Because the moon's orbit is inclined five degrees to Earth's orbital plane, it periodically crosses over the ecliptic. If this happens when the moon is in the new phase—that is, when it's positioned between the Earth and the sun—it will cross directly in front of the sun and cause a solar eclipse.

If the moon crosses over the ecliptic when the moon is in the full phase—that is, when it's on the opposite side of the Earth from the sun, it will pass into the shadow of the Earth, resulting in a lunar eclipse.

Usually when the new moon is in the vicinity of the sun, it passes above or below it, and no eclipse occurs. Similarly, the full moon usually misses the Earth's shadow by sweeping above or below it. Only when all three bodies—sun, Earth, and moon—are in a straight line on the plane of the ecliptic can an eclipse occur. Hence the name ecliptic, the place where eclipses occur.