Crater recognition can be a nightmare! Not only are there so many of them but lots look very similar. Here’s another nightmare. You are in a lecture or presentation when a crater is flashed up on the screen. “Identify that crater”, the audience is challenged. Everyone looks round at you. You are the ‘expert’ aren’t you with all those pictures you take! Many clues can be lost in the picture. Most astronomers get used to observing the Moon either through a reflector or a refractor. The images are then either seen flipped through a vertical axis or a horizontal one. Some (me) adjust them on the computer to be binocular view that seems a sensible idea because that’s how you see it with the unaided eye. However, if you are thoroughly used to one orientation then identifying from another poses an additional, sometimes impossible, difficulty. The image presented might be focused only on the crater with little in the way of surrounding terrain to assist. Fortunately the Moonscape (unlike the Earth) has not changed and will not change significantly for millions of years so we have plenty of time to learn.

Some common large craters are similar. They are roughly circular and have damaged but intact walls and possibly central peaks. So I have selected three of those to begin with. They are to the north of the Moon and are commonly observed because they appear at the terminator at a convenient time in the evening i.e. within a day or two of first quarter after the Sun sets. They are seen and described in the context of surrounding areas in regions 2 and 5 in chapter 9 of The Book.

		Picture 1 Aristoteles. Characteristic of this crater are the beautiful rugged radial ejecta thrown out at impact by its forming asteroid. Most significantly a small but deep crater, Mitchell, shares part of its wall and a pattern of small mountains or peaks reside within.
Picture 2 Eudoxus. In the absence of any surround this would be a very difficult crater to distinguish. Notice how similar it is to Aristoteles but with no wall encumbrances. Perhaps the best identification is the two small craters, one either side, Lamech to the west and Eudoxus A to the east. By convention, west of the Moon is to the left and east to the right; opposite to what you might expect.
		Picture 3 Eratosthenes. Although the crater is very similar in shape and wall structure to the others it has a very characteristic tear-drop shape of mountains to the south. It also sits in a relatively feature-free lava field area, between Mare Imbrium and Sinus Aestuum.
Some craters have apparently smooth floors, apparently because on close inspection almost every inch of the regolith (Moon suface) is strewn with rocks dust and debris. Three well-known smooth-floored craters are chosen here to learn and identify.
Picture 4 Archimedes. This is a beautifully round crater with a flattish Mare on one side and a more rocky terrain on the other. The most distinguishing recognition feature is the rock formation leading from its southern perimeter. Could it be likened to a curly ‘H’? Small table legs? An anvil on its side? You decide and remember. With familiarity, the shadow cast over the smooth floor becomes a recognition factor also. The adjacent pair of tiny craters Beer and Feuillée at around 9 kilometres across are also unmistakable.
		Picture 5 Plato. Very low walls completely broken down in places and somewhat larger than Archimedes. On its lava plain border there is a very discernable ghost of an outline of a buried crater, Bliss, formed from small mountain blocks and a curved wrinkle ridge.
Picture 6 Ptolemaeus. An even larger crater that is more of a walled plain since the walls are very low and completely broken down in many places. There are two main recognition features. Firstly, if only the crater itself can be seen, a prominent small crater, Ammonius at 9 kilometres across, sits starkly within, only prominent due to the absence of other craters. Secondly, it is part of a very easily recognisable pattern of three craters, Ptolemaeus, Alphonsus and Arzachel. Although the floor is relatively smooth picture 6 reveals several dents the largest of which lies next to Ammonius and is labelled Ptolemaeus B.

Advanced recognition

Pictures are not always of perfect quality but good enough for an album. Craters then lose some of the clues to their identity. At unpopular times of observation they can also be in less familiar regions

What is the name of the large crater just above centre left? A good geographical clue is the smaller crater below bearing a tiny, bright crater on its rim. And what is the identity of the huge crater just disappearing at the top of the picture?