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The answer to ‘what is harvest moon’ is …drum roll please…the full moon that occurs closest to the autumnal equinox. This year(2011) the autumnal equinox will be on September 23, so the harvest moon will be on September 12th. The dates change from year to year. You should be able the correct dates for any year on NASA’s website.
All full moons rise close to sunset. The moon normally rises about 50 minutes later each day. At the time of the harvest moon, moonrise is shorter than usual. 30 minutes compared to 50. In the past, this shorter period of darkness was said to help farmers finish harvesting their crops. Aha, that is how the name came about. The reason for the shorter period between moonrises is that the ecliptic makes a narrow angle with respect to the horizon in the evening in autumn.
The harvest moon is the full moon closest to the autumnal equinox. It is simply the first 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.
Occasionally, the night of the harvest moon coincides with the night of the equinox. Then it is called a Super Harvest Moon. There was a Super Harvest Moon in lower 48 of the United States in 2010 for the first time since 1991.
The harvest moon seems to be bigger, brighter, and more colorful than other full moons. The color of the moon is caused by light refracting and reflecting from a greater amount of atmospheric particles. The atmosphere scatters the bluish component of moonlight, but allows the reddish component to travel a straighter path. The Moon appears larger because of the power of perception of objects along the horizon. This applies to many celestial objects, not just the Moon.
There you have it, ‘what is harvest moon’. You should read some of our articles about the phases of the Moon to get a clearer idea of what is going on with our only natural satellite.
We’ve also recorded an entire episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.