when the new moon passes between the Earth and the sun.
When the moon is farther out in its orbit around the Earth, then it's disk is too small to totally cover the sun - and we get an annular eclipse. When the moon is closer to the Earth, then it's large enough to cover the sun's disk for a total eclipse.
Solar eclipses occur when the Moon passes between the Sun and Earth, casting a shadow on Earth and blocking the Sun's light.
