During a total solar eclipse, three key conditions happen at the same time: The moon is in the "new moon" phase; the moon crosses the plane of the Earth's orbit; and the moon is at its closest point to Earth in its orbit.

When those conditions align, the Earth, sun, and moon line up. Then, if you're in the path of totality, the moon appears to mask the sun.

The moon orbits Earth every 29.5 days. But the moon is not in line with Earth's orbit relative to the sun, which is why we don't have a solar eclipse every month. The moon's orbit is tilted about five degrees, which is large enough to keep its shadow off the Earth and the Earth's shadow off the moon most of the time.