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Ask Chicagoist: How Do the Hours of Sunlight in a Day Change?

By Thales Exoo in Miscellaneous on Sep 25, 2008 7:10PM

Do the hours of sunlight each day change in a linear way (the same amount each time each day) or a exponential/other kind of way? In other words, when the hours of sunlight are increasing, does every day get like 3 minutes longer, or do the first few days only get longer by a minute and then towards June every day gets longer by 15 minutes? Does that make sense? Does everyone else know the answer to this/is this totally common knowledge, and I just missed that day?

2008_09_askdaylightWow, worried about the diminishing sunlight already? Fall is the absolute best time of year, as far as we're concerned (especially this year), so we're trying not to think too much about the impending long winter months. Still, good question. We love a little Thursday afternoon trigonometry.

First of all, daylight doesn't increase and decrease in a linear or an exponential manner. If you take a chart of sunrise and sunset times over a year and plot them on a graph (and really, why wouldn't you do that?), you'll see a curved parabolic shape. It's a sine wave, a shape that occurs rather frequently in nature -- ocean waves, sound waves, light waves. It's called a sinusoidal function, and helps illustrate the concept (shown by that graph we're sure you created earlier) that daylight increases or decreases more slowly at the extremes and faster in the middle. Think of it like a swinging pendulum. As the pendulum rises the swing slows down. As it swoops down in the middle, it speeds up. In this situation of the number of hours of daylight, the "middle" parts are the fall and spring equinoxes, and the "ends" are the summer and winter solstices.

Right now, since we're just past the fall equinox (it was September 22), we're at the fast part of the swing -- our daily dose of sunlight is decreasing more quickly now and we're all rapidly hurdling towards our yearly dose of S.A.D. As we get closer to the winter solstice, that rate of change will decrease and then the amount of daylight will start increasing in the opposite direction. December 21, the turning point of the pendulum, is but three short months away. So brace yourselves. Not to mention, once we throw the end of daylight savings time into the mix on November 2, we'll be perceiving the increase of that darkness even more quickly.

The change is sinusoidal because of the tilt of the Earth as it's spinning on its axis and orbiting on an elliptical path around the sun. The elliptical path causes the rate of our yearly journey around the sun to not be consistent. The path is faster as the Earth approaches the sun and slower as it moves away. The tilt of the Earth is what causes the sun to be "higher" with respect to the planet at the summer solstice and "lower" with the winter one.

Make sense? We think the moral of the story is that it's getting darker more quickly these days, and there's not a lot we can do about it.

Image via Math Demos with Positive Impact.

Feeling the darkness closing in on you? Need some advice? Email ask(at)chicagoist(dot)com.