A full harvest moon will linger in the Butler sky for several nights. It's called the harvest moon because it's the closest full moon to the date of the autumnal equinox, or the first day of fall.

“Shine on Harvest Moon,” a song more than a century old, will certainly be playing in my head this weekend and most of the coming week.

If you’re under 50 you may not even have heard of this tune. Google it or check it out on You Tube. I’m not afraid to say that’s one of my favorite lunar tunes.

We now officially have a full harvest moon that will linger in the Butler sky for several nights. It really doesn’t look all that different from any other full moon. It’s called the harvest moon because it’s the closest full moon to the date of the autumnal equinox, or the first day of fall, which is coming next week on Sept. 23.

Astronomically the harvest moon is special because of the unique celestial mechanics this time of year.

Because of its orbit around the Earth the moon constantly migrates eastward among the stars. That causes it to rise about 30 to 60 minutes later from night to night.

Around the date of the autumnal equinox, though, the moon rises only 20 minutes later each night.

The exact details of why this happens are hairy, but it has to do with our Earth’s axis being tilted to its orbit around the sun. It also has to do with the angle of the ecliptic, the sun and moon’s path among the stars, and the celestial equator, a projection in the sky of the Earth’s equator.

This time of year the full moon is at the intersection of the ecliptic and celestial equator, and because of that the full moon’s path among the stars is nearly parallel to the horizon. Confused? Like I said, the mechanical details are really hairy.

Anyway, it’s called the harvest moon because historically 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, and 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 since it rose less than a half-hour 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.

The harvest moon is often depicted as a big bright orange orb, but really any full moon always looks much bigger and more colorful when rising or setting than when overhead.

The reason the moon sports that orange-red color when it rises or sets is because when it’s close to the horizon, we see it through the thicker and dustier layers of Earth’s atmosphere. That scatters away all but the reddish components of the white light of the moon. When the moon is higher and we don’t see the moon through as much of our atmosphere the scattering effect goes away.

As far as full moons looking much larger when they rise or set, that’s simply an optical illusion. Prove that to yourself by holding a nickel out at arm’s length when the full moon is rising and again at its highest point around midnight, and you’ll see that your nickel will cover exactly the same amount of the moon.

For amateur astronomers and stargazers like myself, serious stargazing is shut down during a full moon. There’s just too much light in the sky.

It’s a really good thing that we have the moon up there in the heavens because it provides a wonderful stabilizing effect on the Earth. It keeps our Earth’s axis from wobbling too much.

Now, the Earth’s axis does wobble modestly 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 we wouldn’t be here.

As you gaze lovingly 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.

Shine on Harvest Moon.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net. Contact Mike Lynch at mikewlynch@comcast.net.