The Autumnal Equinox is at 1:44 pm PDT on Sunday, September 22, and officially marks the beginning of the fall season in the northern hemisphere. The name ‘equinox’ comes from the Latin aequus (equal) and nox (night), referring to the 12-hour long day and night that occurs only twice a year, when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun. And it is only on the spring and autumnal equinoxes that the Sun rises due east and sets due west. But, since Earth never stops moving around the sun, these days of equal sunlight and night will change quickly.
The earliest humans spent more time outside than we do. They used the sky as both clock and calendar. They could easily see that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shift in a regular way throughout the year.
Astronomically speaking, the September equinox marks one of the four major turning points in the cycle of seasons. In many regions of North America, the landscape silently explodes with vibrant colors of red, yellow, and orange. The harvest baskets are full of our summer’s work. The leaves begin to drop off the trees, providing endless hours of jumping into leaf piles for kids and raking them back up for parents! Baseball season hits the homestretch, while football season is just warming up. Temperatures begin to drop, nights begin to get longer, and all the woodland critters are storing up for the long haul of winter.
Notice the signs of the autumn equinox in nature… The knowledge that summer is gone, and winter is coming, is everywhere now, on the northern half of Earth’s globe. If you live in the Northern Hemisphere, you can easily notice the later dawns and earlier sunsets. Also notice the arc of the sun across the sky each day. You will find it is shifting toward the south. Birds and butterflies are migrating southward, too, along with the path of the sun. The shorter days are bringing cooler weather. A chill is in the air. In New York City and other fashionable places, people have stopped wearing white. Creatures of the wild are putting on their winter coats. All around us, trees and plants are ending this year’s cycle of growth. Perhaps they are responding with glorious autumn leaves, or a last burst of bloom before winter comes. In the night sky, Fomalhaut, the Autumn Star, is making its way across the heavens each night.
Why do autumn leaves change color?… What type of trees and weather produce the most vivid fall foliage? Not all leaves turn vivid colors in the fall. Only a few of our many species of deciduous trees, notably maple, aspen, oak, and gum, produce stellar performances for our annual autumn spectacular in North America. Several factors contribute to fall color (temperature, precipitation, soil moisture), but the main agent is light, or actually the lack of it. The amount of daylight relates to the timing of the autumnal equinox. As the autumn days grow shorter, the reduced light triggers chemical changes in deciduous plants causing a corky wall to form between the twig and the leaf stalk.
This “abscission layer” eventually causes the leaf to drop off in the breeze. As the corky cells multiply, they seal off the vessels that supply the leaf with nutrients and water and also block the exit vessels, trapping simple sugars in the leaves. The combination of reduced light, lack of nutrients, and no water add up to the death of the pigment chlorophyll, the “green” in leaves.
Once the green is gone, two other pigments show their bright faces. These pigments, carotene (yellow) and anthocyanin (red), exist in the leaf all summer but are masked by the chlorophyll. The browns in autumn leaves are the result of tannin, a chemical that exists in many leaves, especially oaks.