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"The Arctic Circle marks the southern extremity of the polar day (24-hour sunlit day, often referred to as the "midnight sun") and polar night (24-hour sunless night). North of the Arctic Circle, the sun is above the horizon for 24 continuous hours at least once per year and below the horizon for 24 continuous hours at least once per year. On the Arctic Circle those events occur, in principle, exactly once per year, at the June and December solstices, respectively. In fact, because of atmospheric refraction and because the sun appears as a disk and not a point, part of the midnight sun may be seen on the night of the northern summer solstice up to about 50′ (90 km (56 mi)) south of the Arctic Circle; similarly, on the day of the northern winter solstice, part of the sun may be seen up to about 50′ north of the Arctic Circle. That is true at sea level; those limits increase with elevation above sea level although in mountainous regions, there is often no direct view of the true horizon."
Any point directly on the Arctic circle will experience exactly one 24 hour period of daylight/night. But anything North of the arctic circle will never experience an exact 24 hour time period of daylight/night. I have the same confusion with the antarctic circle article.
The articles on the tropic circles seem to be explained better. — Preceding unsigned comment added by Jpaszko (talk • contribs) 20:17, 18 April 2012 (UTC)
I've simplified the text. Bazonka (talk) 17:35, 20 June 2012 (UTC)
June 2016 edits extrapolating the drift to exclude Iceland
@220.127.116.11: As the article points out, the wobble of the Earth's axis is complicated and variable, depending on the motion of the Moon, the movement of water in ocean currents, and numerous other factors. The rate of drift will emphatically not be constant and steady over the next twenty-seven years (read more at Circle of latitude#Movement of the Tropical and Polar circles); this means that your figure based on a linear extrapolation, while technically true, is highly misleading.
Consider this analogy: Today, the stock of Skullcandy (SKUL) has gained roughly 16% in a day's trading. "Should this stock continue to appreciate at approximately 16% per day, by August Skullcandy will have become the world's most valuable publicly traded firm by market capitalization." This is true, but since it is wildly implausible that the stock would continue to gain at that rate for two months (or even for two days), the sentence's implications are extremely misleading.
Now, if you can provide a credible source in which someone who understands this topic well says that the axial tilt is expected to continue to lessen for some decades to come and that the average location of the Circle will move northward at least several hundred meters in that time, then this idea will have a place in the article.-Bryanrutherford0 (talk) 16:03, 8 June 2016 (UTC)
I am not an expert, but based on the formulas presented in Axial tilt, a few decades is easily predictable. It's only when we try to predict thousands or millions of years in advance that it becomes more complicated. --Lasunncty (talk) 01:35, 9 June 2016 (UTC)
Ah! There we are. I hadn't seen that article, but it seems to contain just the sort of sources I was requesting. So, let's see... according to that article, the tenth-order polynomial provided near the end of Axial tilt#Short term gives a good estimate of the mean obliquity for the next several centuries, but it then also clarifies that "periodic motions of the Moon and of Earth in its orbit cause much smaller (9.2 arcseconds) short-period (about 18.6 years) oscillations of the rotation axis of Earth, known as nutation, which add a periodic component to Earth's obliquity" that is not reflected in that polynomial. If this short-term oscillation moves the axis 9.2 seconds back and forth in 18.6 years, then that's roughly one second per year; the linear term in the tenth-order polynomial only gives a rate of .47 seconds per year, meaning that the slower long-term variations will be buried behind the faster short-term variations. If that short-term cycle has an 18.6-year period, then it seems likely that Grímsey might leave and re-enter the Circle repeatedly over the next century, though the longer-term trend over the next several centuries will indeed be for the circle to shift northward and ultimately exclude Grímsey entirely. So, again, the presence of this rapid "nutation" in the axial orientation leads me to feel that the situation is too complicated for the projection of a single tidy date when Grímsey will leave the Arctic Circle; but, we could perhaps expand the existing section on the location of the Arctic Circle to add some of this nuance.-Bryanrutherford0 (talk) 12:41, 9 June 2016 (UTC)