The Full Moon of Saturday, Sept. 21 also carries the title of the Harvest Moon for those living in the Northern Hemisphere. It has a long history of importance for farmers and birdwatchers, which we'll explain below. First, let's look at what's different about a Harvest Moon.

The Moon officially turns full when it reaches a spot opposite (180) to the Sun in the sky, with Earth in the middle. This moment will occur on Saturday at 13:59 Greenwich Time (9:59 a.m. EDT, 6:59 a.m. PDT).

The Moon will seem nearly full, and nearly as dramatic, Friday and Sunday nights, too.

What sets Saturdays 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 Moons light. It rises about the time the Sun sets.

Also, at this time of year, instead of rising its normal average of 50 minutes later each day, the Moon seems to rise at nearly the same time each night. We'll explain why below.

First, the chart below includes example local moonrise times for Sept. 20, 21 and 22, the middle date being that of the Harvest Full Moon.

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 55 minutes.

Escaping winter climates, most North American birds take part in a great pageant of migration, many winging their way southward to winter in South America. Such seasonal migration patterns can commonly cover up to 3,000 miles.

Bird migration patterns may seem an odd topic for a weekly column on SPACE.com, but bird watchers and astronomers have long shared some common ground.

Before the advent of radar, ornithologists studied nocturnally migrating birds by training small telescopes on the full (or nearly full) Moons of September and October to count their quarry passing in fleeting silhouette. "Moonwatching" is an actual technical term used by ornithologists. Its most celebrated discovery answered a long-standing question: Do birds migrate nonstop for hundreds of miles across the Gulf of Mexico? They do.

Although this weeks Harvest Moon will be an obstacle to stargazers, birdwatchers will be carefully watching it to briefly glimpse the silhouette of a tiny waif during a flight that might last many hours.

Even more challenging to students of ornithology and animal behavior is that most songbirds migrate at night. How do these birds find their way?

An independent study conducted back in the late 1960s by Cornell University under the artificial skies of the Longway Planetarium in Flint, Michigan strongly suggested that the Indigo Bunting, a common songbird throughout the eastern United States and Canada, uses the stars as a guide for its annual migratory flights.

Stephen Emlen is the Jacob Gould Schurman Professor of Behavioral Ecology at Cornell University. He is a world authority on the social behavior of animals, particularly birds. Emlen cleverly demonstrated in experiments conducted outdoors and in the Longway Planetarium that indigo buntings use star patterns as a compass. Emlen took full advantage of a behavior pattern shown by migratory birds in captivity during the periods of time when their normal spring and fall migrations occur.

Termed Zugunruhe, a German word meaning migratory restlessness, this behavior is characterized by increased activity, mainly hopping and fluttering of the wings.

Emlen placed captive buntings in cages made out of a funnel of blotting paper with an ink pad as a floor and a see-through top. Using this set-up, he was able to determine that buntings with a view of the night sky oriented their Zugunruhe activity in the appropriate direction in which they would be migrating. Without a view of the stars, the bird's activity became randomly oriented.

This orientation of activity was apparent in the pattern of inked footprints left on the blotting paper sides of the cages.

The experiments indicated that stars located in the northern sky, close to the celestial pole -- quite possibly the Big Dipper -- may provide essential cues for these birds. Apparently, they obtain directional information from the star patterns, much as we do ourselves. Yet other navigational cues might also be of importance.

Scientists observing migrations by radar have often reported large, oriented flights on overcast nights. These birds could not have been using celestial information, but whether they rely on geophysical factors, winds aloft, or some other cues is still under scrutiny as scientists attempt to discover the guidance mechanisms underlying these navigational feats.