[searke]

You might want to use Fourier analysis instead. I did this project but with temperature :

https://searke.github.io/2017/01/08/TemperatureGraph.html

Also, coincidentally enough, for Chicago using Wolfram Language. Great minds think alike I guess.

[phkahler]

>> You might want to use Fourier analysis instead.

Yep. There error plot clearly shows a 3rd harmonic dominating. IIRC the length of day is constant at the equator and the light/time function is a distorted sinewave as you go north or south from there. I am curious about the slope within a day. It seem to "get dark quickly" these day vs July.

[url00]

What a great title! Sending warmth to you this winter from Wisconsin.