midnight sun

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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 23 1-2° to the plane of the eclipticecliptic
, the great circle on the celestial sphere that lies in the plane of the earth's orbit (called the plane of the ecliptic). Because of the earth's yearly revolution around the sun, the sun appears to move in an annual journey through the heavens with the ecliptic as its
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 (the apparent path of the sun through the sky). Thus, at the summer solsticesolstice
[Lat.,=sun stands still], in astronomy, either of the two points on the ecliptic that lie midway between the equinoxes (separated from them by an angular distance of 90°).
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 (about June 22), the sun is still visible on the horizon at midnight at all points along the Arctic CircleArctic Circle,
imaginary circle on the surface of the earth at 66 1-2°N latitude, i.e., 23 1-2° south of the North Pole. It marks the northernmost point at which the sun can be seen at the winter solstice (about Dec.
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, 23 1-2° 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 equinoxequinox
, either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as "the first point of Aries," is the point at which the sun appears to cross the celestial equator from south to north.
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 (about Mar. 21) to the autumnal equinox (about Sept. 23). In the south polar regions the midnight sun is visible along the Antarctic CircleAntarctic Circle,
imaginary circle on the surface of the earth at 66 1-2°S lat., i.e., 23 1-2° north of the South Pole. It marks the southernmost point at which the sun can be seen at the winter solstice (about June 22) and the northernmost point of the southern polar
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 at the winter solstice (about Dec. 22) and south of the Antarctic Circle for longer periods.

midnight sun

[′mid‚nīt ′sən]
(astronomy)
The sun when it is visible at midnight; occurs during the summer in high latitudes, poleward of the circle at which the latitude is approximately equal to the polar distance of the sun.

midnight sun

the sun visible at midnight during local summer inside the Arctic and Antarctic circles