Lunar Eclipses

For a lunar eclipse to occur, the Moon must be positioned behind the Earth so that the Earth lies directly between the Sun and Moon, causing the shadow of the Earth to fall on the Moon.

This diagram shows the relative positions of the Sun, the Earth, and the Moon (as seen from above) necessary for a lunar eclipse. Note the two different regions of shadow. http://www.skyandtelescope.com/

The shadow of the Earth consists of two different parts, the umbra and the penumbra. The umbra is that region behind the Earth where sunlight is blocked completely; in the penumbra, sunlight is only partially blocked. Due to this, three types of lunar eclipses are possible: total lunar eclipses, partial lunar eclipses, and penumbral lunar eclipses.

A total lunar eclipse occurs when the Moon passes completely through the umbra, and a partial lunar eclipse occurs when the Moon passes partially through the umbra. In contrast, a penumbral lunar eclipse occurs when the moon passes only through the penumbra.

The position of the Sun, the Earth, and the Moon (as seen from the side) for the three types of lunar eclipses. http://burro.cwru.edu/denise/Spring03/Jan23/Jan23.html

Looking at the diagrams for a lunar eclipse, it should be noted that the relative positions of the Sun, Earth and Moon are the same as those for a full moon. Why is it that sometimes an observer sees a full Moon from Earth and other times a lunar eclipse? To answer this question, we must remember that the orbit of the Moon is slightly inclined. Instead of orbiting Earth in the same plane in which the Earth orbits the Sun (the ecliptic plane), it orbits at an angle so that it is sometimes slightly above this plane and sometimes slightly below this plane. During the course of each revolution around the Earth, the Moon crosses the ecliptic plane exactly twice. These points at which the Moon crosses the ecliptic plane are called nodes. For a lunar eclipse to occur, the Earth must be positioned directly between the Sun and the Moon, and the Moon must be at a node.

The ecliptic plane is represented by the large, blue rectangle in the illustration above, and the Moon's orbital plane is represented by the smaller blue rectangles. This tilt in orbit is the reason we can see both the new moon and full moon phases and lunar and solar eclipses. A full moon occurs when the Moon is behind the Earth and not at a node. A lunar eclipse occurs when it is in this same position but also at a node so that the Earth's shadow falls upon it. http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap01/FG01_27.jpg

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