A red leaf spirals to the forest floor. Frost lingers on shaded green grass after sunrise. So it begins. Small changes soon add up to a brand new season, one that starts today Sept. 22 at 3:44 p.m. (CDT). Is fall here already?
Also called the autumnal equinox, September 22 is one of two times a year when the sun’s path crosses the celestial equator, an extension of Earth’s equator onto the sky.
Picture the celestial equator as half of an imaginary hula hoop tilting up from the eastern horizon, crossing the southern sky and then arcing back down to the western horizon. The hoop’s other half continues below the horizon and beneath our feet, circling over the opposite hemisphere.
On the first day of fall the sun crosses the celestial equator moving south. That means a higher sun for southern hemisphere observers – and more daylight – and a lower sun (more night) for folks living in the northern hemisphere. Six months from now the sun re-crosses the equator moving north, and the day-night balance reverses.
If you live on the equator, the celestial equator begins at the due east point, passes overhead and meets the horizon again at the due west point. With the sun sitting squarely on the celestial equator tomorrow, it will beam down from directly overhead at local noon.
Today in Quito, Ecuador the forecast is for partly sunny skies and a high of 79 degrees F. Since Quito’s on the equator, residents there will only see their shadows at noon if they make a point of staring straight down at their feet. At the north pole, where the celestial equator rings the entire horizon, the sun circles along the horizon for 24 hours, never setting. Shadows stretch like taffy into the distance. At mid-northern latitudes, the sun shines halfway up in the southern sky at noon, and no one will pay attention to their shadow.
Because the sun continues moving south of the celestial equator, it quickly drops below the horizon at the north pole and doesn’t return for another six months. Naturally, the situation is reversed for the south pole; there old Sol finally climbs out of his dark cave and stays up 24 hours a day for next six months. For those of us living between those extremes, the sun’s southward trek means it drops ever lower in the sky (for the northern hemisphere), bringing shorter daylight hours, longer nights and the end of life as we know it. Just kidding.
We can tip our hats to the 23.5 degree tilt of the Earth’s axis for making the four seasons possible. During a year’s time as we orbit the sun, the height of the noonday sun changes because of the changing orientation of our axis. When the north polar axis is pointed toward the sun, it reaches its most northerly point in the sky. Summer begins, and the combined effects of the sun’s steeper slant on our backs and longer daylight hours make for longer, hotter days.
During northern hemisphere winter, the axis points away from the sun and our star is southernmost and lowest in the sky. Shorter days and a low sun bring cold weather.
Fall and spring are in-between times when neither hemisphere is tipped toward the sun. We all face the our star broadside, and day and night lengths are nearly equal across the globe. That’s the meaning of equinox – a single word that combines the Latin words ‘equal’ and ‘night’.
Fall’s first day brings with it another bit of symmetry. Since the sun sits directly on the celestial equator it will rise due east and set due west tomorrow – everywhere except the north pole.
Everyone has favorite things they like about autumn. For me it’s the colors, the way the air smells, earlier nights, cooler temperatures and not having to mow the lawn. I try to be grateful for the simple yet profound fact that were it not for the tilt of Earth’s axis, there would be no looking forward to the change of seasons.
Note: I apologize for being unable to include several helpful illustrations in today’s blog. I’ve apparently run out of space! Hopefully that will be corrected tomorrow. Click the links below to see the images.