midnight sun, phenomenon in which the sun remains visible in the sky continuously for 24 hr or longer, occurring only in the polar regions. The midnight sun is due to the fact that the plane of the earth's equator is tilted about 231/2° to the plane of the ecliptic (the apparent path of the sun through the sky). Thus, at the summer solstice (about June 22), the sun is still visible on the horizon at midnight at all points along the Arctic Circle, 231/2° of latitude from the North Pole. At points north of the Arctic Circle, the midnight sun is visible for longer than one day, the North Pole having a full six months of continuous sun from the vernal equinox (about Mar. 21) to the autumnal equinox (about Sept. 23). In the south polar regions the midnight sun is visible along the Antarctic Circle at the winter solstice (about Dec. 22) and south of the Antarctic Circle for longer periods.