The Sciences

The Last Quarter

By Bob BermanOct 1, 1993 5:00 AM


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This is the best month to observe the hidden side of the moon’s personality. As if anticipating Halloween’s devilry, the moon doesn’t rise until midnight on October 8, and it doesn’t reach its greatest prominence till dawn, when the average citizen is dealing with little more than REM sleep. This is the moon’s last quarter, a most furtive phase, offering unique treasures to insomniacs and toilers on the graveyard shift.

Some lunar terminology is in order, perhaps, so here’s a brief excursion into a land of astronomical jabberwocky. The two quarter moons-- first and last--are synonyms for the half-moon. (In what other science do quarter and half mean the same thing?) When seeing a half-moon, you are really seeing a quarter moon. Got it? Last quarter and third quarter are also identical terms, like woodchuck and groundhog, and a new moon means no moon because in moontalk something new is something that’s not there at all.

It’s easy to tell the two half-moons apart. The first quarter, the one that everybody always notices because it’s out when darkness falls, has its bright side on the right. The seldom-seen third quarter is lit up on the left.

The best time to point binoculars or a small telescope at the moon is when its surface gets the most dramatic sunlight; illumination makes all the difference. When it’s a thin crescent, the detail that makes the moon such an awesome target is confined to a narrow sliver on the edge. At full moon, the interesting stuff vanishes under the sun’s shadowless whitewash. Only within a few days of half phase does that middle section become highlighted, the play of shadow and sun optimally showcasing the majestic lunar Apennines. That mountain range towers straight up at you like shark’s teeth, its peaks casting inky shadows on the surrounding terrain. Like bubbles in boiling oatmeal, deep craters are scattered everywhere, especially in the moon’s southern region. Even rills appear, canyonlike clefts such as Hadley Rille, near the Apennines, which astronauts David Scott and James Irwin drove near in their four-wheel-drive lunar rover during the Apollo 15 mission in 1971. All this with even the cheapest small telescope!

The last quarter--that is, the third quarter--offers the same striking features as the commonly seen first-quarter phase, but illuminated from the other direction. This gives the observer a wonderful new vista, as if coming upon a familiar earthly scene at sunrise for the first time. The third-quarter moon always floats at a right angle from the sun. In a clear early-morning sky, the resulting contrast creates a vivid white-on-azure lunar portrait. It’s the moon seen by people on their way to work. This eternal morning apparition is most obvious right now because the autumn third-quarter moon is the most prominent of the entire year, hovering high overhead at dawn. Crane your neck to see it at sunrise, or observe it halfway up the western sky at 10 a.m.

Gazing at that moon, you’ll be focusing on a very special spot for another reason: that last quarter lies squarely in front of us in space. A reconnaissance scout, it travels ahead of Earth in its path around the sun. We follow along as if towed like a disabled car. When we look at the last-quarter moon, we are seeing the very spot in space that we ourselves will occupy some three and a half hours later.

Check it out on your way to work and you’ll know where you’ll be at lunchtime.

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