Polar circle

Defining characteristics

Areas within each polar circle experience, at minimum, one day per year when the sun remains continuously above the horizon and at least one day when it remains continuously below the horizon. These events are commonly called the "midnight sun" and the "polar night." Near the boundary, twilight conditions can vary, so a location may have prolonged periods of bright or dim twilight even when the sun does not rise or set normally; see continuous daylight and darkness for the basic phenomenon.

Cause and movement

The polar circles result from Earth’s axial tilt: they mark the most northerly and southerly latitudes at which the sun can be directly overhead at the solstices, and they correspond to the maximum solar declination. Because the tilt (obliquity) of Earth’s axis changes slightly over years and centuries, the exact latitude of each polar circle shifts gradually; the lines are therefore theoretical, not fixed, on any geodetic grid.

Human, ecological and practical significance

The two polar regions differ markedly. The Arctic is an ocean surrounded by land and supports indigenous peoples, towns and economic activities such as fishing and increasingly shipping and tourism. The Antarctic is a high, icy continent governed by international treaty and occupied mainly by temporary research stations. Both regions host distinctive ecosystems adapted to extreme light cycles and cold climates and are sensitive indicators of climate change.

Notable facts and distinctions

• Polar circles are conceptual parallels, used on maps and in astronomy to predict solar behavior.
• They are distinct from the broader terms "polar regions," which refer to ecological, climatic and geopolitical units rather than a single latitude.
• Because the latitude value slowly changes with Earth's tilt, the nominal position of each circle is an approximation used for navigation, education and public signage.