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Grab your popcorn: Solstice coming up December
21 |
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December 18, 2017 |
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| Ian Hennes in Medicine Hat,
Alberta, Canada, created this
solargraphy between a June solstice and
a December solstice. It shows the path
of the sun during that time period. |
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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.
No matter where you live on Earth’s globe, a
solstice is your signal to celebrate!
The solstice happens at the same instant for all
of us, everywhere on Earth. In 2017, the
December solstice comes on December 21 at 8:28
a.m. PST. That’s on December 21 at 16:28
Universal Time. It’s when the sun on our sky’s
dome reaches its farthest southward point for
the year. At this solstice, the Northern
Hemisphere has its shortest day and longest
night of the year.
To find the time in your location, you have to
translate to your time zone.
Click here to translate Universal Time to
your local time.
The earliest people on Earth knew that the sun’s
path across the sky, the length of daylight and
the location of the sunrise and sunset all
shifted in a regular way throughout the year.
They built monuments such as Stonehenge in
England and at Machu Picchu in Peru to follow
the sun’s yearly progress.
But we today see the solstice differently. We
can picture it from the vantage point of space.
Today, we know that the solstice is an
astronomical event, caused by Earth’s tilt on
its axis, and its motion in orbit around the
sun.
Because Earth doesn’t orbit upright, but is
instead tilted on its axis by 23-and-a-half
degrees, Earth’s Northern and Southern
Hemispheres trade places in receiving the sun’s
light and warmth most directly. The tilt of the
Earth – not our distance from the sun – is what
causes winter and summer. At the December
solstice, the Northern Hemisphere is leaning
most away from the sun for the year.
At the December solstice, Earth is positioned in
its orbit so that the sun stays below the north
pole horizon. As seen from 23-and-a-half degrees
south of the equator, at the imaginary line
encircling the globe known as the Tropic of
Capricorn, the sun shines directly overhead at
noon. This is as far south as the sun ever gets.
All locations south of the equator have day
lengths greater than 12 hours at the December
solstice. Meanwhile, all locations north of the
equator have day lengths less than 12 hours.
And here’s a natural phenomenon you might never
have imagined. That is, the sun actually sets
more slowly around the time of a solstice. The
slowest sunsets (and sunrises) occur at or near
the solstices. The fastest sunsets (and
sunrises) occur at or near the equinoxes.
This is true whether you live in the Northern or
Southern Hemisphere. And, by the way, when we
say sunset here, we’re talking about the actual
number of minutes it takes for the body of the
sun to sink below the western horizon.
At the December (or June) solstice, the sun
rises and sets farthest south (or north) of due
east and due west. The farther the sun sets from
due west along the horizon, the shallower the
angle of the setting sun. That means a longer
duration for sunset at the solstices.
Meanwhile, at an equinox, the sun rises due east
and sets due west. That means – on the day of an
equinox – the setting sun hits the horizon at
its steepest possible angle.
For us on the northern part of Earth, the
shortest day comes at the solstice. After the
winter solstice, the days get longer, and the
nights shorter. It’s a seasonal shift that
nearly everyone notices.
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.
The precise date of the earliest sunset depends
on your latitude. At mid-northern latitudes, it
comes in early December each year. At northern
temperate latitudes farther north – such as in
Canada and Alaska – the year’s earliest sunset
comes around mid-December. Close to the Arctic
Circle, the earliest sunset and the December
solstice occur on or near the same day.
By the way, the latest sunrise doesn’t come on
the solstice either. From mid-northern
latitudes, the latest sunrise comes in early
January.
The exact dates vary, but the sequence is always
the same: earliest sunset in early December,
shortest day on the solstice around December 21,
latest sunrise in early January.
And so the cycle continues. |
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