Falling For Fall And Other Seasonal Deceits

Looking east over the the forest near Sawmill Dome along the north shore of Lake Superior on September 30, 2012. Credit: Bob King
The forest on fire with color near Sawmill Dome along the north shore of Lake Superior on September 30, 2012. Credit: Bob King

The first trees are fired up in reds and oranges, and the tomato vines have withered to brown. I like that the sun sets so much earlier and rises fashionably late. Darkness has nibbled more than 4 hours from the day since the first day of summer, and it won’t stop noshing till the winter solstice. Longer and earlier dark hours mean easier access to the sky with the welcome option of going to bed on time.

The sun never stops moving in the sky because the Earth never stops revolving around it. The sun's motion is a reflection of Earth's yearly orbital journey. It drops south of the celestial version of the equator beginning tomorrow morning and won't move back north of it till the first day of spring 2016. Credit: Bob King
The sun never stops moving in the sky because the Earth never stops revolving around it. The sun’s apparent motion is a consequence of Earth’s yearly orbital journey. It drops south of the celestial version of the equator beginning tomorrow morning and won’t move back north of it until the first day of spring 2016. Credit: Bob King

Fall, the midway oasis between the two extreme seasons, begins Wednesday morning, September 23 at 3:21 a.m. CDT (4:21 a.m. EDT, 2:21 a.m. MDT and 1:21 a.m. PDT). At this instant of time, the sun will cross the celestial equator traveling south. You’ve already noticed it’s been dropping lower since summer. In September, the sun’s always in my eyes during an afternoon drive, while in June, it sits higher overhead and stays out of my windows.

Our star’s southward journey across the celestial equator — an extension of Earth’s equator projected onto the sky — is little more than an illusion. The sun’s not moving at all. No, no, no. It stays put during a year’s time, moving but the tiniest fraction of a degree in its orbit around the center of the Milky Way galaxy. It’s the Earth doing the moving, cruising along its orbit at 18.5 miles per second (30 km/sec) and causing the sun to appear to describe a giant circle around the whole sky during the year.

The celestial equator is an extension of Earth's equator into space. When the sun is furthest south of it, we have the winter solstice; when furthest north, the summer solstice. Credit: Joshua Cesa / Wikipedia
The celestial equator is an extension of Earth’s equator into space. When the sun reaches its furthest south of the equator (for the northern hemisphere), it’s the first day of winter. When furthest north, the first day of summer. The yellow circle is called the ecliptic, the sun’s apparent path around the sky during the year. Credit: Joshua Cesa / Wikipedia

And the up and down stuff – the sun dropping into the southern sky during the winter months and blazing from near the zenith in the summer? That’s fake, too. Again, the sun essentially stays put. It’s the tilt of the Earth’s axis that causes the flip-flop of high sun-low sun across the seasons.

The apparent motion of the sun around the sky is a reflection of Earth's yearly cycle around the sun. The seesaw-like up-and-down movement of the sun from season to season is caused by Earth's titled axis. Credit: Thomas G. Andrews / NOAA
The apparent motion of the sun around the sky is really a reflection of Earth’s yearly cycle around the sun. The seesaw-like up-and-down movement of the sun from season to season is caused by Earth’s titled axis and our planet’s changing orientation to the sun during the year. Credit: Thomas G. Andrews / NOAA

We’re tipped off to this by a simple fact: the sun never gets higher than 23.5° above the celestial equator and never gets lower than 23.5° below the equator. I’m sure you recognize that number — it’s the inclination of our planet’s axis. As we orbit the sun during the year the orientation of the axis (not the tilt) changes with respect to the sun, and these changes are reflected in the seasonal “ups and downs” of our star.

Fall’s entry also comes with a special bonus, days and nights that are equal or nearly equal in length. 12 hours of sunshine followed by 12 hours of moonshine. (Don’t take that the wrong way). The word “equinox” originates from the Latin for “equal” and “night”.

The sun’s slow dip southward means it rises later and sets earlier, spending less time in the sky. You can’t change one thing in nature without changing another. Shorten daylight and the temperature drops, rain becomes snow, chlorophyll in leaves breaks down to expose otherwise hidden red pigments, and we start taking that jacket with us to work and school in the morning.

Happy equinox and I hope you find fall as wonderful a season as I do.