Charting the latitude at which the sun stops setting

June 22, 2022 3:53 PM

I would like a chart of the latitude at which the sun stops setting (never dipping below the horizon) over time. This seems like it should be pretty easy to find, but I'm having trouble.

This would show 66.56° on the solstice, and a larger number before/after that. I found this, which is quite close but not really what I want, and lots of sites that tell me sunrise/set times for a given location, but none that do the reverse. I would be happy also with a calculator that lets me put in a single day and spits out the latitude at which the sun stops setting.

(I am currently walking north until I catch the sun, and would like to chart my progress)

posted by wesleyac to Science & Nature (8 answers total)

I believe that this is approximately what the definition of a polar circle is.
posted by sagc at 4:21 PM on June 22, 2022 [1 favorite]

You're right, it's surprisingly hard to find that particular slice of information!

The chart in this Wikipedia article has the info you're looking for, I think. It's not in English, but the caption has enough info to understand the chart. Each arc represents one degree of latitude. The dates on the left show the start and end date of the midnight sun at that latitude (in DD/MM format). The numbers on the right show the total number of days of midnight sun.
posted by mekily at 4:24 PM on June 22, 2022 [1 favorite]

Since the arctic/antarctic circle is where this condition happens on the summer solstice, I think what's being asked for is how far north or south that condition moves every day.
posted by LionIndex at 4:25 PM on June 22, 2022

Best answer: This is not too difficult to calculate, so I've done a spreadsheet.

Something is not quite right around the equinoxes as you'd expect to flip to the other pole right at the equinox and the date I'm calculating seems to be off by a day or two. Not quite sure why that's off, though the whole thing is an approximation anyways, since none of the things we are assuming are round for this calculation are actually round. In any case, this should be close enough for your needs.
posted by ssg at 5:32 PM on June 22, 2022 [2 favorites]

(A related topic is one of Astronomical Twilight - the point where the sun is between 12 and 18 degrees below the horizon. As explained here this equates to an all night lack of full on darkness, at the summer solstice to places with an attitude 48°33′ and 54°33′ North and South. Here in Scotland we don't get a full night of >18 degrees below the horizon darkness between mid May and mid September, for example).
posted by rongorongo at 1:59 AM on June 23, 2022 [2 favorites]

I've done a brief spreadsheet here based on this for calculating sunset/sunrise time.
posted by BigCalm at 2:20 AM on June 23, 2022

One detailed complication; the arctic circle moves every year because the Earth's axis wobbles a bit. It's not a huge effect, about 48 feet a year, but it adds up over the 40,000 year cycle. The drift is nicely visualized here. See also this sculpture project where there's a giant concrete sphere that is moved to follow the change, on the small Icelandic island of Grímsey.
posted by Nelson at 7:44 AM on June 23, 2022

The rather awesome website Suncalc will allow you to work this out and display it in an easy to follow way.
posted by Happy Dave at 7:54 AM on June 23, 2022 [2 favorites]