An eclipse, whether solar or lunar, is defined by the placement of the celestial bodies. While a solar eclipse depends on the placement of the moon, a lunar eclipse depends on the placement of the earth.
How does a lunar eclipse occur?
The moon, constantly moving around the Earth at a rate of 2,300 miles per hour, completes one full rotation in 27.3 days and is called a lunar cycle, but because of the Earth's simultaneous revolution around the sun, a full lunar cycle appears to take 29.5 days to complete. The moon is roughly 238,855 miles from Earth and has a diameter of 2,159 miles. If our planet were described as the size of a basketball, then the moon is comparable to the size of a tennis ball. While the moon orbits the Earth, the Earth orbits the sun and it is this spherical movement that causes a lunar eclipse.
The moon receives its illumination from the sun. The side facing the sun is lit up and the opposite side is dark. While the side that reflects the sun's rays is heated to 243 degrees Fahrenheit, the dark side has a temperature of -272 degrees Fahrenheit. One side of the moon typically faces the sun for about two weeks before its revolution places the alternative side near the sun, causing it to be heated while the other side cools creating a constant cycle of heating and cooling. Lunar eclipses typically occur in clusters of two to three a year when the Earth falls between the sun and the moon and a shadow is cast across the illuminated face of the moon. Following a cluster of lunar eclipses, two or three years may pass without any before another cluster begins.
Types of Lunar Eclipses
Dependent upon the placement of the moon, sun, and Earth, three types of lunar eclipses can occur.
The first, a Penumbral Lunar Eclipse occurs when the moon passes through the Earth's penumbral shadow, penumbra meaning the imperfect shadow in which the sun is only partly cut off. This sort of eclipse is often only visible with the aid of high-powered astronomical viewing devices and typically only observed by members of the astronomical community.
The second type of lunar eclipse is called a Partial Lunar Eclipse and is defined by a portion of the moon passing through the Earth's umbra, which is a perfectly formed shadow in which the sun's light is completely cut off. As such, this sort of eclipse is readily visible to the naked eye.
The final type of eclipse is called a Full Lunar Eclipse in which the moon in its entirety passes through the Earth's umbra, which creates a complete shadow across the moon causing it to seem to disappear from the sky.
Depending on the refraction of the sun's rays through the Earth's atmosphere, the eclipse will sometimes cause the moon to cast off a red, brown, or orange hue. Because an eclipse depends on the placement of the aforementioned bodies, meteorologists can often predict the dates and best viewing times.
