`There is a very prevalent opinion among the lower and uneducated classes that the Harvest Moon always occurs at the time of harvest, let that happen when it may. It is needless, however, to remark that such an erroneous idea can only proceed from persons of an entirely ignorant turn of mind - it being childish to suppose that a change of the moon could in any way influence such an occurrence."
Now we may feel uneasy at the turn of phrase that flowed so trippingly from the pen of Rev Thiselton Dyer in his English Folk-lore, written in 1878, but we must defend to the death the logic of his main thrust. The harvest moon, which you will see tonight just after sunset if the skies are clear, has acquired its name merely because it is a full moon which usually occurs at harvest time.
The attention paid to a full moon at this time of year is merited by its regularity and by its very bright appearance. Moonrise throughout the lunar cycle occurs a little later each day by an amount of time known as the "retardation". The mean value of the retardation is about 50 minutes, but it varies; it is at its maximum of well over an hour around the time of the vernal equinox, and at a minimum in the autumn, so that at this time of the year, to the casual observer, the moon appears to rise at almost the same time for several successive nights.
The harvest moon, traditionally the full moon nearest to the autumnal equinox around September 22nd, displays this characteristic, the moon almost seeming to take over from the sun as it illuminates the early evening sky. It was the custom in olden times for farmers to take advantage of this moonlight by continuing to work after sunset to gather in their crops.
Nothing is as simple as it seems. Although tonight's full moon is the one closest to the autumn equinox, and therefore 2001's harvest moon by definition, some authorities on this important matter prefer to think of the harvest moon as the full moon occurring in September - which this year would have been the one a month ago, on September 2nd. By their reckoning, bearing in mind that in olden times, once the harvest had been gathered in, the hunting season started, tonight's would be the hunter's moon.
And then, to complicate the issue further, the fullness of the moon, astronomically speaking, occurs around 3 o'clock tomorrow afternoon - so perhaps harvest moon, or hunter's moon if you prefer, is the moon tomorrow night, and not tonight.