Schematic diagram of the shadow cast by the Earth. Within the central umbra shadow, the Moon is totally shielded from direct illumination by the Sun. In contrast, within the penumbra shadow, only a portion of sunlight is blocked.

A lunar eclipse is an eclipse which occurs whenever the moon passes behind the earth such that the earth blocks the sun’s rays from striking the moon. This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle.  The type and length of an eclipse depend upon the Moon’s location relative to its orbital nodes.

An eclipse of the Moon (or lunar eclipse) can only occur at Full Moon, and only if the Moon passes through some portion of Earth’s shadow. That shadow is actually composed of two cone-shaped components, one nested inside the other. The outer or penumbral shadow is a zone where the Earth blocks part but not all of the Sun’s rays from reaching the Moon. In contrast, the inner or umbral shadow is a region where the Earth blocks all direct sunlight from reaching the Moon.

Penumbral Lunar Eclipse

The Moon passes through Earth’s penumbral shadow.These events are of only academic interest because they are subtle and hard to observe.

Partial Lunar Eclipse

A portion of the Moon passes through Earth’s umbral shadow.These events are easy to see, even with the unaided eye.

Total Lunar Eclipse

The entire Moon passes through Earth’s umbral shadow.These events are quite striking due to the Moon’s vibrant red color during the total phase .

One can wonder “If the Moon orbits Earth every 29.5 days and lunar eclipses only occur at Full Moon, then why don’t we have an eclipse once a month during Full Moon?”. The Moon’s orbit around Earth is actually tipped about 5 degrees to Earth’s orbit around the Sun. This means that the Moon spends most of the time either above or below the plane of Earth’s orbit. And the plane of Earth’s orbit around the Sun is important because Earth’s shadows lie exactly in the same plane. During Full Moon, our natural satellite usually passes above or below Earth’s shadows and misses them entirely. No eclipse takes place. But two to four times each year, the Moon passes through some portion of the Earth’s penumbral or umbral shadows and one of the above three types of eclipses occurs. Every year there are usually at least two partial lunar eclipses, although total eclipses are significantly less common. If one knows the date and time of an eclipse, it is possible to predict the occurrence of other eclipses using an eclipse cycle like the Saros cycle. Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth.

Unlike solar eclipses, lunar eclipses are completely safe to watch.

Links:

wikipedia

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