If you could see all of Earth's shadow, it would be spectacular: an elongated cone of darkness measuring just under 8,000 miles across at its base and tapering away to a point 864,000 miles into space. Most of the time this zone of gloom vanishes into the void, but on May 15, observers in North America can see it nibble away at the full moon until the moon almost completely fades from view—a lunar eclipse.
The moon's orbit around Earth is tilted about 5 degrees relative to Earth's orbit about the sun, enough of a disparity to allow the moon to dodge Earth's shadow most of the time. As a result, people in North America usually get to see no more than one eclipse a year. Often they see none at all. This year most of them will get two; the other happens on November 8. Such a lunar double play happens only a few times in a century.
The pairing will give you a chance to discover that no two eclipses are alike. This month's event takes place while the moon is in Libra, one of the southernmost constellations of the zodiac. For viewers at mid-northern latitudes, the moon will appear low in the southeastern sky long after the twilight has faded. The November eclipse, in contrast, will kick in at sunset for observers along the East Coast, just as the moon is rising in the northeastern sky. Each eclipse takes on a different color, because Earth's shadow is not truly black. It is painted copper red by the world's sunrises and sunsets, which merge into a fiery ring of refracted light that filters into the sunless zone. The exact color of the shadow—and hence of the eclipsed moon—depends on atmospheric conditions along Earth's rim that day. So the combined effects of pollution, dust, and cloudiness can be seen on the moon's face. Scientists rate the darkness of a lunar eclipse from zero (almost invisibly dim) to four (bright orange) on the Danjon scale, named for French astronomer André Danjon.
The moon will begin to take on weird shapes as it plunges into shadow at 10:03 p.m. EST on the 15th. Even a casual glance will show you that the dark outline of Earth, tapered like an ice-cream cone to just 5,700 miles across, is more than twice the moon's diameter. Contrary to how it may look, our shadow does not invade the moon. Rather, the moon's relatively rapid west-to-east orbital motion overtakes Earth's shadow at 2,112 miles per hour. By coincidence, the moon is 2,160 miles wide, so the time from the first touch of darkness to full eclipse takes almost exactly one hour.
As more and more of the lunar surface goes dark, the coppery color of the shadow becomes easier to discern. Binoculars and camcorders easily capture the eclipse; no skill is required. Telescopes make things worse, because too much magnification obscures the effect. Lunar eclipses are the sky's vox populi, indifferent to expensive equipment and as clearly visible from a city as from the countryside. At 11:14 p.m. EST, the crescent-shaped remnant of sunlight disappears, and the moon becomes fully engulfed by Earth's far-flung shadow.
At mid-eclipse, the moon floats at the antisolar point, the spot in the sky opposite the sun. Civilizations throughout history considered this location special, and they were right. By day, the antisolar point is the center of every rainbow. By night, it is where all the outer planets (Mars and beyond) hover when they are nearest and brightest. It is where interplanetary dust grains faintly reflect sunlight back in our direction like a movie screen, creating a floating patch of light called the gegenschein. And on nights like May 15, the antisolar point is a spot where a cone of nothingness transmutes the moon into a circle of dark fire.
