A plethora of planets and moons may populate the universe, but only one reflects Earth’s light back to our eyes. This unique talent belongs, not surprisingly, to our nearest neighbor--the moon.
Perhaps you’ve already noticed an odd aspect of the crescent moon: the dark portion glows a bit, even though there’s no sunlight there. It’s Earthshine: sunlight bouncing off our shiny world to the moon’s nightside and then back to us. Any citizen of the crescent moon’s dark side would be dazzled by the gibbous Earth.
But there’s another, rarer occasion when our planet stands mirrored by the moon. It’s coming up this month, when the moon plunges into Earth’s shadow.
It’s the first total lunar eclipse in almost three years, and the first since the Mount Pinatubo eruption in the Philippines. For over a year, a pink afterglow has followed sunset--a reflection from the volcano’s stratospheric dust and a vivid confirmation of the air’s pollution. A decade ago, dust from the Mexican volcano El Chichón darkened Earth’s atmosphere, resulting in the bizarre eclipse of 1982, when the moon assumed a weird yin-yang appearance. What effect will Pinatubo have on this month’s eclipse? Nobody knows. But who would miss the chance to find out? We can see the whole picture of Pinatubo’s pollution from a cosmic perspective as our atmosphere’s status is broadcast by the darkened moon on December 9.
That’s because the moon rarely disappears when it falls into Earth’s shadow. The eclipse’s hour-long partial stages are usually followed by a total eclipse that is not black but coppery red. The ruddy tint materializes because Earth casts a red shadow in space. More precisely, our shadow is colored by atmospherically refracted sunlight. The most dramatic way to visualize the situation is to pretend you’re a tourist on the moon during the eclipse. You’d see the sun slowly slipping and vanishing behind Earth. But then what a sight! The ink black cameo of Earth would be surrounded by a brilliant red ring--its atmosphere illuminated from behind by the sun. This crimson band is nothing less than all of Earth’s sunrises and sunsets, forming a continuous halo around the ebony disk.
But not perfectly continuous. Here and there clouds mar the ring; atmospheric dust and pollution darken other sections. The total light striking the moon depends on the clarity of Earth’s air.
The action begins at sunset. The full moon will come up already partially or totally eclipsed as seen from most of the United States and Canada. The eclipse will appear in its entirety just from the easternmost states, the moon rising perfectly full only to have the first black bite taken out of its bottom side soon afterward. Totality starts at 6:08 eastern time.
Only the westernmost states and provinces miss the total phase; their consolation is the strange spectacle of a moon rising partially eclipsed. You’ll see totality if your area’s moonrise occurs before totality’s end, at 7:22 eastern time. Of course, if the moon does blacken it may be hard to spot, a murky phantom in the eastern sky between the horns of Taurus.
Should clouds spoil the show, just two more total lunar eclipses will be widely visible over the United States for the remainder of the century. The next occurs in only a year, on November 29, 1993. After that we wait until September 27, 1996. But neither is likely to offer the tantalizing suspense of this, the great Pinatubo eclipse.