The Moon's dance around the Earth and Sun paints a different face every night. In the early morning of Sunday Dec. 30, one usually invisible transformation will reveal itself, when the Moon dips its southern edge into the Earth's shadow. This will make a dramatic penumbral eclipse; a subtle intersection of the Earth and Moon, seen in the Americas, across the Pacific to Eastern Asia.
"If casual observers get up at 5, they'll notice the lower left-hand side of the Moon is dimmer than the upper right hand side," said Fred Espenak, an eclipse expert at NASA's Goddard Space Flight Center.
The effect will be the most subtle of three types. There are a variety of eclipses because the Earth throws two shadows. There is the darker umbra, the exact silhouette of the Earth carved out of the sunlight. Then there is the penumbra, which is a larger cone of diffuse shade, where the Sun's light and the Earth's shadow mix.
In a penumbral eclipse, the Moon passes through the partial shadow of Earth. The subtle change in lighting on the Moon is usually visible only with a telescope. An umbral eclipse on the other hand, is quite noticeable and varies in degree by how much the Moon drops behind our planet. A total eclipse is the most dramatic and rare, when the moon is fully behind the Earth.
Although Espenak said a penumbral lunar eclipse is, "the least scientifically interesting," he did agree this particular event will be special because of its strength.
"This one, as far as penumbral eclipses, is a very deep one," he said. "The Moon will move very deeply into penumbral shadow, almost to the edge of umbra."
The moon will dip itself 92 percent of the way inside the Earth's penumbral shadow at 5:29 a.m. EST, 2:29 a.m. PST.
The way the Earth and Moon gyrate around each other, with the Moon completing its orbit around the Earth every 29.5 days, it may at first seem like a lunar eclipse should happen about once a month. Instead, they occur roughly 3 times a year.
This is because the plane on which the Moon orbits, is tilted nearly five degrees from the plane of the Earth's orbit around the Sun, Espenak said. The Moon skips over and under the Earth's shadow more easily than if the two orbs were spinning on equal footing.
For the Dec. 30 eclipse though, the moon is going to get caught.
"If you were to view the Moon from space, just its lower edge will be passing through the Earth's shadow," Espenak said.
After this, the next penumbral eclipse will happen on May 26, 2002 when the 72 percent of the Moon enters the Earth's penumbra, and will be invisible to the unaided eye.
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