I recently decided to set my alarm clock to wake me up when it is "dark" out. In the end, I decided to set my clock to the earliest time that nautical sunrise is in my state (Illinois) and stick with that all year.
While doing some research for this, I noticed something that surprised me. Check out this disparity between astronomical sunrise to civil sunrise for the solstices and equinox (the latter two adjusted for daylight saving time):
- Dec 20: 0533 - 0640 (67 minutes)
- Mar 20: 0526 - 0630 (64 minutes)
- Jun 20: 0320 - 0451 (91 minutes)
To be honest, these sets of ranges surprise me for multiple reasons. I clearly don't know what I don't know, but here are some questions I can formulate:
- Why would twilight be longer in summer than winter? Before seeing this data, I had assumed that since the sun makes a more perpendicular path through the horizon in summer that twilight would be shorter in summer than in winter. After all, in winter the sun takes a "slanted" path across the horizon. Wouldn't the summer's path be more direct and therefore quicker?
- OK: seeing this empirical evidence I conclude that something is wrong with my premise that forms my first question. Summer twilight is longer in summer than winter. However, I still would have assumed that March 20 would have had a twilight length in between the two solstices. But it's not! Why does the equinox have the shortest twilight?
I didn't want to leave my original incorrect statement in here without flagging it. As Cheekhu points out below, the sun does not follow a more perpendicular path in summer than winter, as I had erroneously assumed and stated above. See his post for more details.