Solstice (redirected from solstitial)

solstice (sŏl`stĭs) [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°). At the solstices the sun's apparent position on the celestial sphere reaches its greatest distance above or below the celestial equator (see equatorial coordinate system), about 23 1-2° of arc. At the time of summer solstice, about June 22, the sun is directly overhead at noon at the Tropic of Cancer (see tropics). In the Northern Hemisphere the longest day and shortest night of the year occur on this date, marking the beginning of summer. At winter solstice, about Dec. 22, the sun is overhead at noon at the Tropic of Capricorn; this marks the beginning of winter in the Northern Hemisphere. For several days before and after each solstice the sun appears to stand still in the sky, i.e., its noontime elevation does not seem to change from day to day.

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solstice

Either of the two moments in the year when the Sun's apparent path is farthest north or south from Earth's Equator; also, either of the two points along the ecliptic that the Sun passes through at these times. In the Northern Hemisphere the summer solstice occurs on June 21 or 22; the winter solstice on December 21 or 22. In the Southern Hemisphere the seasons are reversed. See also equinox.

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solstice

1. either the shortest day of the year (winter solstice) or the longest day of the year (summer solstice)

2. either of the two points on the ecliptic at which the sun is overhead at the tropic of Cancer or Capricorn at the summer and winter solstices

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solstice [′sälz·təs]

(astronomy)

The two days (actually, instants) during the year when the earth is so located in its orbit that the inclination (about 23½°) of the polar axis is toward the sun; the days are June 21 for the North Pole and December 22 for the South Pole; because of leap years, the dates vary a little.

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Solstice 

the instant at which the center of the sun is at the northernmost or southernmost point on the ecliptic. The term “solstice” is also applied to these points on the ecliptic, as is the term “solstitial point.” When the sun is at the northernmost point on the ecliptic, both the instant in time and the point are known as the summer solstice; the sun’s declination here is +23°27’. When the sun is at the southernmost point on the ecliptic, the instant and the point are called the winter solstice; the sun’s declination in this case is – 23°27’.

Near the solstices, the sun’s declination changes very slowly (seeCELESTIAL COORDINATES), since in this region the sun’s motion along the ecliptic is almost parallel to the equator. As a result, the midday altitude of the sun remains almost constant for several days—hence the term “solstice.”

The interval of time between two successive passages of the sun through the same solstitial point does not coincide with the calendar year. For this reason, the instant at which the sun is at the solstitial point occurs at a different time each year relative to the beginning of the calendar day. In a common year, the solstice occurs 5 hours 48 minutes 46 seconds later than in the preceding year. In a leap year, the solstice occurs 18 hours 11 minutes 14 seconds earlier than in the preceding year. Thus, the solstice may occur on two successive dates. At the present time, in the second half of the 20th century, the summer solstice is on June 21 or 22, and the winter solstice is on December 21 or 22.

The summer solstice is the beginning of the astronomical summer in the northern hemisphere, and the winter solstice is the beginning of the astronomical winter in the northern hemisphere. In the northern hemisphere, the length of the day is greatest at the time of the summer solstice and is least at the time of the winter solstice. In the southern hemisphere, the shortest and longest days, respectively, are observed at these times.