Shine On
The Solar System >
It is time once more to reap liberally from the ripened fields of knowledge. It is the autumnal equinox, which is traditionally a time of harvest, the namesake for next week's full Moon --- the Harvest Moon.
Actually every full Moon of every month has a special name --- like October's Hunting Moon, November's Beaver Moon, or July's Blood Moon --- but the one at this time of the year is probably the best known of the lot.
Perhaps it is such a recognizable name because of that song written over 100 years ago by Nora Bayes and Jack Norworth --- Shine on Harvest Moon. If you are like I am, you are probably already singing it to yourself and are finding it difficult to get out of your head --- make that very difficult. But where did the name come from?
In Europe for hundreds of years during this time of the year, it would be time to harvest the crops. But headed for winter, with daylight hours get shorter and shorter, any extra light during this time, any extra time to work the fields, was welcome. Enter the Harvest Moon.
We define the Harvest Moon as the full Moon closest to the fall equinox, one of two days of the year that the earth is tilted neither toward nor away from the sun; the so-called first day of fall.
What is unique about this full Moon, and what made it a Godsend for farmers in the days of old, is the time and way it rises.
Most readers know that a full Moon rises after sunset. The sun, setting over there in the west, fully illuminates the face of the Moon rising there in the east, on the other side of the Earth. And we all know the full Moon can be awfully bright. This bright light, in days before artificial lighting, could help farmworkers harvest for a longer period of time. Good for business if not for the back.
But there's more to it than that.
The geometry of the Sun, Earth, and Moon at this time plays an unexpected role. The way the three heavenly bodies are oriented at the time of the fall equinox allows the Moon to pull off a nifty little trick as it ascends after sunset.
Instead of coming right off the horizon and launching high into the sky like most would expect, the Moon rises instead at a shallow angle. That one bizarre characteristic is responsible for the real reason the Harvest Moon is so helpful. Time for the thinking cap.
The Moon is orbiting around the earth slowly but surely, taking about a month to crawl out a complete circuit. Doing so means it won't be in the same part of the sky every day. For example, because of its movement the Moon rises in the east on average nearly an hour later each day than the previous. Got that so far?
The dance we have with the Sun and Moon at this time of the year, however, allows the Moon to rise only 20 minutes later each day. And the farther north you go the smaller the time difference. In England, for example, successive Moon risings around equinox differ by only about 15 minutes a day.
Bottom line? For the days just before, during, and just after Harvest Moon, farmers had the equivalence of a daylight extension as a very bright natural light immediately followed the sun to help illuminate the farmstead. Shine on, Harvest Moon!
Keep the ancient tradition alive in a modern way by going out sometime next week after sundown and harvesting the mail or picking a weed out of the lawn by the light of the big, bright, autumn Harvest Moon.
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2004.09.17 12:07 PM | Comments (0)
