3 ms·

A closely related follow-up blog post could be titled: “How Long Twilight Lasts”.

• tasuki 2y ago
  I'd love to read that! It depends on the latitude for sure, but also I think on the time of the year?
  • madcaptenor 2y ago
    Play with timeanddate.com's times. It gives the total amount of civil twilight for the day (which is split in half). For example at Atlanta (https://www.timeanddate.com/sun/usa/atlanta https://www.timeanddate.com/sun/usa/atlanta) there's 58 minutes per day at the summer solstice, 55 at the winter solstice, and 50 at the equinoxes. As you go poleward those numbers increase but it's always summer solstice > winter solstice > equinox. This is basically down to the angle between the sun's path and the horizon and I think I get why that would be different in summer than in winter but I can't get my head around the fall/spring bit.
    • tasuki 2y ago
      Ah yes, a very helpful link! I think the angle under which the sun rises/falls is steeper in spring/autumn. And summer/winter is less steep. And in summer it bends upwards when under the horizon, making summer sunset longer than winter. I'm not sure if these are the right words - it sounded better in my head!
      • madcaptenor 2y ago
        I think you're right about the angles but I can't picture it well enough to understand why.
  • antognini 2y ago
    You'd be able to do it based on the math in the article. Civil twilight lasts from sunset until the Sun is 6° below the horizon. So in the spherical law of cosines in the article, you'd just set a to be -6° instead of 0.