Impact Scars

The Moon’s most famous landforms, of course, are its craters. Practically all of these are the scars of titanic impacts by asteroids or comet heads. Most occurred more than 3.9 billion years ago during the “era of heavy bombardment” early in the solar system’s history. Earth was bombarded just as heavily, but Earth’s wind, water, and geologic activity have erased almost all trace of its early craters. The Moon, on the other hand, is geologically dead. We see on the Moon a record of what happened in the extremely ancient past, right there in stark view. The era of lava flooding that created the maria came later, so the maria bear fewer craters — only those caused by straggler asteroids and comets.

In fact, your telescope will show many places around the edges of the maria where the lava partially flooded preexisting craters. Sometimes the flooding was so nearly complete that only a “ghost crater” remains.

The Moon’s large bright areas — the lunar highlands — are the oldest terrain, as you can see from the thick cratering still preserved here. Craters come in every possible size, from dozens or even hundreds of miles wide, down to tiny craterlets as small as your telescope can show, typically a mile or two across. You can often tell the sequence in which several craters formed by how they overlap.

A large crater often shows a central peak — a mountainous pile created when the surface rebounded after a giant impact. Other big craters, sometimes called walled plains, have very flat bottoms because they became flooded with lava, like small maria.

The youngest craters are surrounded by bright rays that extend far across the surrounding landscape. These are great splashes of rock ejected by the impacts. Unlike most lunar features, rays are best seen when they are illuminated by a high Sun far from the terminator. At full Moon, bright rays from the large, young crater Tycho (only about 110 million years old) can be seen extending far around the Moon’s face.

Among the Moon’s other features are mountain ranges and individual peaks. Canyonlike cracks, or rilles, are sometimes visible, especially around mare edges. Look carefully near the terminator and you’ll see low wrinkle ridges winding across the maria.

How to Use a Moon Map

Every one of these features takes on its own individuality and meaning if you know its name. To do that, you'll need a Moon map and a flashlight to read it by. Many astronomy books include Moon maps, or you can buy one on our Web site. But you’ll need to know a trick or two to compare the map with what you see in the eyepiece — so read on.

Most maps show the Moon oriented more or less how you’ll see it with the unaided eye or binoculars: with its north side up. But here’s the tricky part. Many telescopes give an upside-down view, and many give a mirror-image view. Some telescopes do both. These two effects are entirely separate from each other, and you need to deal with them separately.

If you have a reflector telescope, or a refractor that you’re viewing “straight through” (in a straight line from end to end), you’ll see an ordinary, non-mirror image: a correct image. If you’re using a telescope where the eyepiece fits into a right-angle attachment (a star diagonal), a mirror image is probably what you’ll see.

To check, aim the telescope at a billboard or street sign during the daytime. Twist your head around so the sign appears more or less right-side up, and you’ll see right away whether you’re looking at correct writing or mirror writing.

If you have a correct image, simply turn the Moon map around until its mare patterns match the patterns you see. (Never mind if the printing is upside down or at some weird angle.) You can now compare the map directly to the view in the telescope.

If you have a mirror image, you’ll have to mentally flip the Moon in your eyepiece right-for-left to match the Moon on paper. Alternatively, you can buy a mirror-image Moon map. Small maps like these identify only a few of the thousands of lunar features revealed in an amateur telescope. The next step up is Antonín Rükl’s larger Field Map of the Moon or — for true lunar enthusiasts — our highly detailed and beautiful moon globe, a three-dimensional representation of the moon built with 15,000 actual images from NASA's Lunar Reconnaissance Orbiter.

And because you've read this far, here's a special treat: NASA's animation of the entire 4.5-billion-year history of the moon boiled down into 2.6 minutes: