At 3:02 p.m. Central time today falls begins. The moment is also called the autumnal equinox from the Latin “equal night” — on the first day of fall, day and night are equal at 12 hours apiece. Like the Earth, the sky is divided into hemispheres, north and south. There’s even an equator up there called the celestial equator that divides the northern half of the sky from the southern.
If you live at 40° N latitude, the celestial equator pokes up from the eastern horizon, arcs across the southern sky, reaches it peak height of 50° (the complement of your latitude) at the due south point and then meets the horizon again at the due west point. Today, the sun will ride the equator from sunrise to sunset, allowing us to mentally trace half of that heavenly circle.
But the sun doesn’t stand still. Because Earth spins on its axis, it rises and sets. That’s easy, right. At the same time however, Earth’s revolution around the sun during the course of the year causes the sun to creep about 1° (two moon diameters) eastward each day and move up and down in the sky, lowest in winter and highest in summer. The fall equinox marks the moment the sun crosses the celestial equator moving from north to south. In other words, the sun is dropping lower in the sky, and we’re headed for winter. At the vernal equinox in March, it’s moving the other direction, from south to north, in the direction of summer.
Why does the sun appear to move up and down, crisscrossing the equator? That’s just a reflection of Earth’s tilted axis. Our planet spins on its axis tilted 23.5° from vertical. On the first day of summer, the sun stands 23.5° (about two fists) north of the celestial equator and 23.5° below the equator on the first day of winter. I love this ballet-like dance of the sun over the years, moving across the sky stage, while plunging and leaping.
I’m headed for the Hidden Hollow Star Party in Bellville, Ohio today, held at the Warren Rupp Observatory. It’s one that state’s oldest star parties and this weekend promises hot and clear nights. I can’t wait to meet all the great people there and get a taste of central Ohio skies. As my plane draws its arc across the blue sky, I know the sun will be doing the same in the opposite direction.
Happy equinox! May fall bring you the cooler weather and wildly colorful trees you’ve been hoping for.