The Harvest Moon Effect

© 2000 by Robert C. Moler

The Harvest Moon is the most famous full moon of the year. By definition it is the nearest full moon to the autumnal equinox. The equinox falls on September 22nd this year. The closest full moon this year is September 15th.

There is something remarkable happening to the moon's rise times in the period of several days before and after the full moon itself. That is, the moon rises at nearly the same time. The moon orbits the earth with respect to the sun in about 29 1/2 days. In essence it gains one day in that period. Divide the number of minutes in a day 1440 (24 X 60) by 29.5, and it will give the average daily increase in moonrise and set times at nearly 49 minutes. However at the time of the Harvest Moon, the daily increase in rise times drops to 30 minutes or even 20 minutes.

In the days before electric lights the Harvest moon extended twilight, allowing farmers to work later gathering their crops during the critical harvest time.

The diagram to the left shows the harvest moon this year, plus two days before and after. For clarity I've made the earth transparent. The heavy horizontal line is the Horizon and the "E" is the east compass point. The rest of the lines belong to the celestial sphere, the imaginary sphere the stars appear on. The apparent motion of the sky due to the earth's rotation is upwards to the right parallel to the celestial equator.

The line near the moon's path is the ecliptic, the apparent path of the sun in the sky. It crosses the celestial equator by about 23 1/2 degrees, the tilt of the earth's axis. Astronomers use a fancy term for this angle. It's called the obliquity of the ecliptic, because that's what it looks like in the sky, assuming you could see these lines, which of course you can't. The words eclipse and ecliptic are related. Eclipses can only occur when the moon is near the ecliptic. The sun of course must cooperate by being either at the same spot, for solar eclipses, or at the opposite point in the sky, for lunar eclipses.

The line running perpendicular to the celestial equator is the zero hour line of right ascension. The celestial counterpart of the zero longitude line on the earth that runs through the old Greenwich Observatory outside London.

The point where the zero hour line, the ecliptic, and the celestial equator meet is the vernal equinox point, where the sun was on the first day of spring. When the sun is near the autumnal equinox, the full moon, being opposite the sun, will be near the opposite equinox point. The shallow angle of the moon's path to the horizon causes the shorter than average interval in moonrise times. This full moon the minimum difference is 25 minutes.

This year is not the best the harvest moon effect can get. Note that the moon is diverging south of the ecliptic. Its orbit is tilted about 5 degrees to the ecliptic. Above right is the harvest moon of 1987. There happened to be a penumbral eclipse of the moon that night October 7th. The moon was moving northward from the ecliptic, making an even shallower angle to the horizon than this years harvest moon. The smallest difference in moonrise times occurred a couple of days before full, down to 19 minutes.

If moonrise pokes around when the moon is near the vernal equinox, it really speeds up when the moon passes the autumnal equinox. To the left is a chart of the moonrise from next April 6th through the 10th. The longest difference between daily moonrises is 75 minutes from the 6th to the 7th.

Amateur astronomers who aren't farmers, generally are not happy about the harvest moon effect. As can be seen from this month's Ephemeris of Sky Events a couple pages over. It takes 3 days past full for the moon to finally rise after the end of evening twilight allowing a few minutes of dark skies. The spring full moon moves out much faster.

Having stated the above, I must emphasize that the amount of difference in rise times occur for the moon, whatever its phase, when it is in these positions in the sky. It's just that the moon is most effective illuminating the sky when it is nearly full.

The harvest moon effect on wiping out the dark evening skies is also noticeable the months before and after the Harvest Moon. The next most famous "moon" is the Hunter's Moon, the next full moon after the Harvest Moon. I'm not sure but I think August's full moon might be called the Back To School Moon.

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Uploaded: 08/28/00