Reading the Moon: Step 3

Jeff Kanipe, author of "A Skywatcher's Year," gives advice on exploring the Moon in detail:

To the unaided eye, the Moon's surface appears mottled with irregular patches of gray amid splashes of white. These darker patches are known as maria, Latin for seas, since that is how they appeared to early skywatchers. We know them now as areas where lava pooled on the lunar surface billions of years ago, probably after an asteroid punched through the thin lunar crust.

Together, the light and dark regions arrange themselves into the popular face of the "Man in the Moon." They were interpreted as a "rabbit in the Moon" by the Maya and Aztecs of ancient Mexico, as well as the Mimbres Indians of the southwestern United States.

As sublime as the Moon appears to the eye, a pair of binoculars turns this marbled world into a breathtaking wonder. One of the best times to look at the Moon with any optical aid is during its crescent and quarter phases. At that time, the low, slanting angle of the rising Sun creates deep dark shadows in valleys and crater floors that contrast sharply with bright mountain ranges and crater rims. When the Moon is full, it is essentially "noontime" there, with the Sun overhead. Though this geometry makes the Moon appear very bright as seen from Earth, it lowers the contrast of the lunar features.

With a small telescope, a number of prominent lunar features may be observed at first or last quarter. Some of these may also be seen when the Moon is full, but as mentioned, their contrast will be significantly reduced. Moreover, the Moon is so bright that even at first or last quarter it can dazzle the eye (never mind its glaring appearance at full phase!).

Let's begin by considering the Moon's prominent features at first quarter.

Near the upper eastern (right) limb (as the Moon appears to the naked eye and in binoculars), is the round, dark basin known as Mare Crisium. This region can be seen standing in the full glare of the Sun during the early crescent stages. Our perspective of Crisium on the curve of the Moon's limb often gives it an oval appearance. To the west (left) lies Mare Tranquillitatis, the region of the first manned lunar landing. (By the way, none of the Apollo landing bases can be detected in telescopes from Earth.) Above and slightly to its left is another round basin, Mare Serenitatis, which stands partly in shadow. Serenitatis is rimmed on its western (or left) edge by great mountain chains.

Along the terminator, which is the dividing line between the lit and unlit regions of the Moon, many craters stand out in sharp light and shadow. Most prominent is the trio Theophilus, Cyrillus, and Catharina, all lying just below center. Theophilus and Cyrillus are actually overlapping and look like a figure 8. Catharina lies a little below them. Another prominent crater, Aristoteles, lies along the upper terminator, near the Moon's North Pole. Near first quarter it has a bright west rim, with the floor in deep shadow.

Lined up neatly along the terminator at first quarter, just below the lunar equator, are three prominent walled plains. Ptolemaeus lies furthest north (uppermost) and has a number of craters and pits within it. Adjoined to Ptolemaeus to the south, and slightly smaller, is Alphonsus, site of the robotic Ranger 9 landing in March 1965. Finally, just below Alphonsus, is Arzachel, a very conspicuous crater with terraced clefts along its rim and a prominent central peak.

At last quarter, the western (or left) half of the Moon is lit. One of the more prominent features is Oceanus Procellarum in the upper left quadrant, a sprawling expanse of dark basalt punctuated by the white chalk-like streak of Aristarchus, a very recent impact crater. Observers with telescopes have reported unusual brightness and color changes in this feature, which are considered to be attributable to gas emissions from within the crust.

Further east (right) is Mare Imbrium, a large, round basin that makes up one of the "eyes" of the Man in the Moon. Two bright ray craters, Copernicus and Kepler, lie near the equator in this section. The rays are produced by material ejected when an asteroid or comet struck the surface, splashing out bright fresh material from below. Mare Nubium lies directly below, or south, of Imbrium in the Moon's southern hemisphere. Mare Humorum, one of the smallest of the maria, lies to the west, or left.

Most of the Moon's southern hemisphere is rugged and pockmarked with craters, including the bright ray crater Tycho. To its south lies Clavius, one of the most dominant walled plains in this sector of the Moon. Its rim and floor are pitted with a variety of craters.

You don't have to wait until the Moon is full to appreciate our closest celestial neighbor. As lifeless as it appears in binoculars or a telescope, the shadow show at quarter phase imparts a dynamic quality to the lunar craggy surface. All at once, the Moon ceases to be a beautiful bauble floating over Earth's landscape and becomes a world unto its own.

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