MORE THAN 20,000 craters are visible on the Moon with a small backyard telescope. What saves us from beeeming bored observing them is that these craters have many interesting differences. Like people-watching in a big city, a lunar observer notices both the similarities and differences of unusual features and attempts to understand why they appear the way they do. A good place to start is with the word "round," which is a defining characteristic of nearly all lunar craters.
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