Frost lines a turning leaf. Photo: Bob King
Yikes, fall begins tomorrow night (Sept. 22). Is the autumnal equinox already upon us? Must be the lulling effect a long summer has on the mind, because I didn’t pay attention to the calendar until this morning, the last full day of summer. Here in Duluth, the waning of the season puts many of us in a wistful mood, knowing that we’ll soon be spending five months with our hands cupped around a single candle, snapping icicles off our beards and pulling frozen bodies off the sidewalk. Oh, it’s not like that, really. Well, maybe sometimes.
At 10:09 p.m. Central Daylight time Wednesday, the sun will cross the celestial equator moving south. The celestial equator is simply the earthly version expanded into the sky against the background of stars. If you happen to live on the equator, you’ll see the sun overhead tomorrow around noon, and if you look down at your feet, you’ll see they completely cover your shadow. For everyone on the planet, the sun will rise due east and set due west. Daylight and night will achieve a perfect balance at 12 hours apiece no matter where you live. That’s the meaning behind the word equinox, derived from the Latin words for “equal” and “night”.
Two views of the sun's path at the autumnal equinox which begins tomorrow. The left side shows the view from the equator where the sun passes overhead. The right shows the view from the northern U.S. Credit: Tau'olunga
Depending on your latitude – how far north or south of the equator you are – the sun’s altitude at your location will vary. It shines overhead at the equator, about halfway between the horizon and zenith for mid-northern and mid-southern latitudes and directly on the horizon at the poles. Since the poles are at +90 and -90 degrees latitude, the celestial equator hugs the horizon in all directions.
Because the sun continues moving south of the celestial equator in the days following the fall equinox, it soon disappears below the horizon at the north pole and won’t reappear for another six months. Deep cold will follow quickly enough for high northern latitudes. The situation is reversed for the south pole, where the sun slowly climbs higher and remains above the horizon 24 hours a day for six months.
During Earth's revolution around the sun, we alternately face toward and away from the sun. We're sidelong at the spring and fall equinoxes. Credit: Tau'olunga
This crazy Earth. These variations are all caused by the tip of our axis. In summer, the northern hemisphere is tipped toward the sun, vaulting it high in the sky and making for long hours of daylight and a bounty of heat. During the winter, we’re tipped away, and a low sun means less daylight and subsequent loss of heating. Fall and spring are in-between times when neither hemisphere is tipped toward the sun. We all face it from the side, hence the equal day-night lengths and sun’s east-west path.
Two views of the sun taken around 8 a.m. this morning by the Solar Dynamics Satellite, one in ordinary light that shows the big sunspot #1108, and the other in far ultraviolet light that exposes a large coronal hole. Credit: NASA
While we’re on the topic of the sun, the folks at the NOAA Space Weather Prediction Center, are forecasting minor solar storm levels for the next two nights which could mean a chance of seeing northern lights at high latitudes. The cause of this bit of unsettled weather is a hole in the sun’s atmosphere called a coronal hole. Solar plasma – electrons and protons – are free to stream into the solar system from such holes where they can interact with the magnetic fields surrounding many of the planets and stimulate auroras. There’s also a substantial sunspot group in the sun’s southern hemisphere, but thus far it’s produced no major flares.