Why won't the Sun set for days at N66.2 which is below the arctic circle?

  • Correct me if I am wrong, but if we count sunsets by the center of the Sun apparently crossing the horizon then the Sun is supposed to set every day at latitudes under the arctic circle. (Yes if you count in the disc then adjust by 0.27 degrees.)


    I was playing around with PyEphem (a python library that claims an approximately 1 arcsecond accuracy), and found that according to it the sun stays up for a couple of days even under it ($66.2<90-23.4$). Can someone explain what is going on?


    I created this simple plot to illustrate the issue. The left panel shows the whole period while the right panel shows that the problem is not that the data is under sampled. The plot goes from
    2021-06-13 00:12:58.085383+00:00 to 2021-06-28 23:54:05.880661+00:00.


    enter image description here


    And one more example: Kuusamo is a city at 65°58′N 29°11′E and apparently it also has a few days when the sun doesn't set: LINK1 LINK2


    You've defined the problem in a way that necessarily produces the very anomaly you're asking about. Almost everyone regards a sunset as meaning the Sun has *entirely disappeared below the horizon*, rather than only half of it having "set". Are you allowing for part of the solar disc remaining above the horizon at the time of the northern summer solstice even at latitudes below (but close to) the Arctic Circle?

    Yes, i did the half disc thing consistently. Notice that even 66.2 < 90-23.4-0.27 so I don't think you are right. I also added a an example to the bottom that is surely correctly computed and is even more to the south.

    Normally, you should wait at least a week before accepting an answer. This will allow you to get more answers and look for feedback on existing answers.

    @slowerthanstopped: Sometimes a question has a simple factual answer. There's no reason to delay accepting when someone posts it, in a case like this. You should still keep an eye on your question in case of comments on the answer pointing out that it's not that simple, but if you're pretty sure an answer *fully* answers the question, you should accept it. That doesn't close the question or stop anyone else from answering, and you can even change your accept vote if an even better answer comes in. (You don't want to long-term leave a less good answer pinned to the top, even if sufficient.)

  • notovny

    notovny Correct answer

    one year ago

    From the PyEphem Quick Reference Guide:



    Rising and setting are sensitive to atmospheric refraction at the
    horizon, and therefore to the observer’s temp and pressure; set the
    pressure to zero to turn off refraction.



    It seems likely that, if you're using the default settings, the result returned is including atmospheric refraction, giving the results you would expect to see at the location, where the sun is visible even though a direct line to the disc of the sun would put it below the horizon.


    Image showing atmospheric refraction from TimeandDate.com


    Image is from "What is Refraction of Light" by By Konstantin Bikos and Aparna Kher at timeanddate.com


    This was it. I never expected to have this turned on out of the box. Thanks!

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Content dated before 7/24/2021 11:53 AM