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Three nights of the Harvest Moon

The full moon of Saturday, Sept. 17, also carries the title of the Harvest Moon for those living in the Northern Hemisphere.
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The full moon of Saturday, Sept. 17, also carries the title of the Harvest Moon for those living in the Northern Hemisphere. The moon officially turns full when it reaches that spot in the sky opposite (180 degrees) to the sun in the sky.

This moment will occur on Saturday at 6:01 p.m. EDT (3:01 p.m. PDT).

The Harvest Moon is the full moon that comes the closest to the Fall equinox, so this year it falls in September, although in one out of three years this title can be bestowed upon the October full moon. The 2005 version of the Harvest Moon comes just five days prior to the Autumnal Equinox, although it can occur as early as September 8 (as in 1976) or as late as October 7 (as in 1987).

Many think that the Harvest Moon remains in the night sky longer than any of the other full moons we see during the year, but that is not so. What sets Saturday’s full moon apart from the others is that farmers at the climax of the current harvest season can work late into the night by the moon’s light. It rises about the time the sun sets, but more importantly, at this time of year, instead of rising its normal average 50 minutes later each day, the moon seems to rise at nearly the same time each night.

Below we’ve provided some examples for ten North American cities. The local moonrise times for September 16, 17 and 18 are provided, the middle date being that of the Harvest Moon. All times are local daylight-saving time.

In actuality, over this three-night interval for our relatively small sampling, the rising of the moon comes, on average, just under 26 minutes later each night. A quick study of the table shows that the night-to-night difference is greatest for the more southerly locations (Miami, located at near latitude 26ºN., sees moonrise come an average of 36 minutes later).

Meanwhile, the difference is less at more northerly locations (at Edmonton, Alberta, located at latitude 53.6ºN, the average difference is only 11 minutes).

The reason for this seasonal circumstance is that the moon appears to move along the ecliptic and at this time of year when rising, the ecliptic makes its smallest angle with respect to the horizon for those living in the Northern Hemisphere.

In contrast, for those living in the Southern Hemisphere, the ecliptic at this time of year appears to stand almost perpendicular (at nearly a right angle) to the eastern horizon.  As such, the difference for the time of moonrise exceeds the average of 50 minutes per night.  At Sydney, Australia, for instance, the night-to-night difference amounts to 70 minutes.

Interestingly, for those who live near 60 degrees north latitude, the moon does indeed appear to rise at about the same time each night around the time of the Harvest Moon.  And for those who live even farther to the north, a paradox: the moon appears to rise earlier!  At Reykjavik, Iceland (latitude 64.1ºN), for instance, the times of moonrise on September 16, 17 and 18 will be, respectively, 8:07 p.m., 7:56 p.m. and 7:46 p.m. (GMT).

So from Reykjavik, the moon will seem to rise an average of 11 minutes earlier each night.