The Shadow of the Earth
     As shown in the diagram below, two lines can be drawn from the bottom of the Sun, past the Earth, to define a region where the Earth blocks any view of the bottom of the Sun; and similarly, two lines can be drawn from the top of the Sun, past the Earth, to define a region where the Earth blocks any view of the top of the Sun. Where those overlap, on the night side of the Earth, is the Earth's umbra (Latin for shadow), a region where neither the top nor the bottom of the Sun can be seen (nor, of course, any part in between). Where they do not overlap, is the Earth's penumbra (Latin for partial shadow), a region where part of the Sun is visible, and part of it is blocked from view by the Earth. In the upper part of the penumbra, the Earth is in front of the bottom of the Sun, but an observer could look over the top of the Earth, and still see the top of the Sun; while in the lower part of the penumbra, the Earth is in front of the top of the Sun, but an observer could look under the bottom of the Earth, and still see the bottom of the Sun.

     The Earth's umbra is a cone-shaped region, as large as the Earth at the center of the Earth, and gradually decreasing in size as you move away from the Earth, until -- at a distance of about a million miles, or four times the distance to the Moon -- you reach the tip of the cone. At that place, the Earth appears to be exactly the same size as the Sun, and in exactly the same direction as the Sun, and just barely covers it completely. Moving toward the Earth, the Earth looks larger and larger, and the region where the Sun is completely covered grows from a point, to a circle about one percent of the size of the distance from the tip. Thus, 100000 miles in front of the tip of the umbra, the region where the Earth completely covers the Sun is about a thousand miles in diameter, and at the Moon's distance from the Earth, which is about 750000 miles in front of the tip of the umbra, the shadow is about 7500 miles in diameter, and nearly three times the size of the Moon. Since the Earth's umbra is so much larger than the Moon, during a total lunar eclipse, the entire Moon can be covered by the umbra for more than two hours.

Total Lunar Eclipses
(discussion to follow)

The total lunar eclipse of February 20/21, 2008 (NASA, GSFC, Fred Espenak)

During a lunar eclipse, the eastward motion of the nearly "full" moon carries it through the shadow of the Earth. While it is in the outer shadow, or penumbra, at least part of the Sun is visible to anyone on the sunward side of the Moon, and the Moon looks little different than usual; but as it approaches the central shadow, or umbra, less and less of the Sun is visible from those parts of the Moon near the umbra, and the Moon begins to look slightly fainter, as though there is a faint haze or shadow obscuring our view. When part of the Moon enters the umbra, the "partial" phase of the eclipse begins. For the next hour, more and more of the Moon enters the umbra, and a larger and larger circular "chip" is taken out of the eastern side of the Moon. Since that part of the Moon receives no direct sunlight, it is much darker than the region still outside the umbra, and casual examination makes it appear completely dark; but once all of the Moon is inside the umbra (and the eclipse becomes total), the fact that some sunlight filters through the Earth's atmosphere, and illuminates the eclipsed Moon becomes obvious, and the Moon takes on a ruddy hue, characteristic of the ruddy hue of the setting or rising Sun. (far more to follow)

Partial Lunar Eclipses
(discussion to follow)

A whimsical look at the partial lunar eclipse of September 7, 2006 (Laurent Laveder, PhotoAstronomique.net, apod060909)

In a partial lunar eclipse, the Moon crosses above or below the center of the Earth's shadow, so that only a portion of its surface is covered by the central, darker shadow, or umbra, and the rest is covered only by the outer, lighter shadow, or penumbra. That portion of the Moon covered by the umbra is lit only by light refracted through the Earth's atmosphere, and appears pitch black in comparison to that portion covered by the umbra, which is lit by part or most of the uneclipsed Sun.      Where the Earth's shadow passes through empty space, it is not visible; but in this image, eclipse enthusiasts enjoying the view of the partially eclipsed Moon arranged themselves so that one of them, by holding a hoop just above the Moon, was able to simulate the position of the umbra. The two shadow regions were appropriately darkened and labeled by image-processing software.      Eclipses of the Moon can be differentiated by the depth of the eclipse, which represents the portion of the lunar diameter which is inside the umbra, at greatest eclipse. If the depth is negative, none of the Moon enters the umbra, and we have a penumbral eclipse If it is positive, but less than 1, part of the Moon enters the umbra, but not all of it, and we have a partial lunar eclipse, as shown here, where the depth is 0.19, meaning that 19% of the Moon's diameter was inside the umbra, at greatest depth (also the moment this image was taken). If the depth is more than 1, the entire Moon is inside the umbra at some point, and we have a total lunar eclipse.

Penumbral Lunar Eclipses
(discussion to follow)

An animation of the March 14, 2006 penumbral eclipse of the Moon (Larry Koehn, shadowandsubstance.com)

In a penumbral eclipse, the Moon does not enter the Earth's umbra at all, but sweeps through the penumbra above or below the umbra. Areas on the Moon which are very close to the umbra may become noticeably darker, but areas further away look essentially the same as at any other full moon. Just inside the outer edge of the penumbra, someone on the Moon would see the Earth just barely covering a small part of the Sun, so the Sun's light is hardly dimished; while just outside the umbra, someone on the Moon would see the Earth almost entirely covering the Sun, so the Sun's light is considerably diminished.