A harvest moon is a full moon that occurs closest to the autumnal equinox. It is the first full moon closest to the equinox and once every four years it occurs in October. The harvest moon gets its orange color from the scattering of light from the atmosphere. When the moon is near the horizon, the moonlight has to pass through much more atmosphere than when the moon is directly overhead. The atmosphere then scatters the green, blue and purple components of the moonlight, which is reflected white light from the sun, and allows the reddish components of light to travel to the human eye, thus giving the harvest moon its red orange color.