A lunar eclipse occurs when Earth passes between the Sun and the Moon, casting a shadow on the Moon. A similar action occurs when a person (representing Earth) stands in front of a lighted lamp (the Sun) and someone else (the Moon) walks behind that person and into their shadow. The event called a lunar eclipse is different than a solar eclipse, which occurs when the Moon passes between the Sun and Earth, preventing the Sun's light from reaching planet Earth. One way to remember the difference is that one can only witness a lunar eclipse at night, when the Moon is up in the sky, whereas a solar eclipse occurs during the day, when the Sun is up.
The term "eclipse" means literally the complete or partial blocking of a celestial body by another. Eclipses on Earth happen only when the Sun, the Moon, and Earth are all positioned in a straight line. This situation does not occur often because the plane of Earth's orbit around the Sun is at a different angle from the plane of the Moon's orbit around Earth. Therefore, the Moon is usually located just above or below the imaginary line connecting the Sun and Earth. Only about every six months do the planes of Earth, the Moon, and the Sun all intersect, creating the conditions needed for an eclipse. Thus, a lunar eclipse happens on Earth at least twice each year.
A lunar eclipse can only occur during a full moon, when the Moon lies behind Earth, opposite the Sun, and is fully illuminated. As the Moon crosses into Earth's umbra (the dark, core area of its shadow), it does not become totally hidden. The reason is that molecules of gas in Earth's atmosphere cause the Sun's light to bend around the surface of the planet. Some light still reaches the Moon, giving it a reddish appearance.
If the entire Moon falls within the umbra, the result is a total lunar eclipse. If only part of the Moon passes through the umbra, or if it only passes through the penumbra (the lighter shadow region surrounding the umbra) a partial lunar eclipse occurs. A partial lunar eclipse may be difficult to detect because the Moon dims only slightly.
A total lunar eclipse occurs in stages. As the Moon moves first into Earth's umbra, one edge of the Moon begins to darken. Gradually, the umbra covers the whole Moon and then recedes, leaving a full moon once again.
Lunar eclipses are more common than solar eclipses. With a solar eclipse, the Sun, the Moon, and Earth have to be in nearly perfect alignment. The reason that perfect alignment is necessary is that Earth, the Sun and the Moon all appear to be about the same size in the sky. Thus, if the Moon lies directly in the Sun's path, it will block out the sunlight, and a total eclipse occurs. If the Moon is even slightly above or below the line connecting the Sun and Earth, no more than a partial eclipse will result. Lunar eclipses, however, are a different story. Earth is relatively close to and large compared to the Moon. Thus, any shadow cast by Earth will at least partly cover the Moon.
A lunar eclipse also lasts longer than its solar counterpart. When the sky is clear, a lunar eclipse can be viewed all night. A solar eclipse, on the hand, lasts only a few minutes. Also unlike a solar eclipse, which is only visible along a narrow strip of Earth's surface, a lunar eclipse can be seen from everywhere on the planet where it is nighttime.