Observing the Moon

The Moon is truly one of the most compelling objects in the night sky. The unaided eye can easily discern a mottled surface of bright and gray patches that arrange themselves into the popular face of the 'Man in the Moon,' or, as seen by the Maya and Aztecs of ancient Mexico, as well as the Mimbres Indians of the southwestern United States, a rabbit. The dark regions are known as maria, Latin for seas, since that is how they appeared to early skywatchers. We know them now as areas where lava spilled across the Moon's surface billions of years ago, probably after an asteroid punched through the thin lunar crust.

As sublime as the Moon appears to the eye, a pair of binoculars turns this marbled world into a wonder. One of the best times to look at the Moon using any optical aid is during its crescent and quarter phases, not when it is full. The reason is simple. 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 also lowers the contrast of the lunar features and reduces the shadows cast by mountains, valleys, and craters.

When the Moon is a young or old crescent or at first or last quarter, however, the Sun is rising on the Moon. The shadows and craters cast deep, dark shadows across the surface, and suddenly, the landscape has dramatic relief.

Let's consider 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. 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.

South of Mare Tranquillitatis, many craters stand out in light and shadow. Most prominent are the trio Theophilus, Cyrillus, and Catharina. Theophilus and Cyrillus are actually overlapping and look like a figure 8. Catharina lies a little below them. Another prominent

Last quarter Moon.

angrenus i

First quarter Moon.

angrenus i

First quarter Moon.

crater, Aristoteles, lies above Mare Serenitatis, 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 which is the dividing line between the lit and unlit regions of the Moon, 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, revealing more dark features. Most prominent among them is Oceanus Procellarum in the upper left quadrant, a sprawling expanse of basalt punctuated by the bright feature Aristarchus, a very recent impact crater. Observers with telescopes have reported unusual brightness and color changes in this feature, some of which may 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.

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