It’s Harvest Moon time for 2013. The moon has waxed larger each night this past week, and full moon is tonight (September 18-19) for North America. In traditional skylore, the Harvest Moon is the full moon closest to the autumnal equinox. For us in the Northern Hemisphere, the autumnal equinox comes on September 22. That makes this September full moon the Northern Hemisphere’s Harvest Moon.

Order your 2013 moon calendar today.

A full moon always ascends over your eastern horizon at sunset and stays in the sky all night. But each month there is a time of fullest full moon. That crest of the moon’s full phase happens this month on September 19 at 11:13 Universal Time. That’s 6:13 a.m. CDT in the U.S. Translate UT to your time zone.

Depending upon your time zone, you may regard different nights as the night of the September full moon. From our vantage point in North America, the moon turns precisely full before sunrise, September 19, so we might regard the night of September 18-19 as the night of the full moon. In eastern Asia, where the moon turns full after sunset September 19, people in that region of the globe might say the night of September 19-20 is the night of the full moon.

Day and night sides of earth at instant of full moon

Day and nigght sides of Earth at the instant of the September 2013 full moon (2013 September 19 at 11:13 Universal Time). Image credit: Earth and Moon Viewer

There’s also a name for the next full moon after the Harvest Moon. It’s called the Hunter’s Moon, and it’ll come this year on October 18.

So don’t just look for the Harvest Moon on the nights of September 18-19 or 19-20. At middle and northerly latitudes in the Northern hemisphere, look for the moon to be bright and full-looking for a few nights. If you live far enough north – for example, northern Canada or Alaska – the Harvest Moon will shine from dusk until dawn for several nights. This procession of moonlit nights is what characterizes the Harvest Moon.

Harvest Moon rising above the Little Florida Mountains in Deming, New Mexico as seen by EarthSky Facebook friend Dan Gauss. Dan takes a photo of the full moon rising each month from this same spot. See more favorite 2012 Harvest Moon photos here.

Harvest Moon is just a name. It’s the name for the full moon nearest the autumnal equinox. In the Northern Hemisphere, you’ll always see the Harvest Moon in either September or October. In the Southern Hemisphere, a moon with these same characteristics always comes in March or April.

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.

But the Harvest Moon is more. Nature is particularly cooperative around the time of the autumn equinox to make the full moonrises unique around this time.

Here’s what happens. On average, the moon rises about 50 minutes later each day. But when a full moon happens close to the autumnal 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. Why? The reason is that the ecliptic – or the moon’s orbital path – makes a narrow angle with the evening horizon around the time of the autumn equinox. The narrow angle of the ecliptic results in a shorter-than-usual rising time between successive moonrises around the full Harvest Moon.

These early evening moonrises are what make every Harvest Moon special. Every full moon rises around sunset. After the full Harvest Moon, you’ll see the moon ascending in the east relatively soon after sunset for a few days in a row at northerly latitudes. The lag time between successive moonrises shrinks to a yearly minimum, as described in the paragraph above. Because of this, it seems as if there are several full moons – for a few nights in a row – around the time of the Harvest Moon.

Want to know the time of moonrise in your location? My favorite source of that information is this Custom Sunrise Sunset Calendar. Once you get to that page, be sure to click the box for ‘moon phases’ and ‘moonrise and moonset times.’

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

Not necessarily, but the actual size of the Harvest Moon depends on the year. The Harvest Moon has the reputation of being especially big and bright and orange. But it isn’t really the Harvest Moon’s size or brightness that distinguishes it from other full moons. In fact, the 2013 Harvest Moon is pretty close to an average-sized full moon.

Harvest Moon on September 29, 2012 as seen by EarthSky Facebook friend Domenico D’Erchia in Italy. See more favorite 2012 Harvest Moon photos here.

Still, you might think otherwise. 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 lengthy explanations of the Moon Illusion by googling those words yourself.

How the Harvest Moon got its name

So why is this moon – the moon closest to the autumnal equinox – called the Harvest Moon?

The shorter-than-usual 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.

EarthSky Facebook friend Zhean Peter Nacionales in the Philippines camped on the night of the 2012 Harvest Moon with other scout friends. He writes: Me with with my scout friends salute for the Harvest Moon. Thanks, Zhean! See more favorite 2012 Harvest Moon photos here.

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.

Bottom line: The Harvest Moon in 2013 comes on the night of September 18-19 for us in the U.S. The Harvest Moon is the full moon closest to the autumnal equinox, which in 2013 comes on September 22. The exact time of fullest full moon for this month is September 19 at 11:13 UT, or 6:19 a.m. CDT in the central U.S. The night of September 19 will have a beautiful bright full-looking moon, too. The Harvest Moon is not really bigger, brighter or more pumpkin-colored than other full moons, but it’s special because, at this time of year in the Northern Hemisphere, the time between successive moonrises – from one night to the next – is shorter than usual. Enjoy!