Astro Bob: Winter solstice brings hope during the longest night of the year

Winter officially begins on Monday, December 21st at 4:02 a.m. Central Time. During the longest night of the year we find ways to be hopeful.

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The sun rises with two sundogs in tow over the ice-encrusted shore of Lake Superior in Duluth. (Bob King)

We've finally arrived. Or I should say that the sun has finally arrived? Tomorrow at 4:02 a.m. it reaches its lowest point in the sky in 2020, marking the start of winter. The lower the arc the sun makes across the sky the less daylight. Here in Duluth, Minnesota, the sun rises at 7:51 a.m. and sets at 4:23 p.m., dishing up just 8 hours and 32 minutes of daylight while leaving us stranded in 15 1/2 hours of darkness.

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On the winter solstice the sun climbs to about 20° at noon in Duluth, Minn. (latitude 47° N) but only 2° in Fairbanks at latitude 64°. North of 66.5° N, the latitude of the Arctic Circle, the sun doesn't rise on the solstice. (Stellarium with additions by the author)

I can't complain. In Fairbanks, Alaska, the sun rises at 10:59 a.m. and sets at 2:40 p.m. — a 3 hour, 41 minute performance. And while the sun rides a low arc in Duluth, climbing to just 20° (two fists) at noon, it manages only a measly 2° in Fairbanks. While long hours of darkness sometimes can get you down, especially when coupled with gloomy, gray days, there's an upside for sky watchers. We can enjoy the night sky right after dinner and get to bed at a decent hour. In June, night arrives between 11 and midnight when we should really be in bed. Most amateur astronomers will also quickly point out that winter also means no mosquitos.


Seasons Earth orbit Sonoma U_S.jpg
During northern hemisphere winter, the Earth's axis points away from the sun, causing it to describe a short, low path in the sky. In summer, we "lean into" the sun (left), and it appears high in the sky. At the equinoxes our planet sits sideways to the sun so the entire globe experiences equal amounts of sunlight and darkness. Seasons are reversed in the southern hemisphere. (Courtesy of Sonoma State University)

As I discussed in this recent blog , the sun's yearly swing from its summertime high to wintertime low is a reflection of the 23.5° tilt of Earth's axis. Midway between those extremes lies the celestial equator, an imaginary extension of the Earth's equator into the sky. Picture it as a big circle starting at the due east point on the horizon, extending across the southern sky and touching the horizon again at the due west point. It continues around the backside of the Earth before returning to complete the circle at the eastern horizon.

From mid-latitudes the celestial equator stands about halfway up the southern sky. The sun crosses this circle moving north on the first day of spring, then crosses it again moving south on the first day of fall. The crossing points are called equinoxes.

Sun moves south of celestial equator after fall equinox.jpg
The celestial equator is a great circle in the sky that lies midway between the sun's solstice points. On Monday's winter solstice, the sun dips to 23.5° below the celestial equator. Six months from now, it will shine 23.5° north of the equator and reach its highest point in the sky. The annual cycle is caused by the tilt of Earth's axis. (Stellarium with additions by the author)

On the first day of summer the sun stands 23.5° north of the celestial equator, while on the first day of winter it's 23.5° south of it.

Look at that number. It's no coincidence that it's the same as our planet's axial tilt. If the Earth's axis were tipped 3°, the sun's highs and lows would be restricted to just 3° above and below the celestial equator, a change in altitude of just 6° across the year. That would mean no seasons. Or maybe it would be better to say there would be just one season — a year-long spring-fall we'll call sprawl.

If the tilt were 40°, the sun's range in altitude across the year would be 80°, and the seasons would be far more extreme. Uranus is tilted 98° and takes 84 years to orbit the sun. During summer it's north pole points almost directly at the sun and receives 21 years of summertime sunshine, while the south pole points away and experiences 21 years of winter darkness. That's what I call extreme!


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Earth's tipped axis causes the sun to swing up and down in the sky during the year. It reaches its high point on the June solstice and low point on the winter solstice, coming up on Dec. 21. As the sun approaches the low point, it's moving both south and east. After the solstice, it moves east and north. (Bob King)

Once the sun bottoms out in the sky on the winter solstice it can only go one way. Up. The moment it reaches its lowest point, it slowly begins moving northward again. As the sun climbs higher daylight begins to nibbles away at darkness. After a 3-month slog it will arrive at the celestial equator on March 20, and we'll celebrate the start of spring, when day and night are equals again. Three months after that the ticks and mosquitos return. For joy.

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See anything odd in this photo? Orion is upside down! This photo was taken at the Carnegie Las Companas Observatory in Chile. (Yuri Beletsky)

Let's not forget the southern hemisphere. While the northern hemisphere is tipped away from the sun on Dec. 21, the south polar axis nods toward the sun, making Monday the summer solstice for our friends Down Under. For them, Orion and Canis Major are summer stars associated with steamy temperatures, a jump in the pool and firefly-studded nights.

If darkness and COVID-19 feel insurmountable at times, the speedy arrival of vaccines and the inevitability of spring are currently "in conjunction" and offer hope in the days and weeks ahead. Wishing you a happy and hopeful solstice!

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at .

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