Harvest Moon to Grace Evening Sky

The moon of Wednesday, Sept. 26 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 diametrically opposite (180 degrees) to the sun in the sky. This moment will occur on Wednesday at 19:45 Greenwich Time (3:45 p.m. EDT or 12:45 p.m. PDT). Wednesday's full moon is the one that comes the closest to the September 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 (as was the case in 2006).

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 Wednesday'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.

In analyzing local moonrise times for Sept. 25, 26 and 27 in 10 locations across North America, the rising of the moon comes, on average, less than 27 minutes later each night. The night-to-night difference is greatest for the more southerly locations. (Miami, located at near latitude 26-degrees N., sees moonrise come an average of 37 minutes later). Meanwhile, the difference is less at more northerly locations (at Edmonton, Alberta, located at latitude 53.6-degrees N, the average difference is just 12 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 rise time difference amounts to about 71 minutes.

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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.