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Tuesday, September 15, 2009

The Solstices and The Equinoxes

(click on the images to enlarge) 
     Solstice refers to either of the the two days 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. This occurs on June 20 or 21 (summer solstice), when the North Pole is tilted toward the Sun; and on December 21 or 22 (winter solstice) when the South Pole is tilted toward the Sun. The adjectives summer and winter, used above, refer to the Northern Hemisphere; seasons are reversed in the Southern Hemisphere. The name is solstice is derived from the Latin sol (sun) and sistere (to stand still) because at the solstices, the Sun stands still in declination i.e. the apparent movement of the Sun's path north or south comes to a stop before reversing direction.

     The two equinoxes occur roughly midway between the solstices: the autumnal equinox (September 22 or 23) and spring/vernal equinox (March 20 or 21). The term equinox is derived from the Latin for equal nights, indicating that the day and night are of equal duration.
     The two solstices and the two equinoxes mark the beginning of four seasons.

* The dates drift with the difference between the actual solar years and 365 days, and are corrected by leap years
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