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.
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.
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.
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!
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.
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 duluthnewstribune.com/astrobob.