Midnight sun and polar night
Relationship of Earth's axial tilt (ε) to the tropical and polar circles
The Antarctic Circle is the northernmost latitude in the
Southern Hemisphere at which the sun can remain continuously above or partially below the horizon for twenty-four hours; as a result, at least once each year at any location within the Antarctic Circle the sun is
visible at local midnight, and at least once it is partially obscured at local noon.
Directly on the Antarctic Circle these events occur, in principle, exactly once per year: at the
solstices, respectively. However, because of
atmospheric refraction and
mirages, 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 southern
summer solstice up to about 50
minutes (′) (90 km (56 mi)) north of the Antarctic Circle; similarly, on the day of the southern
winter solstice, part of the sun may be seen up to about 50′ south of the Antarctic 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. Mirages on the Antarctic continent tend to be even more spectacular than in Arctic regions, creating, for example, a series of apparent sunsets and sunrises while in reality the sun remains under the horizon.