An equinox occurs twice a year, around 20 March and 22 September. The word itself has several related definitions. The oldest meaning is the day when daytime and night are of approximately equal duration. The word equinox comes from this definition, derived from the Latin aequus (equal) and nox (night). The equinox is not exactly the same as the day when period of daytime and night are of equal length for two reasons. Firstly, sunrise, which begins daytime, occurs when the top of the Suns disk rises above the eastern horizon. At that instant, the disks center is still below the horizon. Secondly, Earths atmosphere refracts sunlight. As a result, an observer sees daylight before the first glimpse of the Suns disk above the horizon. To avoid this ambiguity, the word equilux is sometimes used to mean a day on which the periods of daylight and night are equal. Times of sunset and sunrise vary with an observers location (longitude and latitude), so the dates when day and night are of exactly equal length likewise depend on location. - Wikipedia
Posted on: Tue, 16 Sep 2014 01:29:37 +0000
Trending Topics
Recently Viewed Topics
© 2015