus at a temperature of 2,000°C. Needless to say, this prompted much debate and criticism and there has never been any modern evidence to support the claim. Indeed, many modern astronomers, amateur and professionals, have concluded that the late Mr. Kozyrev was "not sailing with a full crew," was "knitting with one needle," or was "two bricks short of a full load." However, it did reserve Alphonsus and Kozyrev a place in history, albeit a dubious one! The space probe Ranger 9 landed not far from Alphonsus' central peak on March 24, 1965, and not far from the northern tip of the westernmost of the two rilles that meander around the eastern side of Alphonsus' floor.
The third crater in the distinctive trio is the 97-kilometer-diameter crater Arzachel. Slightly smaller than Alphonsus, Arzachel is the most rugged in appearance of the trio and features some spectacularly terraced walls four kilometers in height, a substantial central peak, a rille on the eastern part of the floor, and even an internal 10 kilometer craterlet. Arzachel may be the smallest and youngest crater of the trio but, to me, it is the most interesting. An exceptionally fine webcam shot of Arzachel, by Jamie Cooper, is shown in Figure 10.28.
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