Copernicus [97 N 2GG W

Copernicus (Figure 10.19), situated in the Oceanus Procellarum, must be the most obvious crater on the Moon to the binocular user. Although it is certainly not the largest lunar crater, it is situated close to the center of the disc and away from any

Figure 10.18. The crater Gassendi and the intricate rilles on its surface. Orion Optics 250-mm f/6.3 Newtonian and 5x TeleVue Powermate. September 9, 2004. Image: M. Mobberley.
Figure 10.19. The crater Copernicus on March 30, 2004, imaged with a 20-cm Newtonian working at f/18 and a ToUcam Pro webcam. Only 22 frames were stacked for this result. Image: Mike Brown

rugged highlands. On a waxing gibbous Moon the feature is the most obvious crater a few days after the first quarter phase. The walls of Copernicus are truly magnificent and, to me, it is the only lunar crater that looks just like the top of a volcano. Of course, it is not, but it is a deep and impressive impact crater. The walls rise to 4 kilometers above the inner floor and the diameter across the crater rim is 90 kilometers. When the illumination is just right the crater walls can appear illuminated with the whole interior still in shadow, resembling a jet-black lake. Because of the rubble on the inner crater floor, below the wall ramparts, the internal floor diameter is little more than 65 kilometers. With the lunar terminator a few degrees away, i.e., just after sunrise, or just before sunset, the crater looks as deep as a bucket, but this, of course, is an illusion. One of the first instances of near-perfect atmospheric seeing that I witnessed was on October 21, 1981. It was just before dawn and a last quarter Moon was high in the sky. The view of Copernicus' floor was stunning; far more detailed than I had ever seen it. In fact, the view was so stunning that, for one crazy moment, I thought of knocking on all the neighbours' doors and dragging them from their beds for a look. (Somehow, as the temperature was close to freezing, I doubt they would have shared my enthusiasm!) It was obvious on that night, and quite a few since, just how nonsmooth the floor of Copernicus is. The terraced walls were spectacular, as was the fine detail on the floor. The renowned lunar observer T.G. Elger described Copernicus as "The Monarch of the Moon": an apt description. The outer slopes of Copernicus, in fact, the whole region within 200 kilometers of the crater center, shows the result of the impact that took place some 800 million years ago. The radial pattern looks like that of a stone thrown into mud and there are small, secondary impact craters everywhere. A good test for a lunar photograph used to be resolving the chain of "Stadius craterlets," near to the ghost ring Stadius and the magnificent, but smaller crater, Eratosthenes. However, in the webcam era, resolving the chain of secondary impact craters midway between Copernicus and Eratosthenes is not too difficult, even with a modest telescope.

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