Why is the Harvest Moon special?
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.
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 minutes later daily for several days before and after the full Harvest moon. Why? The reason is that the ecliptic – or path of the sun, moon and planets – 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 several days in a row. 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 several 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, this year’s Harvest Moon counts as one of the smaller full moons of 2011, because the moon will be just a few days away from
apogee, its farthest point from Earth for the month.
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.
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 will come early in 2011. The Harvest Moon is the full moon closest to the autumnal equinox, which in 2011 comes on September 23. So the full moon of September 11-12 is 2011′s Harvest Moon. September 13 will have a beautiful bright full-looking moon, too. The Harvest Moon is not really bigger, brighter or yellower than other full moons, but it’s special for ushering in the year’s grandest procession of moonlit nights. Enjoy.
http://earthsky.org/astronomy-essentials/harvest-moon-2
