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You might think that an eclipse should happen every month since the Moon's orbit, depending on how its defined is between 27 and 29 days long. But our Moon's orbit is tilted with respect to Earth's orbit around the Sun by about five degrees. So, sometimes the Moon's shadow misses too high and someimes too low to cause a solar eclipse. Only when the Sun, Moon and Earth line up close to the "lines of node," the imaginary line that represents the intersection of the orbital planes of the Moon and Earth, can you have an eclipse.
For more on how an eclipse works and different types of eclipses, visit NASA's ECLIPSE 101 page, part of their Eclipse2017 site.
NASA's Goddard Space Flight Center provided this (0:45) video.
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