Its Brightness Enables Farmers to Gather in Their Crops During the
Night—The Natural Phenomena Which Make September “The Month of
Moonlight.”
September is “the month of moonlight.” Poets and impressionists at this
season of the year, have, from time immemorial, flooded the world with
harvest-moon imagery. Pictures of moonlight lovers strolling along
moonlit lanes, rowing on moonlit rivers, in moonlit boats, moonlight
gleaners and the harvest home have been painted over and over again in
word and color, but the why and wherefor of the extraordinary brilliance
of the Queen of Night during the period that she is known as the
“harvest moon” has been completely lost sight of by the great majority.
Those who observe the ordinary astronomical phenomena of daily
occurrence are familiar with the time variations in the moon’s rising
and setting. This is due to the direction of the moon’s apparent path
with reference to the horizon, of whatever place it is viewed from. Its
distance from the earth and its daily motion eastward in right
ascension.
For the first few days in every lunar month the moon rises or sets
twenty-three or twenty-four minutes later for three or four successive
evenings, after which the retardation varies from that time to an hour
and seventeen minutes, and sometimes more.
In the latitude of New York the maximum retardation is seventy-seven
minutes and the minimum is twenty-three.
When the retardation is a minimum at the time of the full moon, the
light is very powerful, and farmers have often taken advantage of the
practically all-night brilliancy for several days, to harvest their
grain. September 21 being the autumnal equinox and the full moon
occurring nearest that date being usually in the height of harvest
time, it is called the harvest-moon.
To understand the action of the causes which produce this phenomenon
it is necessary to remember that at the time of the autumnal equinox
the sun sets exactly in the west, and the southern half of the
ecliptic, or the sun’s apparent annual path in the sky, will then be
wholly above the horizon and the northern half entirely below; the
ecliptic, therefore, making the least possible angle with the horizon.
In high northern latitudes, as in Alaska, British Columbia, Norway and
Sweden and the north of Scotland, the moon’s path at such times is
almost parallel with the horizon, and for more than a week she rises
at very nearly the same time, giving the farmers ample light and time
to garner their crops.