Alternatively, when the axis tilts towards the sun, as it does between June and September, it is summer in the northern hemisphere but winter in the southern hemisphere. December 21 is called the winter solstice in the Northern Hemisphere and simultaneously the summer solstice in the Southern Hemisphere. On June 21 the solstices are reversed and summer begins in the northern hemisphere. On December 21, there are 24 hours of daylight south of the Antarctic Circle (66.5° south of the equator) and 24 hours of darkness north of the Arctic Circle (66.5° north of the equator).