Eclipses, whether solar or lunar, occur because of the periodic alignments of the sun, Earth, and moon. These three bodies, orbit in space in very predictable paths (yes, the sun orbits too. It orbits the galaxy once every 200 million years!). Ever since the days of Kepler and Newton, we have been able to predict the motion of planetary bodies with great precision. So, why do eclipses happen?
Solar eclipses happen when the moon moves between Earth and the sun. You might think that this should happen every month since the moon’s orbit, depending on how it is defined is between about 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. Not much, you say? Yes, but the moon, itself, is only about ½ degree in width in the sky, about ½ the width of your pinky finger held at arm’s length. So, sometimes the moon misses too high and sometimes too low to cause a solar eclipse. Only when the sun, moon, and Earth line up close to the “line of nodes”, the imaginary line that represents the intersection of the orbital planes of the moon and Earth, can you have an eclipse.
This is true for both solar and lunar eclipses. This situation is somewhat unique as no other moon in the solar system orbits roughly in the plane of the “ecliptic”, Earth’s orbital plane, that the planets more or less follow.
The Moon orbits the Earth in the months prior to the August 21, 2017 total solar eclipse. Viewed from above, the Moon's shadow appears to cross the Earth every month, but a side view reveals the five-degree tilt of the Moon's orbit. Its shadow only hits the Earth when the line of nodes, the fulcrum of its orbital tilt, is pointed toward the Sun.
When the moon does eclipse the sun, it produces two types of shadows on Earth. The umbral shadow is the relatively small in diameter point on Earth where an observer would see a total eclipse. The penumbral shadow is the much larger area on Earth where an observer will see a partial eclipse. Here, the sun is not completely covered by the moon.
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NASA's Goddard Space Flight Center