Many of us have heard about the "Harvest Moon." Astronomers define Harvest Moon as the full Moon nearest the autumnal equinox (the beginning of autumn). Why does it have special importance? Read on . . .
The Usual Moon
The usual behavior of the Moon is to rise distinctly later each night -- an average of about 50 minutes later. This is because the Moon's orbital motion (combined with the larger orbit of the Earth around the Sun) carries it farther eastward among the constellations of the zodiac from night to night. At any one moonrise, the Moon occupies a particular place on the celestial sphere (the great dome of the heavens), but when the Earth turns toward that point 24 hours later, the Moon has moved off to the east about 12 degrees, and it takes an average of 50 minutes longer for the Earth to rotate toward the Moon and for the Moon thus to "rise." Think of it as a giant Slinky in which each loop, representing one lunar orbit of the Earth, advances the orbit a bit farther along the spiral path.
The Harvest Moon
But around the date of the Harvest Moon, the Moon rises at almost the same time for a number of nights in our intermediate northern latitudes. Why is the Harvest Moon different? Well, remember that the zodiac is the band of constellations through which the Moon travels from night to night. The section of the zodiac band in which the full Moon travels around the start of autumn is the section that forms the most shallow angle with the eastern horizon.
Because the Moon's orbit on successive nights is more nearly parallel to the horizon at that time, its relationship to the eastern horizon does not change appreciably, and the Earth does not have to turn as far to bring up the Moon. Thus, for several nights near the full Harvest Moon, the Moon may rise as little as 23 minutes later on successive nights (at about 42 degrees north latitude), and there is an abundance of bright moonlight early in the evening, a traditional aid to harvest crews. By the time the Moon has reached last quarter, however, the typical 50-minute delay has returned.
By contrast, the full Moon near the start of spring is in the section of the zodiac that has the steepest angle with respect to the eastern horizon, and the opposite applies. For several days bracketing the full Moon nearest the vernal equinox, the delay in moonrise is as much as 75 minutes (at 42 degrees north latitude).
Here is another way of expressing what happens with the Harvest Moon: It is in this part of the zodiac that the Moon's eastward (orbital) motion has its largest northward component. For observers in Earth's Northern Hemisphere, the farther north an object is in the heavens, the longer an arc it makes across the sky, and the longer a time it is visible above the horizon. Thus, to say that the Moon is getting rapidly farther north each night around the time of the Harvest Moon is to say that, for northern latitudes on Earth, it will keep rising distinctly earlier than would otherwise be expected -- nearly the same time as the night before.
How nearly the same is "almost the same time" each night? This varies with latitude, for the farther north you are, the shallower the angle of the zodiac is with respect to your horizon. In most of the United States and southern Canada, the Harvest Moon rises 25 to 30 minutes later each night. The effect is less noticeable the farther south you go. But going north makes the Harvest Moon more extreme. According to astronomy author Guy Ottewell, the idea of the Harvest Moon originated in Europe (average latitude about 50 degrees north), where the Harvest Moon rises only ten to 20 minutes later each night. It must have seemed a boon that just when days were getting rapidly shorter and the Sun seemed to go down all too soon, the Harvest Moon arrived to extend the hours that harvesting could be done.
Enjoy a Mooncake!
As a final note, I should add that it is not just western civilization that has given special importance to the Harvest Moon. This Moon is for Chinese people everywhere the occasion for the Festival of the August Moon (the "August" is through a calendar discrepancy) or Mid-Autumn Festival (in some cultures, the equinoxes and solstices have been considered the middle of the seasons). This festival is celebrated with joyful games and the eating of "Mooncakes." I remember vividly being invited to one such celebration and singing songs and playing my guitar to a circle of friendly faces in the light of the rising Harvest Moon.