Clavius [584 S 144 W

It is tempting to think of Clavius (Figure 10.11) as the largest crater on the near side of the Moon. In fact, this is not the case. But, undoubtedly, it is the largest spectacular crater on the lunar surface and it is a truly awesome sight. Larger,

Figure 10.11. An excellent image of the huge crater Clavius, taken with a 250-mm f/6.3 Orion Optics Newtonian, working at f/24. Image taken with an ATiK 1HS webcam on March 19, 2004. Image: Jamie Cooper.

much older craters, do exist but are harder to spot as a complete crater; they appear as a "ruined plain," ruined by impacts after they formed. The largest and deepest basin on the Moon is, in fact, the massive southern polar Aitken Basin, which is 2,500 kilometers deep and 12 kilometers below the mean sphere! However, from the viewpoint of Earth-based visual and webcam observers, the ruined crater Bailly, with a diameter of 287 kilometers (situated at 66.5° S, 69.1° W) is the largest named crater, but is so foreshortened that it is in no way comparable with Clavius. Clavius itself lies at 58.8° S and 14.1° W in the rugged southern highlands and is an awesome 245 kilometers in diameter. It is only 200 kilometers northwest of Moretus. The crater first becomes illuminated a day or so after first quarter and the Sun does not set on the formation until after last quarter. This factor, coupled with its sheer size, gives Clavius the illusion of being an almost permanent feature through the eyepiece. There are regions of the Clavius floor that have, for decades, been a test for the lunar photographer and are now tests for the small-aperture webcam user. I can remember sitting in the legendary Horace Dall's study in Luton in 1984, when he proudly showed me his best photographs of a horseshoe-like pattern of small craterlets on Clavius' floor. The craterlets, only 2 kilometers across, were at the very limit of resolution in the film era (even with Dall's 39-cm Dall-Kirkham Cassegrain). Recently, Damian Peach showed me a webcam composite he had taken with an 80-mm apochromat that was every bit as good as Dall's image! The webcam really has opened our eyes to what is possible.

Clavius is so large that there are numerous decent sized crater's within its walls! The most distinctive is the 50-kilometer crater Rutherfurd and the crescent-shaped curve of diminishing crater's (Clavius D, C, N, and J) which lead off from it. Another 50-kilometer crater called Porter breaks into Clavius' northeastern wall. The only trouble with Clavius is fitting it onto the webcam chip. If you are a planetary imager who frequently increases the telescope's effective focal length to almost 10 meters you may have to review your strategy. Clavius spans over two arc-minutes in length and so an effective focal length of five meters or less is needed to comfortably squeeze it onto a webcam's CCD chip.

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