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When Earth and the moon were very young, they were much closer together. The moon was so close, it took up a huge part of Earth’s sky. When it hung overhead in the daytime, it cast such a shadow on the surface of Earth that in many places it would have been almost like night.
Over Earth’s long history, the moon has drifted a lot farther away. Now it just so happens that the moon takes up almost exactly as much of the sky as the sun does. That is because the sun is 400 times wider (larger in diameter) than the Moon, but it is also 400 times farther away than the moon. Isn’t that awesome? What are the odds? Actually, nobody knows.
What this happy coincidence allows is a perfect total eclipse of the sun. That means the moon, when it is just in the right spot at the right time, can hide the Sun, exactly covering its disc. When this event happens, the sun’s corona, or atmosphere “pops out,” looking like a broad halo of fire.
The moon’s orbit around Earth is not an exact circle, however. Sometimes it’s a little closer to Earth than at other times. The closer it is, the bigger it looks to us. If a solar eclipse occurs when the moon is farthest from Earth, the moon looks smaller. This smaller looking moon does not quite cover the disc of the sun. This type of eclipse is called an annular eclipse. The Sun’s disc shows as a thin “ring of fire” around the edges of the moon.
What a great year for eclipses 2012 was! We had two solar eclipses: An annular eclipse on May 20 and a total eclipse on Nov. 13. These eclipses looked different depending on your location. If you were right in the moon’s shadow as it moved across Earth’s surface, you would see the annular or total eclipse. If you were outside the shadow you would have seen only a partial eclipse, with some or most of the sun still peeking out from “behind” the moon.
Find out more about solar and other kinds of eclipses, including the rare Venus transit of 2012 at spaceplace.nasa.gov/venus-transit.
This article was written by Diane K. Fisher and provided through the courtesy of the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.