Astro Bob: Here comes the Harvest Moon
This weekend's special moon brings a double dose of light to the ever-lengthening night.
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.
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.
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.
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?
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!
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.