The Harvest Moon comes soon before or soon after the autumnal equinox. It is simply the full moon closest to that equinox. About once every four years it occurs in October (in the northern hemisphere), depending on the cycles of the moon. Currently, the latest the Harvest Moon can occur is on October 7.
In 2010, in the United States, the Harvest Moon happens in the early morning hours of Sept 23, only 5 1/2 hours after the autumnal equinox.
Often, the Harvest Moon seems to be bigger or brighter or more colorful than other full moons. The warm color of the moon shortly after it rises is caused by light from the moon passing through a greater amount of atmospheric particles than when the moon is overhead. The atmosphere scatters the bluish component of moonlight (which is really reflected white light from the sun), but allows the reddish component of the light to travel a straighter path to one's eyes. Hence all celestial bodies look reddish when they are low in the sky.
The apparent larger size is because the brain perceives a low-hanging moon to belarger than one that's high in the sky. This is known as a moon illusion and it can be seen with any full moon. It can also be seen with constellations; in other words, a constellation viewed low in the sky will appear bigger than when it is high in the sky.
photographs by Edouard Plongeon
officially a full moon.