Editing Polar circle (section)

==Correspondence to polar night and day{{anchor|Effect of atmospheric refraction|Effect of the angular diameter of the Sun}}==
[[File:axial tilt vs tropical and polar circles.svg|thumb|400px|Relationship between Earth's axial tilt (ε) to the tropical and polar circles]]
The polar circles would almost precisely match the boundaries for the zones where the [[polar night]] and the [[Midnight sun|polar day]] would occur throughout the [[winter solstice]] and [[summer solstice]] day respectively. They do so loosely due to two effects. The first one is [[atmospheric refraction]], in which the Earth's atmosphere bends light rays near the horizon. The second effect is caused by the [[angular diameter]] of the Sun as seen from the Earth's orbital distance (which varies very slightly during each orbit). These factors mean the ground-observed boundaries are {{convert|80|to|100|km}} away from the circle.{{cn|date=February 2022}} A further global factor for this numerical range is [[Earth's nutation]], which is a very small change in tilt.  Observers higher above sea level can see a tiny amount of the Sun's disc (see [[horizon]]) where at lower places it would not rise. For the [[Arctic Circle]], being 80&ndash;100 km north of the circle in winter, and 80&ndash;100&nbsp;km south of the circle in summer; the inverse directions apply to the other circle.<ref>Swedish Astronomic calendar 2003 (or any other year) at the times of the winter and summer solstices, around 22 June and 22 December</ref>