When I am going to run in the afternoon I usually check to see what time the sunsets. Most of the trails close 30 minutes after sunset. They call this twilight. After the sun sets it is still light because the sun is just below the horizon but it is still illuminating the sky. There are actually three different twilights. The one we are most accustom to is civil twilight which is about 30 minutes after sunset. Then after that is nautical twilight which is about 30 minutes after civil twilight. After another 30 minutes comes astronomical twilight. Here are the twilight times for today:
Sunset: 5:31
Civil Twilight: 5:59 (When the human eye can clearly distinguish objects)
Nautical Twilight 6:30 (When sailors can navigate based on the horizon)
Astronomical Twilight: 7:01 (After this astronomers can see the most stars)
Twilight depends on how far the sun is below the horizon. Civil twilight is when the sun is 6 degrees below the horizon. Nautical twilight is from 6 to 12 degrees below the horizon. Astronomical twilight is 12 to 18 degrees below the horizon. After that it is officially night.
If the parks system would go by astronomical twilight then we would have an extra hour to run trails in the evening.
In July when Kathy and I ran the Get Your Butt Kicked Race from 6 to midnight we did not need our headlights until long after sunset. We must be sailors or astronomers.
Today's run was the Castlewood Cup Course which is a 15K. I started about 4:30. Shorts weather yesterday, snow flurries today. It was 30 degrees. I was dressed for 30 degrees but I was not dressed for 30 degrees with a wind chill of 19. I was cold most of the run. Due to the fact that I had to go by civil twilight, I only ran about 6 miles. I did not want to get stuck on the trails in the dark.
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