Is it ensure, insure, or assure?
1550s, in reference to a celestial circle, a line around the sky which, in any location, bounds the stars which are ever-visible from that latitude (in the Northern Hemisphere, this is focused on the celestial north pole); the concept goes back to the ancient Greeks, for whom this set of constellations included most prominently the two bears (arktoi), hence the name for the circle (see arctic). Of Earth, the circle 66 degrees 32 minutes north of the equator, marking the southern extremity of the polar day, it is recorded from 1620s.
Imaginary circle around the Earth about three-quarters of the way from the equator to the North Pole. North of this line is the “Land of the Midnight Sun,” where the sun never sets on the summer solstice.
Note: The Arctic Circle corresponds to the Antarctic Circle in the Southern Hemisphere.