updated 4/22/2005 7:38:30 PM ET 2005-04-22T23:38:30

Just over two weeks after its dark silhouette crossed in front of the sun for parts of the Pacific and the Americas, it is the moon’s turn to undergo an eclipse.

Early on Sunday morning, April 24, the moon will quietly slip into the Earth’s outer shadow, known as the penumbra. While a penumbral eclipse is less dramatic than partial or total eclipses, avid skywatchers will be setting their alarm clocks early.

North and South America are in the best position to see this event.

In a penumbral eclipse, no part of the moon enters the dark umbral shadow of the Earth, so no part of the moon shows the distinct outline of the Earth's shadow. Since it will pass through the outer extremities of the Earth’s shadow, this is a pale eclipse that will do little to moderate the Moon’s light. That part of the moon closest to the much darker shadow of the Earth (called the umbra) may exhibit a sensible darkening, but it might not catch the eye.

The penumbral shadow is usually faint and difficult to perceive unless at least two-thirds of the Moon’s disk is immersed within it. Also, one edge of the moon should closely approach the much darker umbral shadow.

If seen from the moon, the Earth would appear to partly eclipse the sun.

In order to see it, Americans will have be up during the wee hours of the morning, as our timetable indicates. The best views will be for Alaska, Hawaii and western North America where the moon will be high in the sky. Farther east, the moon will appear lower in the sky and for the eastern U.S. and Canada, morning twilight will be in progress as the Full moon descends the western sky.

The timetable provides the moment when the moon will start to enter the penumbral shadow, but absolutely nothing unusual will be noticed on the lunar disk at that time. At the time of the darkest phase of the eclipse, the eclipse magnitude – that is, the percentage of the Moon’s diameter that is within the lighter penumbral shadow – will equal 89.0 percent.

The moon will be passing through the southern part of the Earth’s penumbra and its uppermost edge will miss touching the umbra by about only about 237 miles. Perhaps for about 25 minutes before and after this time, that uppermost portion of the moon should appear lightly "smudged" or shaded, especially through binoculars and low-power telescopes.

The eclipse will also be visible from eastern Asia, Indonesia, Australia and New Zealand, however for these regions, the eclipse is a Sunday evening event; the moon will already be within the penumbra as it rises, except for eastern Australia and New Zealand where the eclipse starts after moonrise. For the rest of the world, this event will happen during the daylight hours with the moon below the horizon, hence the eclipse will not be visible.

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