Welcome to You Ask Andy

Greg Burley, age 11, of Beaconsfield, Quebec, Canada, for his question:

What is meant by the Harvest Moon?

In the fall of the year, the full moon lends a helping hand with the harvesting. It rises earlier than usual after sunset and adds a few more hours of brightness at the end of the day. Generations of farmers in northern latitudes called it the harvest moon because it gives them extra time to gather in their crops. The favor lasts only during the full moon phase, but the next full moon is almost as kindly. It is called the hunter's moon because in bygone days it gave people a chance to catch stores of wild game before the frosty winter arrived.

On the average, the moon rises 50 minutes later every calendar day. But the average varies a great deal with the seasons and in different latitudes. In northern latitudes, the friendliest full moon is the one that comes nearest to September 23. This is the date of the fall equinox, when the sun's path crosses the equator on its way down to the Southern Hemisphere. For a few evenings, this full moon rises soon after sunset and sheds its bright light on the fields of ripened corn.

When the crops are ripe, they refuse to wait and harvest time is the hurry scurry season of the year    at least in the farmer's world. The summer days have grown shorter and there is no time to waste. Then right on schedule, the stately harvest moon appears. It rises more slowly than usual, at a smaller angle to the horizon. The next night it does not wait the average 50 minutes to rise again. So during the rush season, farmers can work a few extra hours by the light of the full harvest moon.

This helping hand from on high is governed by the lunar orbit, plus motions of the earth and moon as they orbit the sun. The earth's axis and the moon's orbit are tilted and their orbits are not perfect circles. These irregularities create variations in the paths of the heavenly bodies as they appear to parade over our skies.

It's easier to track these paths if we visualize our planet inside a huge, hollow celestial sphere. Astronomers cross hatch this celestial sphere with great circular lines of latitude and longitude to match those on the earth. The celestial poles are above the earth's poles, the celestial equator is above the earth's equator. This neat grid provides a background for the paths of the sun and the moon. But because the earth's axis is tilted 23 1/2 degrees, so is the yearly path of the sun. This  circular path is the eclipic. And the tilted eclipic intersects the great circle of the celestial equator at the two opposite equinox points. On September 23, the sun reaches the point of autumnal equinox and the moon is at the opposite point of vernal equinox. This is when the full moon helps the harvesters.

It takes a full year for the sun and moon to inch around their seasonal circles. Each month they complete one 12th of the schedule. One month after the harvest moon they are not far from the equinox points. So the next full moon also rises steadily soon after sunset. This one is called the hunter's moon. It adds a few bright hours at the end of the day but it does not do quite so well as the helpful harvest moon.

 

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