American Meteorological Society (AMS). 1999. Glossary of meteorology. 2nd ed. Edited by Todd S. Glickman. Boston, Massachusetts USA: American Meteorological Society (AMS).
Definition: The line of latitude 66°34'S (often taken as 66.5°S). Along this line the sun does not set on the day of the summer solstice, about 22 December, and does not rise on the day of the winter solstice, about 21 June. It is defined here as 66.56°S.