In this video, Exploratorium Senior Scientist Paul Doherty explains why we don't have a total solar eclipse every month. A total solar eclipse happens when the moon crosses between the sun and Earth and casts its shadow onto our planet. But the orbit of the moon is tilted relative to the orbit of the Earth around the sun, so the moon often passes below or above Earth. At those times, it does not cross the line between the sun and the Earth, and therefore does not create a solar eclipse. There are just two times a year in the Earth's orbit when there is a possibility of a total solar eclipse.