Take a Moon Walk Tonight
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.
The Moon's Changing Phase
Each month as the Moon circles the Earth, we see it go through its cycle of phases. Starting from “new Moon,” when it is nearly in our line of sight to the Sun, the Moon grows, or waxes, to a crescent, then to first quarter (half lit), gibbous (somewhat football-shaped), and full. Then the Moon wanes back through gibbous, last-quarter, and crescent phases to new again. When waxing, the Moon is visible mostly in the evening. When waning, it’s best seen in the early morning hours.
In every phase except full Moon, the lunar globe is divided by the terminator, the line separating the Moon’s day and night portions. Along the terminator, the Sun is rising during the Moon ’s waxing phases, and setting when the Moon is on the wane.
Near the terminator, the lunar landscape stands out in stark relief. Mountains, craters and valleys here look especially steep and rugged,because the low Sun makes every low hill cast a long, dramatic shadow. As you look away from the terminator onto the Moon’s day side the surface appears smoother, because it’s lit by a higher Sun that casts few shadows.
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’s 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 Moon map here 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 rotation period became locked to its orbital period around Earth. (Earth’s gravity got hold of the Moon’s most massive hemisphere and keeps it facing us all the time.) This “spin-orbit locking” is common among moons through-out 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.