The summer's been so hot it's hard to imagine that fall has arrived. But celestial bodies don't look at thermometers. They move on. And that's why we're headed for frosty mornings, like it or not. For the record, I love autumn and its special attributes, including the pleasantly cool nights, quiet clatter of falling leaves and no bugs.
Fall begins when the sun crosses the celestial equator (CE), moving south. It's easy to picture this great circle in the sky. To do it we'll take an imaginary journey to a city near the real equator — Nairobi, Kenya.
The celestial equator is an extension of Earth's equator into space. From Nairobi, the CE starts at the due east point on the horizon, passes directly overhead, and touches the horizon again at the due west point. It continues around the other side of the Earth to make a complete circle.
On the first day of fall, the sun sits squarely on the celestial equator, rising in the east, passing directly overhead at local noon and setting in the west. For Kenyans and other equatorial residents, shadows are now at their shortest. But they'll soon lengthen. Already, the day after the equinox, sun shines a degree south of the overhead point high in the southern sky. This trend continues until the first day of winter, with the sun 23.5° below the CE, as far south as it gets.
Chances are you don't live on or near the equator. But because the celestial equator resides in outer space, high above Earth's equator, it still crosses your sky. To imagine it, subtract your latitude from 90°. That number will be the altitude of the CE at its highest point in the southern sky.
Let's use Chicago as an example. The city is located at latitude 42° North. Subtract 42° from 90° and you get 48°. On Wednesday, the sun will stand 48° above the horizon at it peak altitude around 1 p.m. local time. By the way, although the altitude of the CE changes with latitude, it intersects the horizon at the due east and due west points no matter where you live on the planet.
Here in Duluth, Minnesota its altitude will be 43° because the city's latitude is 47°, about 5° north of Chicago.. From the North Pole, the CE's altitude is 90°-90° or 0° high. What? Yes, at either pole, the celestial equator makes a complete ring along the horizon.
Imagine standing at the pole on the first day of fall. The sun would crawl through a complete 360° circle at the horizon. The following day, moving south, it would disappear below the horizon. Instead of sunshine you'd experience 24 hours of bright twilight. From then on, the sky would get darker and darker as the sun continued to sink southward. It won't return at the horizon until next spring, the reason the poles experience six months of twilight and darkness.
From the northern hemisphere, any time the sun heads south, it drops lower in the sky. It began this southward slide on the first day of summer, and days have been getting shorter ever since. The sun reaches its lowest point — 23.5° below the CE — on December 21st. Then it turns around to the north and climbs back up the sky, crossing the CE again on the first day of spring. Any time the sun moves north, days lengthen and nights shorten. It reaches its northernmost point in the sky (highest elevation) 23.5° north of the celestial equator on the summer solstice.
Now, you have some context to appreciate why the sun's southward crossing of the celestial equator is relevant. The equator is the midway point between the extremes of winter and summer.
Of course, the bigger question is why the sun moves at all. I know you know the answer to that. It's all the Earth's doing! The sun's bouncing up and down the sky during the year is simply a reflection of Earth's tilted axis paired with its orbital motion.
The planet's axis steadily maintains that tilt throughout its orbit, but the orientation of the axis to the sun varies across the year, the reason the sun dips up and down. In summer, the north pole end of the axis tilts toward the sun as if in a bow, and the sun stands high in the sky. In winter it tilts away as if bending backwards, and the sun rolls low. On the first day of fall and spring, it tilts neither toward nor away. Because the Earth is sideways to the sun the day is split equally — every place gets 12 hours of dark and 12 hours of light.
There's that word. Equally. It's the root of equinox, Latin for equal plus night. For stargazers, the start of fall means not having to stay up late to see the stars and the opportunity to catch a morning sunrise at a not-ungodly hour. Living things respond to the decrease in daylight by heading south, hibernating, dropping leaves and splitting firewood. It's all so simple really. And equally profound.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.