The Sciences

The Year's Best Moon

By Bob BermanMar 1, 1995 6:00 AM


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The moon, like Nehru jackets, is out of fashion. With astronomers’ pursuit today of quasars and supernovas, our nearest neighbor seems too familiar to hold their interest. Observatories all but close up during its full phase, when lunar brilliance washes out the universe beyond.

Beginners pointing telescopes at the full moon are disappointed because the sun then shines straight down on the moon like a flash camera, erasing all shadows and highlights.

But not all phases are created equal. The year’s best and highest moon for observers occurs between March 8 and 11, when it suddenly explodes with breathtaking detail for anyone with binoculars or a small telescope. The phase responsible for these sudden riches is the first quarter.

This is a moon packed with misconception. Even its name is misleading. How many realize that a quarter-moon is the same thing as a half-moon? Or that of all the moon’s phases, the first quarter stands highest at nightfall, posing prominently at the convenient time of 6 to 8 P.M.? And only a half-moon aims its terminator, the day-night line that is home to all the juicy detail, straight at Earthly observers. Highlighted craters, like actors hamming it up in the footlights, stand out sharply in the stark interplay of light and shadow.

Even the half-moon’s degree of brightness is surprising. You’d think it would be half as bright as a full moon, right? Oddly enough, a half-moon is only one-tenth as bright as a full moon.

For years astronomers believed that a phenomenon called shadow hiding explained the full moon’s lopsided luminosity. When the moon isn’t full, this theory went, its powdery surface casts innumerable little shadows, reducing the light reflected to Earth. But at a full moon, lunar shadows disappear; sunlight shines straight back at us. A little over a year ago, though, astronomers showed that a more subtle process is at work.

Not all sunlight that hits the moon is reflected. Some passes through tiny transparent grains stuck to larger lunar particles. Like minute prisms, these grains bend sunlight slightly. As the light shuttles from grain to grain, some of it bends back toward the sun--and us. It is this backscattered light, astronomers now believe, that makes full moons so bright, because sometimes two backscattered light waves reinforce each other, creating a light ray brighter than two separate rays. Sunlight always backscatters from the moon, but we see it only during a full moon, when Earth, sun, and moon align.

But it’s the half-moon we’re interested in. Look at it with ordinary binoculars and see the Apennines, mountains near the moon’s middle, whose jagged peaks tower straight up at you. The gaping crater near the center is Copernicus, suddenly popping out of the moon’s nightside on March 10, and more dramatic the next night. Its spectacular terraced walls make it one of the most famous of the 30,000 craters visible from Earth. The scene changes strikingly each night as the terminator slithers over the moon’s surface at ten miles an hour. (A lunar jogger with enough stamina could keep nightfall at bay!)

On March 13 the gibbous moon hovers just below Mars. The planet’s ruddy glow serves as a stoplight, warning that the moon has now become too swollen to reveal much detail. Our lunar revue ends with neither bang nor whimper as the ides of March ushers in that brilliant and romantic underachiever, the full moon.

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