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So shine on, shine on harvest moon |
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September 27, 2017 |
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By Deborah Byrd
EarthSky.org
The moon reaches its first quarter phase on
September 28, 2017, at 7:54 p.m. A first quarter
moon shows half of its lighted hemisphere – half
of its day side – to Earth. The moon is now
waxing toward full moon – and the Harvest Moon,
or closest full moon to the September equinox –
on October 5.
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Quarter moon |
At quarter moon, the moon’s disk is
half-illuminated by sunlight and half-immersed
in the moon’s own shadow.
We call this moon a quarter and not a half
because it is one quarter of the way around in
its orbit of Earth, as measured from one new
moon to the next.
Also, although a first quarter moon appears
half-lit to us, the illuminated portion we see
of a first quarter moon truly is just a quarter.
We’re now seeing half the moon’s day side, that
is.
Another lighted quarter of the moon shines just
as brightly in the direction opposite Earth!
The Northern Hemisphere’s autumn equinox was
September 22. Harvest Moon – the full moon
closest to this equinox – is coming up October
5.
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.
Is the Harvest Moon bigger, or brighter or more
colorful? Not necessarily.
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| In autumn,
the angle of the ecliptic – or sun and
moon’s path – makes a narrow angle with
the horizon. |
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| 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. Images via
classicalastronomy.com. |
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.
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.
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, "Shine On Harvest Moon." |
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