Craters

A casual glance at the Moon reveals that her landscape can be divided into two general types of terrain, "highlands" covered with the ring-shaped formations called "craters" and relatively smooth areas, the "mare," the lunar "seas." The highland area, particularly the Moon's southwest quadrant, is a paradise for a Moon lover, as it is composed of unending numbers of shoulder-to-shoulder craters of all types, sizes, and shapes.

These seemingly innumerable craters are the product of eons of bombardment of the airless Moon by debris left over after the formation of the Solar System. Lunar craters range in size from tiny, less than an inch across, to great dishes hundreds of miles in diameter. One of the first things a beginning observer wants to know is, "What's the smallest crater I can see?" A well-collimated 8-inch SCT (or other CAT) under excellent atmospheric seeing can reveal craters 1/2 mile across, which is smaller than what should be theoretically possible for an 8-inch scope. Contrast is the reason; shadows created by crater walls at lunar sunset and sunrise help the SCT resolve detail that would normally be too small for it to make out. A 12-inch CAT might be able to distinguish craters somewhat smaller than 1/2 mile in diameter, but bigger telescopes usually won't do too much better than an 8-inch because atmospheric seeing limits their better resolving power.

Some craters that deserve the novice's attention are Tycho, Copernicus, and Plato. Tycho, 85 km across, is prominent because it's young, "only" about 108 million years old. For that reason it's sharply defined, standing out well in its crowded area. What makes it truly wonderful, however, is its huge system of "rays." Tycho's rays are the ejecta thrown from the crater during the impact of the body that formed it. This debris is lighter than the landscape it's deposited on and stands out starkly as bright radial streaks emanating from the parent crater. Tycho's rays, which are especially brilliant at full Moon time, stretch nearly 1,500 km across the surface. Copernicus (Plate 49), 93 km in size, has a prominent ray system, too, if not as an extensive a one as Tycho. What's special about Copernicus is the crater itself. It features a "terraced" inner wall and an intricate system of central peaks. Plato is strikingly different from the other two. It's located away from the highlands in the northwestern quadrant and doesn't have a ray system. If it ever did have rays, they were extinguished long ago by the lava floods that formed nearby Mare Imbrium. Under good conditions (good seeing and low Sun angle), a C8 will reveal that Plato's floor is littered with numerous small craterlets.

Plate 49. (Copernicus)

The great crater Copernicus as seen by an 8-inch SCT. Credit: Author.

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