Harvest Moon

This Friday, September 16, you can enjoy the Harvest Moon! So why is it called the Harvest Moon anyway?

Traditionally, the full moon closest to the autumnal equinox is called the Harvest Moon. This year, the autumnal equinox is on September 22, so our Harvest Moon will rise this Friday. Some people think that the Harvest Moon will look different than a normal full moon, because it has a different name. Some people even say it will be as orange as a pumpkin. If you see the moon looking orange, it is usually right as it is rising or setting. The thick layers of the Earth's atmosphere near the horizon absorb more blue light, making the Moon appear orange. (This is also why the Sun looks more orange or red around sunrise and sunset.) This can happen anytime, not just at the time of the Harvest Moon.  But the story of how the Harvest Moon got its name is really interesting. 

Dreamy full moon by astrophotographer Jerry Lodriguss. Image Credit: APOD and Jerry Lodriguss.

Long ago, when most people farmed to survive, they had to pay close attention to the night sky. The changing night sky helped farmers know when to plant or harvest their crops. The equinoxes (the two times of the year with equal amounts of day and night) mark the beginnings of spring and fall. I will explain more about the upcoming equinox in a future blog post, so stay tuned!

The narrow angle of the ecliptic means the moon rises farther north on the horizon from one night to the next. So there is no long period of darkness between sunset and moonrise. Credit: Earthsky.org and classicalastronomy.com.

On average, the Moon rises 50 minutes later each day than it did the day before. However, around the time of the autumnal equinox, the ecliptic (the path that the Sun, Moon, and planets follow in our sky) forms a narrow angle with the horizon. This causes the Moon to rise only 30 minutes later than it did the day before. Therefore, the Moon lights up the early evening skies right after sunset for several days before and after the full moon, providing farmers with more light in the day to bring their crops in from the field. Thus its name, Harvest Moon. Actually, every full moon has a name. Have you heard of the Wolf Moon?
 

Harvest Moon in Turin, Italy in 2011. Here the layers of atmosphere are making the Moon appear pinkish red! Image credit: Stefano De Rosa and APOD.

So look up! Notice the Harvest Moon as it illuminates our sky this week. And if you see the moon replaced by the Great Pumpkin, maybe you should put down the pumpkin ale and lay down for a bit. 

"What's that?! What's that?!"

As always, learn more about the Moon, constellations, space and science at the Dome at the Peoria Riverfront Museum.