A new NASA video explains what determines when Earth experiences solar eclipses — and why they don't happen every month, as the moon crosses between the sun and the Earth.
To prepare for the 2017 total solar eclipse, which will cross the continental United States on Aug. 21, NASA illustrated how the moon orbits the Earth and why the natural satellite's shadow sometimes lines up just right to move across Earth's surface, effectively blotting out the sun for a short period of time.
Because the moon's orbit wobbles up and down with respect to the Earth, the satellite's shadow can be too high or too low to cause a solar eclipse. If the moon partially covers the sun for a given location, that area experiences a partial solar eclipse. But if the moon fully covers the sun, it will cause either a total solar eclipse or an annular solar eclipse; that's when a bright ring of light is still visible in the sky around the dark silhouette of the moon.