The full moon of September, called the Harvest Moon, will occur on Sept. 7. On the same night, there will also be a total lunar eclipse visible from many parts of Earth.
Full moons are reckoned when the moon is exactly on the opposite side of Earth from the sun – using the moon's celestial longitude (a projection of Earth's longitude lines on the sky) the moon has to be exactly 180 degrees from the sun. The moon's orbit is slightly tilted relative to the plane of Earth's orbit, by about 5 degrees, so usually the moon is fully illuminated by the sun at that point.
