I know this dates me, but every time I see a harvest moon around this time of year, the old song “Shine on Harvest Moon” plays in my head.
I know there are more hip songs about the moon, but I’m not afraid to say that’s one of my favorite lunar tunes.
Tonight and Monday, we have a full harvest moon that honestly doesn’t look all that different than any other full moon.
By definition, the full moon closest to the date of the autumnal equinox, the first day of fall, is called the harvest moon. Because we have a full moon every 29.5 days, and because the autumnal equinox is a week from Monday, the harvest moon is shining upon us.
Astronomically, the harvest moon is special because of the unique celestial mechanics this time of year.
Most of the year, the moon migrates eastward among the stars each night, its orbit around Earth causing it to rise about 30 to 60 minutes later from night to night.
In the autumn, however, the moon rises only 20 minutes later each night, because the sun and the full moon rise and set directly in the east and west.
The exact details of why this happens get a little complicated. It has to do with the angle of the ecliptic, the sun’s path among the stars, and where it intersects the horizon. We’ll just leave it at that.
It’s called the harvest moon because it helped farmers get their crops in this time of year. In the days before lights on tractors, farmers used every speck of daylight they could to bring in the crops.
When they got the full moon they used that dim light as well, and pulled “all nighters.” The beauty of the harvest moon for them was that since it rose only about 20 minutes later each evening, there wasn’t much of a gap between sunlit days and moonlit nights, at least for a few days after the exact date of the full moon.
For amateur astronomers and stargazers like me, serious stargazing is shut down during a full moon. There’s just too much light in the sky.
It is fun, though, to train binoculars or a telescope at the moon and check out all the dark and relatively flat lunar plains known as the maria, the high mountain tops, and all those craters made by meteors slamming into the moon during the chaos of the early developing solar system.
One of the biggest craters we can easily see is Tycho, found toward the lower limb of the moon, as you can see from the picture I took of a recent full moon. Tycho was named after the famous 16th century Danish astronomer, Tycho Brahe. The crater Tycho is about 50 miles in diameter, with a crater wall more than two miles high.
Our moon was formed more than 4 billion years ago when a large object about the size of the planet Mars crashed into our Earth, knocking out a huge chunk of the developing crust and mantle of the Earth.
The debris that got knocked out of the Earth, and what was left of the projectile that collided with it, eventually coalesced gravitationally and became our moon.
That was a good thing, because the moon provides a wonderful stabilizing effect on the Earth. It keeps our Earth’s axis from wobbling too much.
The Earth’s axis does wobble modestly now, in a 26,000-year cycle. However, if our moon wasn’t around with its stabilizing gravitational force, it’s thought that the gravitational force of Jupiter as well as other planets would cause the Earth’s axis to wobble more chaotically and quickly, causing Earth’s seasons and our climate to be thrown way out of whack.
Maybe without the moon here we wouldn’t be here. As you gaze on the harvest moon, think of that and say thanks to the moon, but don’t let anybody hear you — you don’t want to be called a lunatic.
One more lunar note: On Friday night, the waning gibbous moon will rise after 9:30 p.m. Until 11 p.m. it will be partially covering the bright star cluster Pleiades, otherwise known as the Seven Sisters. It should be quite a sight to begin your weekend with.
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis and author of the book, “Washington Starwatch,” available at bookstores and at his Web site www.lynchandthestars.com
The Everett Astronomical Society welcomes new members and puts on public star parties. The Web site is members.tripod.com/everett_astronomy.
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