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).

Saturday's Full Moon is the one 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 Full Moon.

Location Sept. 16 Sept. 17 Sept. 18

Albuquerque

6:42 p.m. MDT

7:12 p.m. MDT

7:41 p.m. MDT

Chicago

6:33 p.m. CDT

6:58 p.m. CDT

7:21 p.m. CDT

Denver

6:41 p.m. MDT

7:07 p.m. MDT

7:33 p.m. MDT

Edmonton

7:38 p.m. MDT

7:49 p.m. MDT

7:59 p.m. MDT

Houston

6:50 p.m. CDT

7:24 p.m. CDT

7:57 p.m. CDT

Los Angeles

6:28 p.m. PDT

6:59 p.m. PDT

7:29 p.m. PDT

Miami

6:44 p.m. EDT

7:21 p.m. EDT

7:56 p.m. EDT

Montreal

6:42 p.m. EDT

7:03 p.m. EDT

7:23 p.m. EDT

New York

6:36 p.m. EDT

7:02 p.m. EDT

7:26 p.m. EDT

Seattle

7:03 p.m. PDT

7:22 p.m. PDT

7:39 p.m. PDT

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.

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

DEFINITIONS

1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees.

Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe.

Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90).