The harvest moon is the moon at or about the period of fullness that is nearest to the autumnal equinox. The harvest moon is often mistaken for the modern day hunter’s moon.

All full moons rise around the time of sunset. However, although in general the moon rises about 50 minutes later each day, as it moves in orbit around Earth, the Harvest Moon and Hunter's Moon are special, because around the time of these full moons, the time difference between moonrise on successive evenings is shorter than usual which means that the moon rises approximately 30 minutes later, from one night to the next, as seen from about 40 degrees N. or S. latitude, for several evenings around the full Hunter's or Harvest Moons. Thus there is no long period of darkness between sunset and moonrise around the time following these full moons. In times past this feature of these autumn moons was said to help farmers working to bring in their crops (or, in the case of the Hunter's Moon, hunters tracking their prey). They could continue being productive by moonlight even after the sun had set. Hence the name Harvest Moon. The reason for the shorter-than-usual rising time between successive moonrises around the time of the Harvest and Hunter's Moon is that the ecliptic —the plane of Earth's orbit around the sun—makes a narrow angle with respect to the horizon in the evening in autumn.

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. Often, the Harvest Moon seems to be bigger or brighter or more colorful than other full moons. These effects are related to the seasonal tilt of the earth. 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 clelestial bodies look reddish when they are low in the sky.
The apparent larger size is because the brain perceives a low-hanging moon to be larger 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.

January – Wolf moon, Hunger moon, Old moon
February –Snow moon, Ice moon
March – Worm moon, Sap moon, Sugaring moon, Crow moon, Storm moon
April – Pink moon, Egg moon, Grass moon, Rain moon, Growing moon, Wind moon
May – Flower moon, Planting moon, Milk moon, Hare moon
June – Strawberry moon, Rose moon, Honey moon, Mead moon
July – Buck moon, Thunder moon, Deer moon, Hay moon
August – Sturgeon moon, Corn moon, Fruit moon, Barley moon
September – Harvest moon
October – Hunter’s moon
November – Beaver moon, Frosty moon, Snow moon
December – Cold moon, Long Night moon, Winter moon

The third full moon in a season with four full moons is called a blue moon, as described in the Maine Farmers’ Almanac. Until recently it was commonly misunderstood that the second full moon in a month was the blue moon. However, it was recently discovered bySky & Telescope magazine and reported on NPR that the interpretation of a blue moon as the second full moon of the month was erroneously reported in an issue of Sky & Telescope dating back to 1946 and then perpetuated by other media.
In modern usage, when a second full moon occurs during any calendar month, it is called a Bblue Moon.
In some cultures, individuals whose birthdays fall on or near a harvest moon must provide a feast for the rest of the community.
Folklore gave each moon a name according to its time of year. A moon that came too early had no folk name, and was called a blue moon, retaining the correct seasonal timings for future moons.
The Farmers’ Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.
Recent popular usage defined a blue moon as the second full moon in a calendar month, stemming from an interpretation error made in 1946 that was discovered in 1999. For example, December 31, 2009 was a blue moon according to this usage.
A "blue moon" is also used colloquially to mean "a rare event", reflected in the phrase "once in a blue moon".
Blue Moon is different from the monthly or seasonal moon names as it isn't restricted to a time of year. It is a movable feast.
At least once during August or September this year [1993], you probably heard through the media about the "blue moon." People have been saying that "according to folklore" the second moon in a calendar month is a "blue moon." So, they say, this is the origin of the phrase "once in a blue moon." Don't believe them! "Once in a blue moon" is old, about 150 years old, but the two-full-moons-in-a-month meaning of "blue moon" is a lot younger than that. The older meaning may be wishy-washy and the newer one solid and technical, but don't let anyone tell you they have replaced one with the other.

Naomi Sherer