for National Geographic News
Tomorrow's full moon, the harvest moon, will flood the twilight sky with natural light just after sunset, providing folks a few extra hours to complete outside chores.
Unlike the rest of the year, the nearly full moon will continue to rise shortly after sunset for the next several days.
"With the early full moon rise at pretty close to the same time at a number of nights around the full moon, it gives farmers a little extra light to bring in the harvest," said Geoff Chester, a spokesman for the U.S. Naval Observatory in Washington, D.C.
These consecutive evenings of moonlight-filled twilight sky prompted ancient European farmers to name this particular full moon the harvest moon, explained Chester. The moniker is given to the full moon that falls closest to the autumnal equinox.
Peter Geiger, editor of the Farmers' Almanac in Lewiston, Maine, said that artificial lighting long ago reduced the importance of the harvest moon to farmers and gardeners but that the name has stuck.
"There are some 25 million people who have their own gardens, are they out in the middle of the night pulling crops? Probably not," he said. "The development of [full moon] names was based on what was going on hundreds of years ago."
Regardless, Geiger added, people today still watch for full moons because they know the extra light will allow them to do things outdoors that they couldn't otherwise do.
Matter of Geometry
Throughout the year, the moon generally rises about 50 minutes later each day. But around the time of the harvest moon, the lapse between consecutive moonrises in the Northern Hemisphere is about 30 minutes and even shorter at more northern latitudes like Anchorage, Alaska.
"What is happening is that the angle at which the moon rises with respect to local horizons varies through the course of the year depending on where the moon is in its orbit around the Earth," said Chester.
During the autumnal equinox, which falls on September 23 this year, the intersection between the ecliptic plane of the moon's orbit and Earth's eastern horizon makes a shallow angle.
The result is that the moon does not move that far from night to night with respect to the horizon, and it rises at about the same time said Chester: "This purely is a case of geometry. It's one of those things that happen year in and year out."
SOURCES AND RELATED WEB SITES