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Shine on, Harvest Moon!

Did you see the moonrise last night? Gorgeous. And – for those in the Northern Hemisphere, now near our autumnal equinox – we’re now due for lovely Harvest Moon rises over the next several evenings.

Image Credit: Dan Bush of Missouri Skies

A Harvest Moon via Dan Bush of Missouri Skies

Here in the Northern Hemisphere, we call the closest full moon to the autumn equinox the Harvest Moon. In 2017, the September equinox came on September 22. And the September full moon came early in the month, on the night of September 5-6 for the Americas. Thus, for the the Northern Hemisphere, the full moon of October 5 – the closest full moon to our autumn equinox – is our Harvest Moon. Follow the links below to learn more.

What is a Harvest Moon?

Is the Harvest Moon bigger, or brighter or more colorful?

When is the Harvest Moon in 2017?

How did the Harvest Moon get its name?

When is the Southern Hemisphere’s Harvest Moon?

What are some other full moon names?

Tonight's sunset and moonrise - September 19, 2013 - as seen by EarthSky Facebook friend Andy Somers in Noumea, New Caledonia.  One of the characteristics of the Harvest Moon is that it rises around the time of sunset for several evenings in a row.  Thank you, Andy.

Harvest Moon sunset and moonrise – September 19, 2013 – as seen by EarthSky Facebook friend Andy Somers in Noumea, New Caledonia. One of the characteristics of the Harvest Moon is that it rises around the time of sunset for several evenings in a row.

What is a Harvest Moon? In skylore, the Harvest Moon is the full moon closest to the autumn equinox. Depending on the year, the Harvest Moon – by the usual definition of that name – can come anywhere from two weeks before to two weeks after the autumn equinox.

Harvest Moon is just a name. But this full moon does have special characteristics, related to the time of moonrise. Nature is particularly cooperative in giving us full-looking moons near the horizon after sunset, for several evenings in a row, around the time of the Harvest Moon.

Here’s what happens. On average, the moon rises about 50 minutes later each day. But when a full moon happens close to the autumn equinox, the moon (at mid-temperate latitudes) rises only about 30 to 35 minutes later daily for several days before and after the full Harvest moon. For very high northern latitudes, there’s even less time between successive moonrises. It happens because the ecliptic – or the moon’s orbital path – makes a narrow angle with the evening horizon near the autumn equinox.

The difference between 50 minutes and 35 minutes may not seem like much. But it means that, in the nights after the full Harvest Moon, you’ll see the moon ascending in the east relatively soon after sunset. The moon will rise during or near twilight on these nights, making it seem as if there are several full moons – for a few nights in a row – around the time of the Harvest Moon.

In autumn, the angle of the ecliptic – or sun and moon’s path – makes a narrow angle with the horizon. Image via classicalastronomy.com.

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

Jarred Donkersley caught this photo of 2016’s Harvest Moon at the Vincent Thomas Bridge in San Pedro, California.

Is the Harvest Moon bigger, or brighter or more colorful? Not necessarily.

Because the moon’s orbit around Earth isn’t a perfect circle, the distance of the Harvest Moon is a bit different every year. But the 2017 Harvest Moon does not count as a bigger-than-average full moon. Instead, the moon’s perigee for September is the 13th; for October, it’s the 9th. The September and October full moons aren’t particularly close full moons.

Still, in any year, you might think the Harvest Moon looks bigger or brighter or more orange. That’s because the Harvest Moon has such a powerful mystique. Many people look for it shortly after sunset around the time of full moon. After sunset around any full moon, the moon will always be near the horizon. It’ll just have risen. It’s the location of the moon near the horizon that causes the Harvest Moon – or any full moon – to look big and orange in color.

The orange color of a moon near the horizon is a true physical effect. It stems from the fact that – when you look toward the horizon – you are looking through a greater thickness of Earth’s atmosphere than when you gaze up and overhead. The atmosphere scatters blue light – that’s why the sky looks blue. The greater thickness of atmosphere in the direction of a horizon scatters blue light most effectively, but it lets red light pass through to your eyes. So a moon near the horizon takes on a yellow or orange or reddish hue.

The bigger-than-usual size of a moon seen near the horizon is something else entirely. It’s a trick that your eyes are playing – an illusion – called the Moon Illusion. You can find many lengthy explanations of the Moon Illusion by doing an online search for those words yourself.

Worldwide map via the US Naval Observatory. Day and night sides of Earth at the instant of the full moon (2017 September 6 at 7:03 UTC). The shadow line passing to the left of Africa depicts sunrise and the shadow line to the right of Australia represents sunset.

When is the Harvest Moon in 2017? More often than not, the September full moon is the Northern Hemisphere’s Harvest Moon. The Harvest Moon is usually defined as the full moon closest to the autumn equinox, which – in the Northern Hemisphere – comes on or near September 22 each year.

2016’s Harvest Moon fell in September. 2018’s Harvest Moon will, too.

But, in 2017, the September 6 full moon happened too early to be the Northern Hemisphere’s official Harvest Moon, according to the most widely accepted definition of the term. That’s because the full moon of October 5, 2017, will fall closer to this year’s September 22 equinox. The October 2017 full moon will be this year’s Harvest Moon, while the September 5-6 full moon carried its ordinary monthly full moon name of Fruit Moon in the Northern Hemisphere (and Worm Moon, Lenten Moon, Crow Moon, Sugar Moon, Chaste Moon or Sap Moon in the Southern Hemisphere). Read more about full moon names.

However, in most respects, the September 2017 and October 2017 full moons could be regarded as Harvest Moon co-stars. By that we mean that they share the characteristics of a Harvest Moon. The primary Harvest Moon characteristic has to do with the moonrise. On the average, the moon rises some 50 minutes later with each passing day. Around the time of the full Harvest Moon, the lag time between successive moonrises is reduced to a yearly low.

In 2017, there’s no appreciable difference between the lag in moonrise times associated with September and October full moons. In both of these months, the moon rises a shorter-than-usual time after sunset for several evenings in a row, following the date of full moon.

Ed and Bettina Berg in Las Vegas, Nevada contributed this image of the 2016 Harvest Moon.

How did the Harvest Moon get its name? The shorter-than-usual lag time between moonrises around the full Harvest Moon means no long period of darkness between sunset and moonrise for days in succession.

In the days before tractor lights, the lamp of the Harvest Moon helped farmers to gather their crops, despite the diminishing daylight hours. As the sun’s light faded in the west, the moon would soon rise in the east to illuminate the fields throughout the night.

Who named the Harvest Moon? That name probably sprang to the lips of farmers throughout the Northern Hemisphere, on autumn evenings, as the Harvest Moon aided in bringing in the crops.

The name was popularized in the early 20th century by the song below.

Shine On Harvest Moon
By Nora Bayes and Jack Norworth (1903)

Shine on, shine on harvest moon
Up in the sky,
I ain’t had no lovin’
Since January, February, June or July
Snow time ain’t no time to stay
Outdoors and spoon,
So shine on, shine on harvest moon,
For me and my gal.

And don’t miss this more recent version of the song by Leon Redbone.

When is the Southern Hemisphere’s Harvest Moon? For the Southern Hemisphere, the autumn equinox falls in March. So the Southern Hemisphere always has a full moon with these same characteristics – rising shortly after sunset for several nights in a row – in March or April.

What are some other full moon names? Every full moon has a name. The names vary in cultures around the world, and they particularly vary between the Northern and Southern Hemispheres. One of the most famous full moon names – other than Harvest Moon – is Hunter’s Moon. That’s the name for the full moon after the Harvest Moon. For the Northern Hemisphere, this year’s Hunter’s Moon comes on October 16. Read more about full moon names here.

Bottom line: According to folklore, the closest full moon to the autumn equinox is the Harvest Moon. In 2017, the autumnal equinox for the Northern Hemisphere comes on September 22. So this hemisphere’s Harvest Moon is the full moon of October 5.

Deborah Byrd

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