All Things Being Equal, It Must Be Fall

Overlapping sugar maple leaves create a kaleidoscopic display of fall color. Bob King

Happy first day of spring! That’s what they’ll be saying tomorrow in the southern hemisphere. Here in the north the first moment of fall ticks off Monday morning Sept. 23 at 2:50 a.m. (Central Time). The sun has been slowly sinking southward since the first day of summer, when it shone from its highest point in the sky. At the fall equinox, it crosses the celestial equator, an imaginary circle in the sky that’s an extension of Earth’s equator into the heavens.

Earth’s equator is an imaginary circle with a latitude of 0° located midway between the poles. Stretch the equator out into the sky and it becomes the celestial equator. NASA

If you’re standing somewhere along Earth’s equator, say not far from the city of Singapore, the celestial equator begins at the due east point of the eastern horizon, passes directly overhead and meets the horizon again at the due west point. Picture it as a giant invisible hoop stretching from one horizon to the next. It continues below the western horizon around the back side of the planet until it meets up again at your eastern horizon to make a complete circle.

On Monday, the sun will rise due east, pass directly overhead and set due west. At local noon, with the sun shining from the zenith, equatorial residents will stand squarely in their own shadows. If you were out walking that day you wouldn’t notice your own shadow unless you looked directly down at your feet!

The diagrams show the position of the sun at local noon on the Sept. 23rd fall equinox from the equator (top) and Des Moines, Iowa. Stellarium with additions by the author

From mid-northern latitudes in cities like Minneapolis, Chicago, Atlanta and San Francisco, the celestial equator touches the east and west points of the horizon the same as at the equator but arcs across the southern sky. Its highest point in the south is equal to your city’s latitude subtracted from 90°.  In Minneapolis for instance (latitude 45° N) the sun is 90°-45° = 45° high, exactly halfway between the overhead point and the southern horizon at local noon. In Atlanta (latitude 34°N) the sun climbs to 56° altitude on the equinox. From the southern hemisphere the celestial equator arcs across the northern sky.

Equinox means “equal night,” a reference to the the days and nights being of equal length (12 hours apiece) nearly everywhere across the planet. The sun will also rise due east and set due west nearly everywhere. The only exceptions are the north and south poles. There the height of the sun is 90°-90° = 0 degrees high. In other words the sun sits exactly at the horizon and skims along parallel to it all day and all night.

Because the sun continues moving south toward its low point in the sky, a.k.a. the winter solstice, it will drop completely out of view at the poles this coming week. Six months of twilight and darkness will follow until the sun returns to view — moving north — on March 19, 2020, the spring equinox.

For skywatchers at mid-northern latitudes, the sun is about four and half fists above the southern horizon at noon and steadily dropping south each day. The lower the sun’s altitude, the briefer the arc it makes in the sky and the shorter the days become. Short days lead to less solar heating and falling temperatures, and that spells one thing: winter is coming. The lessening of light and return of the cold is felt by all creatures. Leaves stop producing sugar. Chlorophyll breaks down, exposing other colors that have been in hiding all summer. Animals gather and store food, birds migrate to warmer regions, and turtles and bears hibernate.

On the first day of autumn, the sun crosses the celestial equator moving south. The sun’s apparent motion across the sky over the year is caused by Earth’s revolution around the sun. Bob King

But I don’t mean to rush you by bringing up winter so soon. Fall should be treated like a delicious meal where we make a deliberate effort to savor each bite. Make time for a walk in the forest to wrap yourself in the colors of the season. Breathe deeply to fill your nose with the tang of leaf decay. Feel the low sun on your face, its light sharpened by a new transparency in the air. Cock your ear to listen to the delicate clatter of leaves falling to the ground on a still, frosty morning.

Earth’s axis maintains a 23.5 tilt as it orbits the sun, but its changing position in orbit causes the axis to point toward, away and sideways to the sun during the year, the reason behind the seasons. Sonoma University with additions by the author

All this up and down movement of the sun is completely fictitious by the way. It’s Earth’s doing — the sun is just sitting there in the same place as far as the planets are concerned. It’s the tilt of our planet’s axis writ large in the sky that makes the sun appear to sink low in winter and climb high in summer. As the Earth wings around the sun on its tilted axis, first one hemisphere faces toward the sun (summer) and then away (winter). The in-between times, when sunlight falls equally across both hemispheres, mark the first days of spring and fall. The equinoxes are the tipping points to the more extreme seasons. A time to appreciate the subtler things.