MOLINE, Ill. — We are less than a week away from the autumnal equinox which will officially mark the end of astronomical summer and signal the beginning of astronomical fall. By now, you've likely noticed the amount of daylight we experience is quickly shrinking each day. In fact, September logs the biggest loss of daylight for the entire year, losing an average of about 2.5 minutes of it each day all the way through late October. So, why does September feature this unique free fall in the amount of daylight? Let's dig in!
Gain & loss is not constant
The rate at which we gain and lose daylight each year is not linear, meaning if we were to plot the rate on a graph over time, it wouldn't be a steady, straight line. It would look more like the path of a pendulum. This is due to the tilt of Earth's axis along with its revolution around the sun. So, when we approach both the spring and autumnal equinoxes, those two periods will feature both the biggest gain and loss in the amount of daily daylight. Also, the further north you go in the United States, the more daylight loss, and gain you'll see, also!
Definition of sunrise and sunset
The way we define sunrise and sunset also plays another role in all of this in that on the equinoxes themselves we don't observe exactly 12 hours of daylight and 12 hours of darkness evenly. Sunrise is defined as the point in time in which you see the top of the sun above the horizon, while at sunset you wait for the top to go below the horizon.
Our last sunset after...
7 p.m. is September 21st.
6 p.m. is October 29th
5 p.m. is November 4th (due to Daylight Saving Time ending)
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