Harvest Moon This Weekend: Late But Still Gorgeous
Just as the sun is setting in the west this weekend, look to the east and see the harvest moon and Jupiter rising.
Credit: Starry Night Software

This weekend's full moon will be a somewhat special one in that it will also carry the title of "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 and this moment will occur on Sunday at 06:10 Greenwich Time (2:10 a.m. EDT . . . or in the Pacific Time zone, it actually falls on Saturday night, October 3 at 11:10 p.m. PDT).

It turns out that this full moon is also the one that comes closest on the calendar to the September equinox. In 2009 this circumstance comes later than usual . . . into the month of October, as opposed to the more traditional month of September.?

Between 1970 and 2050, there are 18 years when the harvest moon comes in October.? The last time was in 2006 and next time will be in 2017.? The 2009 version of the harvest moon comes unusually late, although it can occur as early as September 8 (as in 1976) or as late as October 7 (as in 1987).

Not a few people are under the impression that the harvest moon remains in the night sky longer than any of the other full moons we see during the year, but that's not so.? What sets the harvest 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 evening, the moon seems to rise at nearly the same time each night.?

For example, at New York City, moonrise on Oct. 3 is at 5:58 p.m. On Oct. 4, moonrise is 6:24 and on Oct. 5, it's 6:53.? So compared to the normal 50 minutes per night, around the time of the harvest moon moonrise comes about 27 minutes later.

In actuality, the night-to-night difference is greatest for more southerly locations.? For example: Cocoa, Florida, located near latitude 28.4-degrees N, sees moonrise come an average of 34 minutes later.? Meanwhile, the difference is less at more northerly locations; at Priddis, Alberta, Canada located at latitude 50.9-degrees N, the average difference is 19 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 Canberra, Australia (35.3-degrees S), for instance, the night-to-night difference amounts to 63 minutes.

Interestingly, for those who live near 60-degrees north latitude, the moon does indeed appear to rise at 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 64.2-degrees N), for instance, the times of moonrise on Oct. 3, 4 and 5 will be, respectively, 5:58 p.m., 5:54 p.m. and 5:50 p.m.? So from Reykjavik, the moon will seem to rise four minutes earlier each night.?

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.