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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.

Harvest Moon September 2005: Successive Rising Times by Latitude
(Average, rounded-off figures for five days following full Moon)


LatitudeMoonrise
60 degrees Northsame time/daily
45 degrees North+24 minutes later/daily
0 degrees (equator)+50 minutes later/daily
45 degrees South+78 minutes later/daily
60 degrees South+104 minutes later/daily


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:

September 2005: Successive Moonrises & Moonsets by Latitude
(Average, rounded-off figures for five days following full Moon)


LatitudeMoonriseMoonset
60 degrees Northsame time/daily104 minutes later/daily
45 degrees North24 minutes later/daily79 minutes later/daily
0 degrees (equator)50 minutes later/daily50 minutes later/daily
45 degrees South78 minutes later/daily23 minutes later/daily
60 degrees South104 minutes later/dailysame time/daily


copyright 2005 by Bruce McClure


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August 2005 Feature * October 2005 Feature


Past Harvest Moon Features:

September 2003: The Harvest Moon
September 2002: The Harvest Moon

Past Equinox Features:

September 2004: Southbound Sun
September 2003: Equinox Geometry