THE LIGHT OF THE HARVEST MOON.
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.