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The Winter Solstice Is Coming, But What Exactly Is It?

20 Dec 2017, 5:12 pm

We have already seen plenty of snow in the Cascades, the Sierra Nevada, and the Rockies this year.  And rare December snowflakes have flown in the southeast as well.  But winter has not officially arrived yet.  That happens on Thursday, December 21st, at 11:28am Eastern time.  In the Northern Hemisphere, this is known as the winter solstice.  Many people call it the “shortest” day of the year.  But technically, that’s not correct.  December 21st will still be 24 hours long.  It’s actually the “darkest” day of the year in the Northern Hemisphere.  The day with the least amount of daylight hours.  It’s been a long time since many of us have been in a science or astronomy class.  So let’s take a brief look at the seasons, why they change, and specifically, the winter solstice.

The earth rotates on it’s axis once every 24 hours.  This is what makes day and night possible.  Obviously, as a location on the earth spins away from the sun, it experiences sunset and then nightfall.  Sunrise and daylight return as the location spins toward the sun.  We all know that.  But seasons are possible, because the Earth is tilted on it’s axis at an angle of 23.5 degrees, and it revolves around the sun.  This varies the amount of sunlight a location receives during different times of the year.  The duration of sunlight differs, along with the warming power of the sun.  But the changes happen at the same time every year.  That’s why here in the U.S. we ski in the winter months, and go to the beach in the summer months.

During the winter solstice, the Northern Hemisphere is pointed away from the sun.  The sun’s rays shine directly on the Tropic of Capricorn.  This has a latitude of 23.5 degrees south of the Equator.  See the correlation?  This latitude matches the angle of the Earth’s tilt.  All locations north of the Equator have less than 12 hours of daylight.  All locations south of the Equator have more than 12 hours of daylight.  Places along the Equator will have equal amounts of day and night, (give or take a few minutes for reasons that are beyond the scope of this article).  But as you travel north of the Equator, you’ll see less and less daylight, until you reach the Arctic Circle.  From that point to the North Pole, there is no daylight on the winter solstice.  As you progress south of the Equator, locations will see more and more daylight, until the Antarctic Circle.  From that point to the South Pole, there are 24 hours of daylight.  December 21st is called the winter solstice in the Northern Hemisphere, but it’s called the summer solstice in the Southern Hemisphere.  That’s because people in the Northern Hemisphere are skiing and using their fireplaces, while people in the Southern Hemisphere are swimming and surfing.

Winter Solstice:

In June, everything is reversed.  The Northern Hemisphere is tilted towards the sun.  The sun’s rays are directly over the Tropic of Cancer, 23.5 degrees north of the Equator.  And the Southern Hemisphere is pointed away from the sun.  It’s summer in the north, and winter in the south.  That’s why in June, the Northern Hemisphere has its summer solstice, and the Southern Hemisphere has its winter solstice.  Areas north of the Equator have more than 12 hours of daylight, while areas to the south have less than 12 hours of daylight.  There are 24 hours of sunlight from the Arctic Circle to the North Pole.  There is no daylight from the Antarctic Circle to the South Pole.

Summer Solstice:

In between the winter and summer solstice, the Northern hemisphere gets a little more daylight each day, while the Southern Hemisphere sees a bit less daylight each day, usually by a minute or so.  In March, the vernal equinox takes place.  The sun’s rays are directly over the Equator, and there is approximately 12 hours of daylight and darkness at all locations around the Earth.  After the vernal equinox, the Northern Hemisphere continues to grow lighter, and the Southern Hemisphere becomes darker, until the Summer Solstice in the Northern Hemisphere in June.  The cycle then begins to reverse, with less daylight each day in the Northern Hemisphere, and more daylight in the Southern Hemisphere, until the winter solstice in December.  In September, the autumnal equinox marks the halfway point of this transition.

Vernal and Autumnal Equinox:

So in summary, the Winter Solstice will mark the beginning of winter for the United States, and the day with the least amount of daylight.  But just remember, after December 21st, each day will get a little more daylight, and before long, spring and summer will be here once again.  Not so uplifting if you like the cold and the snow.  But certainly a positive if you prefer the beach and backyard barbecues.

For WeatherNation:  Meteorologist Matt Monroe

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