September 2005 Feature: the Harvest Moon

Very simply, the Harvest Moon is the closest full Moon to the autumn equinox. In our Northern Hemisphere, the autumn equinox comes annually in September; and in the Southern Hemisphere, it happens yearly in March. This year's September equinox arrives on the 22nd, at 6:23 p.m. Eastern Daylight Time (10:23 p.m. Universal Time), ushering autumn into the Northern Hemisphere and spring south of the equator.

The Harvest full Moon falls on September 17, at 10:01 p.m. EDT (September 18, 2:01 a.m. UT). This September full Moon -- like any full Moon -- rises around sunset, lights up the sky throughout the night, and sets around sunrise. As a general rule, the moon rises (and sets) about 50 minutes later with each passing night -- though this yearly average varies greatly, depending on the season. For instance, after the Harvest full Moon, the moon rises unusually close to sunset for many days in succession, ushering in a grand procession of moon-filled nights, unlike those seen at any other time of the year. In olden times -- before the advent of electricity -- farmers took advantage of these moonlit nights to finish harvesting their crops!

The Harvest Moon phenomenon is most noticeable at the higher latitudes; and no Harvest Moon ever shines at the Earth's equator (0 degrees latitude). In the tropics, the Harvest Moon effect is rather low key. If you're at the equator for the September full Moon, you'll notice consecutive moonrises occurring some 50 minutes later each night thereafter. Appreciably north of the equator (where summer is giving way to autumn) successive moonrises come considerably SOONER than the average 50 minutes later; and south of the equator (where winter is giving way to spring) successive moonrises are considerably MORE than 50 minutes later.

Even though the rising and setting times of the Moon vary wildly by latitude, there is a wonderful symmetry that takes place. The number of minutes the moon RISES later/daily on one side of the equator virtually equals the number of minutes the moon SETS later/daily for a like-numbered latitude on the opposite side of the equator. The table illustrates:

copyright 2005 by Bruce McClure

August 2005 Feature * October 2005 Feature

Past Harvest Moon Features:

Past Equinox Features: