Skywatching Columnist - The moon of Wednesday, Sept. 26 also carries the title of the Harvest Moon for those living in the northern hemisphere.
The moon officially turns full when it reaches that spot in the sky diametrically opposite (180 degrees) to the sun in the sky. This moment will occur on Wednesday at 19:45 Greenwich Time (3:45 p.m. EDT or 12:45 p.m. PDT). Wednesday's full moon is the one that comes the closest to the September equinox so this year it falls in September, although in one out of three years this title can be bestowed upon the October full moon (as was the case in 2006).
Many think that the Harvest Moon remains in the night sky longer than any of the other full moons we see during the year, but that is not so.
What sets Wednesday's full moon apart from the others is that farmers - at the climax of the current harvest season - can work late into the night by the moon's light. It rises about the time the sun sets, but more importantly, at this time of year, instead of rising its normal average 50 minutes later each day, the moon seems to rise at nearly the same time each night.
In analyzing local moonrise times for Sept. 25, 26 and 27 in 10 locations across North America, the rising of the moon comes, on average, less than 27 minutes later each night. The night-to-night difference is greatest for the more southerly locations. (Miami, located at near latitude 26-degrees N., sees moonrise come an average of 37 minutes later). Meanwhile, the difference is less at more northerly locations (at Edmonton, Alberta, located at latitude 53.6-degrees N, the average difference is just 12 minutes).
The reason for this seasonal circumstance is that the moon appears to move along the ecliptic, and at this time of year when rising, the ecliptic makes its smallest angle with respect to the horizon for those living in the Northern Hemisphere.
In contrast, for those living in the Southern Hemisphere, the ecliptic at this time of year appears to stand almost perpendicular (at nearly a right angle) to the eastern horizon. As such, the difference for the time of moonrise exceeds the average of 50 minutes per night. At Sydney, Australia, for instance, the night-to-night rise time difference amounts to about 71 minutes.
The Harvest Moon is the full moon nearest to the autumnal equinox, which occurs (in the northern hemisphere) on or about September 23rd, and in the southern hemisphere on or about March 21st. Its physical characteristics - rising time, path across the sky - are similar to those of the Hunter's moon.
All full moons have their own special characteristics, based primarily on the whereabouts of the ecliptic in the sky at the time of year that these moons are visible. The full moons of September, October and November as seen from the northern hemisphere - which correspond to the full moons of March, April and May as seen from the southern hemisphere - are well known in the folklore of the sky. 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. In other words, 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 (or Hunter's) 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 - or 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 can come before or after the autumnal equinox. It is simply the full moon closest to that equinox. About once every four years it occurs in October, depending on the cycles of the moon. Currently, the latest the Harvest Moon can occur is on October 8. Between 1900 and 2010 the Harvest Moon falls on October 7 in 1930, 1949, 1987, 2006, and on October 8 in 1911.
Many cultures celebrate with gatherings, festivals, and rituals that are intricately attuned to the Harvest Moon or Hunter's Moon.
It is claimed by some that the Harvest Moon seems to be somehow bigger or brighter or yellower in color than other full moons. This is an illusion. The yellow or golden or orangish or reddish color of the moon shortly after it rises is a physical effect, which stems from the fact that, when you see the moon low in the sky, you are looking at it through a greater amount of atmosphere than when the moon is overhead. The atmosphere scatters the bluish component of white moonlight (which is really reflected sunlight) but allows the reddish component of the light to travel a straighter path to your eyes. Hence all moons (and stars and planets) look reddish when they are low in the sky.
As for the large size of a full moon when seen low in the sky, it is true that the human eye sees a low hanging moon as being larger than one that rides high in the sky. This is known as a Moon Illusion and 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.
The Harvest Moon is also known as the Wine Moon, the Singing Moon and the Elk Call Moon. In myth and folklore the full moon of each month is given a name. There are many variations but the following list gives the most widely known names:
January - Wolf moon
February - Ice moon
March - Storm moon
April - Growing moon
May - Hare moon
June - Mead moon
July - Hay moon
August - Corn moon
September - Harvest moon
October - Hunter's moon
November - Snow moon
December - Winter moon
The third full moon in a season with four full moons is called a blue moon, as described in the Maine Farmer's Almanac. Until recently it was commonly misunderstood that the second full moon in a month was the blue moon. However, it was recently discovered by Sky and 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 and Telescope Magazine dating back to 1946 and then perpetuated by other media.