By Night Sky Columnist
updated 9/24/2004 6:32:27 PM ET 2004-09-24T22:32:27

The full moon of Tuesday, Sept. 28 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.

This moment will occur Tuesday at 13:08 GMT (9:08 a.m. EDT or 6:08 a.m. PDT).

The harvest 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. The 2004 version of the harvest moon comes relatively close to the equinox -- slightly less than five days after it -- although it can occur as early as Sept. 8 (as in 1976) or as late as Oct. 7 (as in 1987).

Why it is special
Many think 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 Tuesday’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 sunsets, 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 actuality, for those living at mid-northern latitudes, the rising of the moon comes, on average, roughly 25 minutes later each night. The night-to-night difference is greatest for more southerly locations, while the difference is less than the average at more northerly locations.

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.

Dress rehearsal for an eclipse
The next full moon on the calendar, after this month’s harvest moon, comes on the night of Oct. 27-28. There will be a bonus that night: A total eclipse of the moon.

For viewers in most of the Americas, this shady drama will happen in the early-to-mid evening hours of Wednesday, Oct. 27. Along the West Coast the eclipse gets underway at dusk, only minutes after the sun has set and as the moon is rising. The eclipse will also be visible from Europe, but from there it will occur in the hours before dawn breaks on the morning of Thursday, Oct. 28.

It is not too early to begin making preparations for viewing the eclipse.

One of things to consider is the moon’s location in the sky during the eclipse. For those who live along the West Coast of the United States and Canada this will be an important factor, since initially the eclipse will be rather low in the east-northeast sky. Will tall trees or nearby buildings block your view of the moon?

You can get a very good idea about where the moon will be in the sky during the eclipse by looking for it on the night of Sept. 30-Oct. 1.

During that Thursday night and early Friday morning, the moon – 2.5 days past full – will be very near (within a couple of degrees) to the region of the sky where it will also be on the night of Oct. 27-28 during the eclipse.

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