The Harvest Moon

Autumnal moonlight was a highly valued commodity before the invention of electric lights, particularly in the northern parts of the world when crops had to be gathered from the fields before the first frosts. The Moon's extra light allowed workers to harvest into the evening hours, long after the Sun had set. In fact, this Moon has come to be known as the Harvest Moon.

By loose definition, the Harvest Moon is the full Moon nearest the autumnal equinox. Most years, the title goes to the full Moon in September, since the equinox occurs around September 22. Sometimes, however, the interval of days between the full Moon in September and the autumnal equinox is greater than that of the equinox and the full Moon in October (as was the case in 1998). In such a case, then, the October full Moon is decreed the Harvest Moon.

Some people insist on calling the October Moon the Harvest Moon, no matter where it falls in relation to the equinox. The October Moon, however, is more properly known as the Hunter's Moon because, according to lore, its extra light helps hunters track animals at night among the husks and chaff of harvested fields.

At any rate, you may well ask how the Moon can provide 'extra' light around the equinox. The answer lies both in the clockwork motions of the solar system and the Earth's 23.3° tilt.

Harvest moonrise paths from various latitudes.


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In autumn, the setting Sun is at the autumnal equinox, a point in the sky that lies halfway between its northernmost and southernmost positions for the year. In the meantime, on the opposite side of the sky the rising full Moon is at the vernal equinox, or where the Sun will be on the first day of spring. As it so happens during this time of year, the ecliptic -the apparent annual path across the sky followed by the Sun and Moon -is more inclined toward the horizon the further north you go. At latitude 30°, for example, the ecliptic makes a 45° angle with the eastern horizon, but at latitude 50° that angle shrinks to 20°.

The shrinking ecliptic angle means the Moon's eastward motion does not take it as far below the eastern horizon from day to day as it does at other times of the year. The effect is particularly noticeable at high northerly latitudes. At latitude 30° N, the daily difference in moonrise times around Harvest Moon is about 40 minutes. At 50°, the difference dwindles to about 30 minutes, and at 60° the difference amounts to only 20 minutes.

Thus, there is less difference in moonrise times from night to night for about a week at latitudes above 40° N around the autumnal equinox. This means that the Moon is up in the sky at nearly the same time on successive evenings and is in the sky all night, providing extra light.

If a shallow ecliptic angle can create a Harvest Moon phenomenon in the Northern Hemisphere, a similar Harvest Moon effect must also occur in the Southern Hemisphere for the same reason. Indeed it does, but it occurs in March (i.e., southern autumn), when the Moon is at the autumnal rather than the vernal equinox, and that part of the ecliptic is most inclined toward the eastern horizon.

As in the Northern Hemisphere, the effect is more pronounced the further south you go. In places like Sydney and Melbourne (33°S and 38° S, respectively) successive full Moon rises differ by about 30 to 35 minutes. At latitude 42° S, the difference amounts to about 25 minutes. To really notice the Moon rising each night at nearly the same time, however, you'd have to be on a ship at sea or standing on Antarctica. At latitude 60° S, for example, the difference in consecutive moonrises is only 20 minutes.

The Harvest Moon is not strictly an American tradition. In Chinese communities in the United States, the Harvest Moon festival is a significant holiday that, apparently, can even influence death rates. Carl Sagan, in his book The Demon Haunted World, cites one study that showed that deaths in the Chinese community fell by 35 percent during the week preceding the festival but jumped by that amount the following week. A close examination of the data found that the fluctuation in the death rate occurred mostly among women 75 years and older. Traditionally, the Chinese Harvest Moon festival is presided over by the oldest women of the house. Apparently, these women managed to ward off death in some psychosomatic way for a couple of weeks in order to fulfill their ceremonial duties. Such is the control of the Moon in our lives.

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