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Astro Bob: Here comes the Harvest Moon

This weekend's special moon brings a double dose of light to the ever-lengthening night.

Full moon lighthouse
The Full Sturgeon Moon rises from behind the Duluth Harbor North Pier Light in Duluth, Minn., on Aug. 11. The Harvest Moon will rise nearly due east and shine just to the right of brilliant Jupiter on Sept. 10.
Contributed / Bob King
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Thank you, big, luminous moon. Thank you for giving us time to sleep. All of the aurora watchers who stayed up well past their bedtimes are grateful for your light.

If the moon were nearby, it would only illuminate a small part of the Earth at a time. Like a streetlight, you could walk away from it and still find a dark sky. But at nearly a quarter-million miles away and more than 2,000 miles across, it lights up half the planet at a time. There's no easy escape. That's especially true at full moon. No matter where you might flee, the full moon rises around sunset everywhere and shines all night. Forget about finding a dark sky.

Full moon bed
One of my favorite things is seeing the moonlight up spaces inside the house. It always feels a little magical. Here, the August moon softly illuminates a homemade quilt on a bed.
Contributed / Bob King

Saturday, Sept. 10, marks the night of the Harvest Moon, the full moon that occurs closest to the autumnal equinox or the start of fall. This year, fall begins officially at 8:03 p.m. CDT on Thursday, Sept. 22.

Moon moves diameter per hour 12 degrees anno S.jpg
The moon orbits around the Earth at an average speed of 2,300 mph (1 kilometer per second). This causes it to move eastward across the sky at the rate of one full-moon diameter per hour, or about 12°-13° each day.
Contributed / Bob King

Each night, the moon moves about 12°-13° to the east as it orbits the Earth. That's about the width of a horizontal fist held at arm's length to the sky. Typically, this motion delays successive moonrises by 50 minutes to an hour a night.

Full moons always lie opposite the sun in the sky and rise around sunset. Right now, that's 7:30 p.m., give or take. The night after full, we'd expect the moon to rise about an hour past sunset, and it normally does.

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Moon angle spring fall Harvest Moon GENERIC equinoxes S.jpg
In late summer and early fall the full moon's path lies at a shallow angle to the horizon. For several nights in a row it never dips very far below the horizon, so moonrise times are just 20 minutes apart. The opposite occurs in spring when the path slants at a steep angle, delaying successive moonrises by more than an hour.
Contributed / Bob King

But in September and October, the full moon's path at the eastern horizon lies at a very shallow angle. When it slides 13° to the east each night, much of that distance is to the left (north) and nearly parallel to the horizon, with only a small portion directed downward below the horizon.

For several nights in a row, the nearly full moon dips only about 5° below the horizon instead of 13°. That means the Earth only has to spin for about 20 minutes instead of an hour to ferry the moon back up for the next moonrise. To the casual observer, the full or nearly full moon shines in nearly the same spot in the eastern sky at nearly the same time for several nights in a row. Pretty cool, right?

Harvest Moon George Mason public domain S.jpg
This is part of a larger painting, "The Harvest Moon," by George Mason, that was exhibited in 1872. It depicts a harvest scene by moonlight.
Contributed / George Mason, public domain

Before electric lights were invented, our ancestors employed the Harvest Moon's early and bountiful light to bring in the harvest well into the night. Life then was more closely tied to natural rhythms than it is today, when we tend to do what we want when we want.

The angle the full moon's path makes to the eastern horizon changes throughout the year. To demonstrate this, let's compare the fall and spring full moons. In March and April, the moon's track slants at a steep angle to the eastern horizon. While it still slides 13° to the east each day, most of that movement is angled downward toward the horizon. Compared to the Harvest Moon it moves only a little bit sideways.

Instead of dipping just a few degrees below the horizon from one night to the next, the spring full moon plunges some 12° below the horizon each night — more than twice September's depth. The Earth has to spin an hour or more to carry the moon back to rising. With successive moonrises spaced so far apart, you can see why a spring full moon is far from ideal for nighttime activities. Too much in a hurry to leave!

Full moon pillar and arc
Always be alert for interesting phenomena that occur around the time of a bright moon. Clouds passing by the full moon last August created a pillar of moonlight (center) and a small, bright arc directly above the moon.
Contributed / Bob King

To compare night-to-night moonrise times for your individual city, use this moonrise-moonset calculator . And if you're still in need of sleep, you can catch the early moonrise then hit the hay.

Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at nightsky55@gmail.com.
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