Daylight ebbs to a minimum on Wednesday's winter solstice, but not for long. The very next day, the Sun turns back north and the cycle of light begins again.
"Limbo lower now. Limbo lower now. How low can you go?" The Sun's been doing the limbo since last June, when it stood highest in the sky on the first day of summer. The following day, it began slipping southward along its yearly circle 'round the sky called the ecliptic. And on Wednesday morning, December 21st, at 5:44 a.m. Eastern time (10:44 UT), it limbos lowest, reaching the nadir of its arc.
How low is low? Depends on your latitude. At the north pole, the Sun dropped below the horizon after the first day of fall, not to return until next spring. The pole experienced the last vestige of twilight in mid-November and won't sense the Sun's presence again until about January 28th, when twilight returns.
In Anchorage, Alaska, the Sun tops at 5.5° altitude at local noon on the solstice — that's just three fingers held together at arm's length above the horizon. Daylight lovers there face a severe deficit with just 5 hours 28 minutes separating sunrise from sunset.
We complain here in Duluth, Minnesota, about driving around in a "tunnel of darkness" for much of the winter, but have it easy in comparison, with a more "tropical" 8 hours 32 minutes of sunshine and a 20° high Sun. It gets better as you travel south: in Duluth, Georgia (latitude 34°), the Sun stands more than three fists high (33°) high and days are longer yet.
But no matter where you live in the Northern Hemisphere, the winter solstice marks the Sun's lowest point in the sky and shortest day of the year. Conversely, in the southern hemisphere, the Sun reaches the highest point of its arc — summer solstice — and the nights are shortest.
Solstices have always had something of a split personality, bright side and dark side, as it were. Dearth of daylight defines the dark side, but we all know that when you've reached bottom, the only way to go is up. That's exactly what happens the moment after the Sun passes through its low point. It begins moving northward up the ecliptic and back toward summer.
Rockin' the Light
Our ancient ancestors, who liked a good party as much as we do, celebrated the return of the light at this darkest time of the year with big holiday bashes like Dies Natalis Solis Invicti or "birthday of the unconquered Sun" (Rome), Saturnalia (Rome), the Yule (Norway), and other pre-Christian festivities. The December 25th date for Christmas, the time of celebrating the birth of Jesus, may have been chosen to tap into the spirit of these earlier celebrations, but with a more sacred take.
Calendars for the same year sometimes show different dates for the solstice, typically December 21st or 22nd, because of time zone differences. For instance, if the solstice occurs at 2 a.m. in London, England on December 22nd, it happens in Boston at 9 p.m. on December 21st.
While December 21st and 22nd are the most common dates, the solstice can also occur as early as December 20th and as late as December 23rd. The spread in dates has to do with our calendar system, which measures time uniformly across the year, versus the slippery clock that is the real Earth. The time it takes for the Sun to return to the solstice and equinox points varies year to year according to the orbital and daily rotational motion of the Earth, which are acted upon by the Sun, Moon and planets. Even precession, the wobble of Earth's axis caused by the tug of the Moon and Sun, contributes to the shifting date of the winter solstice from the norm of December 21–22 to its extremes.
Solstice is really a two-part word forged from the Latin "sol" (sun) and "sistere" (to stand still). That's not a bad description of what really happens. The Sun, which has been racing south along the ecliptic since late summer, appears to slow down and practically stand still around the time of solstice as it travels along the bottom of its descending arc. Changes in sunrise and sunset times almost come to a halt as if the Sun were stuck in place.
Sticky Sunsets
Counter to intuition, the Sun sets earliest before the winter solstice (and rises latest after the solstice), not on the date itself. Once again, it’s because our clocks keep steady, uniform time compared to the more “elastic” timekeeping system the Sun follows.
A man-made clock will always measure the interval from one noon to the next as exactly 24 hours, but the solar clock can vary by a fraction of a second up to a half minute from one noon to the next due to the tilt of Earth’s axis and the planet’s changing orbital speed.
Earth’s rotation causes the Sun to rise in the east and set in the west, while Earth’s revolution around the Sun causes the sun to shift a short distance eastward each day. We sense the Sun’s eastward drift in the recent reappearance of Jupiter in the morning sky. Two months ago it rose in bright twilight; since then, the Sun has moved off to the east, leaving Jupiter hanging high in a dark sky before dawn.
When Earth and Sun are closest in December and January, the Sun appears to move eastward a bit faster, covering more sky each day compared to the other seasons. This creates a delay in sunrise time, making for later sunrises. A later sunrise also means the Sun arrives at the sunset point later. Let's use this fact to explore the disconnect between the solstice and earliest sunsets.
In late November and early December, the Sun is sinking to its southernmost point in the sky, so its daytime arc is getting shorter and shorter, lower and lower. This southward slide pairs up with the Sun’s flight east, delaying sunrise even more.
Through late November, the Sun's southward motion more than cancels out its eastward movement, causing it to keep setting earlier and earlier. But beginning around December 2nd, the Sun’s speedier eastern motion (due to Earth's increasing orbital speed) overpowers the tiny bit of southern travel remaining at the bottom of its curve, causing it to slow down, “stall,” and then start setting later. That’s why the earliest sunsets occur in early December and grow later by mid-month before the solstice.
At the solstice, the Sun begins moving north again. Its arc steepens and days begin to get longer. But that extra eastward movement still keeps it rising later for a time until the effect is canceled out by the Sun's steady and steepening northward creep up the ecliptic. Result? Sunrises start coming earlier. This happens in early January from the mid-northern latitudes, when the Sun begins to both set and rise later. To see how the cycle of the Sun’s rising and setting plays out for your city, click here and type in your location.
With later sunsets and earlier sunrises working in tandem, keen-eyed skywatchers easily sense the increase in daylight as early as mid-January.
Despite the cold and brief sunshine, solstice time does have distinct advantages. With 4:30 p.m. sunsets, skywatchers can get out early to enjoy the night and return to the blessed warmth of their homes by 8:00. Late sunrises also mean many of us can observe Jupiter and other sky sights before going to work in the morning.
For the properly-dressed amateur astronomer, an abundance of darkness is reason to cheer on this special day. Let me be the first to wish you a happy solstice!
Comments
Anthony Barreiro
December 20, 2016 at 5:39 pm
Thanks very much, Bob, for all the beauty, wonder, and wisdom you have shared with us S&T readers during Earth's most recent trip around the Sun. Best wishes for a joyful solstice.
My understanding is that for ancient people who synchronized their calendars through direct observation of the Sun, the term solstice was quite literal. To the naked eye without sophisticated surveying equipment, the Sun appears to rise at the same point on the horizon for five days. Day three was marked as the solstice. It was only when we got too lazy to get up at dawn and keep vigil that we needed to start messing around with leap days, etc.
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Bob KingPost Author
December 21, 2016 at 1:51 am
Thank you for such kind words, Anthony. I always appreciate your comments. You add interesting material and personal reflections on the topic and also point out occasional flubs, which keeps me on my toes 🙂 Happy solstice!
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Anthony Barreiro
December 22, 2016 at 1:59 pm
Thanks!
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Monica
December 21, 2016 at 10:38 am
Informative, but perhaps you can be more even-handed than your suggestion that the date for Christmas as "may have been chosen to tap into the spirit of these earlier celebrations, but with a more sacred take." That's one way to describe efforts at converting the "pagans." Let's not obscure history while enlightening others on science.
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Marco Merlo
December 24, 2016 at 10:24 am
Thank you Bob, but I can't understand why the motion of the sun is eastward. For observers in the northern hemisphere, the revolution of the earth around the sun is counterclockwise, that is eastward. Every night, stars moves westward. Shouldn't the sun follow the same pattern? Where is my mistake?
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Bob KingPost Author
December 24, 2016 at 11:14 am
Hi Marco,
That's such a great question. The reason the sun appears to move eastward is because we're revolving around it. Let's try a mental experiment. Shoot a line from the Earth through the sun and extend it so it points to the background zodiac constellations. In December, the sun appears in Sagittarius, so you can imagine the line pointing that way. Now set the Earth in counterclockwise motion about the sun. As time passes, the arrow moves to the left or east (ie. the sun appears to "move" east from Sagittarius to Capricornus and so on), keeping pace with Earth's orbital motion. Kind of like a hand on a clock.
Now, as the sun appears to travel east, it leaves Sagittarius behind, making it look like Sagittarius is returning at dawn to a dark sky. Six months from now, the sun will have left it so far behind that the constellation will appear in the evening sky directly opposite the sun.
This apparent westward drift of Sagittarius -- and all the other star groups -- is caused by Earth's motion around the sun.
I hope this helps 🙂
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Marco Merlo
December 24, 2016 at 3:01 pm
thank you and ...greetings from Italy!!!
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Bob KingPost Author
December 24, 2016 at 4:53 pm
Marco,
Greetings and a Merry Christmas from snowy Duluth, Minn. U.S.!
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