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Longest night of the year approaching |
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December 9, 2016 |
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| Image via
rhythmsofplay.com |
| At the
December solstice, the Northern
Hemisphere of Earth is leaning most away
from the sun. Above the Arctic Circle,
there is 24 hours of darkness.
Meanwhile, the Southern Hemisphere and
Antarctic are bathed in sunlight.
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By Deborah Byrd
EarthSky.org
Late dawn. Early sunset. Short day. Long night.
For us in the Northern Hemisphere, the December
solstice marks the longest night and shortest
day of the year.
Meanwhile, on the day of the December solstice,
the Southern Hemisphere has its longest day and
shortest night.
This special day is coming up on
Wednesday, December 21, at 10:44 UTC (December
21 at 2:44 a.m. PST). No matter where you live
on Earth’s globe, a solstice is your signal to
celebrate.
For all of Earth’s creatures, nothing is so
fundamental as the length of daylight. After
all, the sun is the ultimate source of all light
and warmth on Earth.
If you live in the northern hemisphere, you can
notice the late dawns and early sunsets, and the
low arc of the sun across the sky each day. You
might notice how low the sun appears in the sky
at local noon. And be sure to look at your
noontime shadow. Around the time of the December
solstice, it’s your longest noontime shadow of
the year.
In the Southern Hemisphere, it’s opposite. Dawn
comes early, and dusk comes late. The sun is
high. It’s your shortest noontime shadow of the
year.
The December solstice marks the shortest day of
the year in the Northern Hemisphere and longest
day in the Southern Hemisphere. But the earliest
sunset – or earliest sunrise if you’re south of
the equator – happens before the December
solstice. Many people notice this, and ask about
it.
The key to understanding the earliest sunset is
not to focus on the time of sunset or sunrise.
The key is to focus on what is called true solar
noon – the time of day that the sun reaches its
highest point, in its journey across your sky.
In early December, true solar noon comes nearly
10 minutes earlier by the clock than it does at
the solstice around December 22. With true noon
coming later on the solstice, so will the
sunrise and sunset times.
It’s this discrepancy between clock time and sun
time that causes the Northern Hemisphere’s
earliest sunset and the Southern Hemisphere’s
earliest sunrise to precede the December
solstice.
The discrepancy occurs primarily because of the
tilt of the Earth’s axis. A secondary but
another contributing factor to this discrepancy
between clock noon and sun noon comes from the
Earth’s elliptical – oblong – orbit around the
sun.
The Earth’s orbit is not a perfect circle, and
when we’re closest to the sun, our world moves
fastest in orbit.
Our closest point to the sun – or perihelion –
comes in early January. So we are moving fastest
in orbit around now, slightly faster than our
average speed of about 30 kilometers (18.5
miles) per second.
The discrepancy between sun time and clock time
is greater around the December solstice than the
June solstice because we’re nearer the sun at
this time of year. |
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