The full Moon occurs when the Moon is at the point in its orbit on the side of the Earth opposite the Sun. Another way of describing this, is to say that the Earth is directly between the Sun and Moon when the Moon is full (fully lit). The Earth, like all objects upon which sunlight falls, the casts a shadow. This shadow extends into space in the Moon’s direction at full Moon. The Moon usually misses the Earth’s shadow by passing just a little above or below it at full Moon. But the Moon will pass through the Earth’s shadow this month in the early morning hours of April 15th. The Moon enters the penumbra (the lightest part of the Earth’s shadow) at 12:37am EDT, but the best viewing begins at about 2am when the Moon enters the umbra (the darkest part of the Earth’s shadow). Totality occurs when the Moon is completely inside the umbra from 3:06am until 4:27am, at which time it begins to re-enter the penumbra. This stage of the eclipse is the beginning of the Moon’s exit from the Earth’s shadow. The Moon completely exits the umbra at 5:30am, and the eclipse is completely over when the Moon exits the encumbrance at 6:30am.
There are two really neat things to note about the eclipse. First, the “sunset” effect. The portion of the Moon within the umbra will have a distinct reddish cast as a result of the Sun’s rays passing through the edges of the Earth’s atmosphere. As the eclipse progresses, the umbra can first be seen on the eastern edge of the Moon. Over the next hour or so, the portion of the Moon covered by the umbra will grow larger, until the Moon is completely engulfed by the umbra. The reverse will occur as the Moon slides back out of the umbra. Here’s the other neat thing to note. What the observer is actually watching as the Moon passes through the Earth’s shadow is the Moon moving along it’s orbital path around the Earth.