Take a Moon Walk Tonight

The moon is one celestial object that lives up to expectations the first time it’s seen in a telescope, and it never fails to impress. It’s our nearest celestial neighbor — big, bright, beautifully bleak, and just a quarter million miles away. That’s fewer miles than you may have ridden in cars, and 100 times closer than our next nearest major astronomical neighbor (Venus) ever gets.

This makes the moon a wonderful target for even the most humble astronomical instrument. You can spot and name at least a dozen of its surface features with the unaided eye. Binoculars show scores more, and a telescope can keep you busy on the moon forever. Of course, just looking and not knowing what you’re seeing will grow old pretty fast. As in all of astronomy, the rewards come from recognizing and understanding what you find, and from planning neat things to seek out. Let’s get started.

Seas of Lava

The moon’s biggest and most obvious features — visible even to the naked eye — are its large, flat, gray patches called maria (MAH-ree-a). This is the Latin plural of mare (MAH-ray), which means "sea." Early telescope users thought these markings might be similar to Earth’s bodies of water. In 1651 the Italian astronomer Giambattista Riccioli gave them fanciful names such as Mare Tranquillitatis ("Sea of Tranquillity") and Oceanus Procellarum ("Ocean of Storms"), generally for the imagined astrological influences of the moon at its various phases on the weather. Astronomers soon realized, however, that the moon has no water — but the names stuck. In fact, the “seas” are ancient lava flows that flooded most of the moon’s lowlands between 3.8 and 3.1 billion years ago.

The full-moon photo at left identifies the major maria. These are the moon’s most important geographical features, and even the smallest binoculars are enough for learning them. Make a point of memorizing a couple more of their names each night, and soon the geography of this new world will become as familiar as the continents of Earth.

This is especially easy to do because the moon always shows us the same face. It does so because long ago, the moon’s spin rate became locked to its orbital period around Earth. This happened because Earth’s gravity began holding the moon’s most massive hemisphere to face us all the time. This "spin-orbit locking" is common among moons throughout the solar system. The downside of this situation is that we never get to see the moon’s far side, unless we send spacecraft around back to look. When this was first done in 1959, the lunar far side turned out to be almost free of maria.

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 since then 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 well-preserved record of what happened in the extremely ancient past, right there in stark view. Because the era of lava flooding that created the maria came later, the maria bear fewer craters — only those caused by straggler asteroids and comets that hit in more recent times.

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.

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© 2005 Reprinted with permission from Sky Publishing Corp.