Beginner's Guide to the Moon

Original title: The Moon in June
© 1998 by Robert C. Moler

With the long midsummer twilight lasting past midnight by month's end and no planets in sight it's time to turn our attention to the bright celestial object only a quarter of a million miles away, the moon.

Earth's only natural satellite has always been a favorite of the public, and new telescope owners. However, to many amateur astronomers, the lure of the vast darkness of the deep sky turns the moon into a street light that can't be shielded. So lets visit, or for some of us, revisit the moon.

To the unaided eye the moon looks like a flat white disk with splotches of gray that make up the image of the man in the moon. The ancient Greeks thought it to be a perfectly reflecting sphere, being part of the perfect heavenly precincts. The splotchy markings were merely a reflection of the imperfect earth.

The phases of the moon, as it waxes from new to full, and wanes back again can be duplicated with a ball. Pick a time before sunset or after sunrise when the moon is still visible. Toss the ball so it can be seen next to the moon, and the ball, in sunlight, will have the same phase as the moon.

With binoculars, the moon begins to show enough detail to become a real place. The gray blotches become the dry seas the early telescopic observers saw. The seas are vast overlapping craters filled by molten lava. The line between light and dark, called the terminator, that shifts to cause the moon's phases is the sunrise line when the moon is between new and full, and the sunset line after full. It is here that the sun appears low in the moon's sky, making for long shadows which helps delineate the moon's features.

As the moon waxes from new to first quarter, the first large telescopic feature appears. This is Mare Crisium or Sea of Crises at the moon's upper right edge. The position of Mare Crisium with respect to the limb or edge of the moon is a dramatic indicator of an effect called libration. The moon's axis and rotation rate are fixed and constant. However the moon's apparent orbit of the earth is an ellipse, in which it moves at varying speeds. Therefore the constant rotation gets behind or ahead the moon's revolution about the earth. We then can see a bit of the far side to the east or west. This the position of Mare Crisium lying near the limb is the perfect indicator, at least during the moon's waxing phase.

The large crater Langrenus is prominent south of Mare Crisium. In the next few days more maria are exposed. First Mare Foecunditatis (Fertility) and Mare Tranquillitatis (Tranquillity) appear along with the small Mare Nectaris (Nectar). A large deep crater with a sharp central peak lies between the two later seas called Theophilus. Then the scallop shaped Mare Serenitatis (Serenity) is uncovered to the north. The southern part of the moon is a vast bright highlands saturated by craters.

Near first quarter three large craters appear in a line north to south, located just south of the moon's center. They are, from north to south Ptolemaeus, Alphonsus, and Arzachel They lie on the shore of Mare Nubium (Clouds). A day after they appear a telescope can look north of them, off the shore of Mare Nubium at a thin black line near first quarter, or a bright line near last quarter. It is the Straight Wall a straight cliff a hundred miles long and 600 feet high.. It is visible for a day or so around the optimal times.

In the north the lunar Alps ring the shore of Mare Imbrium (Showers) which start to appear after first quarter. A large flat crater called Plato appears on the north edge of that sea. It's neat to spend an evening watching the sun rise and the shadows of the crater wall retreat across the floor of Plato.

In the north at this time the crater Tycho will move into sunlight. But it isn't spectacular until the moon nears full when it appears as a white ring with three double rays appearing to stream away from it across the face of the moon. A day after Tycho appears, the huge nearby crater Clavius edges into sunlight. In a telescope the crater floor at a low sun angle shows to be slightly convex, not concave as one expects. This is due to the moon's own spherical curvature. An arc of craters of ever decreasing size, in Clavius' floor also make for an interesting sight.

Several days after first quarter the great crater Copernicus appears at its right center position on the moon. Unlike the streak rays of Tycho, Copernicus has a uniform circular ray pattern of short distance.

The next day a hook will appear jutting into the predawn blackness of the upper left edge of the moon. It is a large ruined crater, now the Bay of Rainbows in the sea of showers, called Sinus Iridium. One crater wall was washed down, not by a wave of water, but waves of molten lava.

The next day will bring the great Oceanus Procellarum, the Ocean of Storms into view. South of that is the small sea Mare Humorum (Moisture), connected to the ocean by a 'strait' with a trio of highland 'islands' in a triangular formation within.

In the ocean is also the brightest spot on the face of the moon, the crater Aristarchus. Telescopes can also detect the sinuous Schröter's Valley nearby.

Along the eastern edge (as we look at the moon) of Oceanus Procellarum are several large craters. The darkest of these is Grimaldi. Gassendi and Schickard lie south of Grimaldi. In a telescope look near Schickard when it's near the terminator. Next to it lies the crater Wargentin, a crater filled to the rim, and looks like a circular plateau.

I hope this quick tour of the moon will whet your appetite to spend a summer's evening with the Queen of the Night.

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Uploaded: 5/31/98