A solar eclipse occurs when the moon gets between Earth and the sun, and the moon casts a shadow over Earth. A solar eclipse can only take place at the phase of new moon, when the moon passes directly between the sun and the Earth and its shadows fall upon the Earth’s surface. The moon formed about 4.5 billion years ago, it has been gradually moving away from the Earth (by about 1.6 inches, or 4 centimeters per year). Right now the moon is at the perfect distance to appear in our sky exactly the same size as the sun. Depending on where it is at in its orbit, the eclipse can be a total or annular eclipse. The next total solar eclipse in the United States will be August 21, 2017.