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September 2002:The Harvest Moon

The Harvest Moon refers to the Full Moon that comes closest to the autumnal equinox, which arrives annually on or near September 23. Depending upon the year, the Harvest Moon can come anywhere from two weeks before or after this date. This year, the Harvest Moon reaches full phase on September 21.

The autumnal equinox occurs at the moment that the Sun passes directly overhead at the Earth's equator on its way from the Northern to the Southern Hemisphere. Coming at 12:55 a.m. on September 23, it's actually the spring equinox for people living south of the equator, so the term September equinox better applies on a worldwide scale. In the Southern Hemisphere, the Harvest Moon is the Full Moon occurring closest to the March equinox.

Before the advent of artificial lighting, humanity planned nocturnal activities by the light of the Moon; and for that reason, people of olden times were much more in tune with lunar behavior than we are today. It was common knowledge that at the vicinity of Full Moon, the Moon pretty much rises at the same time as sunset. Each day thereafter, the Moon rises on the average 50 minutes later. But it must be emphasized that this average 50 minute figure varies monstrously, depending upon the season and your latitude.

Our ancestors noted that the lapse of time between successive moonrises shrinks dramatically at falltime, ushering in a glorious parade of moonlit nights. With only twilight intervening between sunset and moonrise for nights on end, farmers could harvest their crops under what seemed like a suspended Full Moon.

The Harvest Moon is really a phenomenon of far northern and far southern latitudes. At the Earth's equator, there is no Harvest Moon at all; and at the tropics, there's not enough of one to say so. In northern New York, the Harvest Moon rises 20 (not 50) minutes later for several nights in succession. This effect, however, is even more pronounced at latitudes further north: at Calgary, Canada, the Harvest Moon rises 14 minutes later daily; whereas at Talkeetna, Alaska, the Moon rises at the same time every day for a week! North of Talkeetna, you can actually see the Moon rising earlier than it did the night before!

Now for an explanation of why this happens. On September 20, the Moon rises at roughly the same time as sunset. If you can catch the rising Moon, note its position on the horizon from night to night. Here's how it works in our Northern Hemisphere: if the Moon rises north (or left) of where it rose the night before, it'll rise sooner than it normally does; on the other hand, if the Moon rises south (or right) of where it rose the night before, it'll rise later. You'll notice the Harvest Moon continually rising more and more north along the horizon as the nights go by - and the further north you live, the more northbound the successive moonrises.

copyright autumn 2002
by Bruce McClure

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