 Equinox is a point in time, not a day
Although the word "equinox" implies equal length of day and night, this is not true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal. Those days are commonly referred to as the "equiluxes" to distinguish them from the equinoxes.
'Equilux' is when the day is nearest in duration to the nightEquinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart. This way, you can refer to a single date as being the equilux, when, in reality, it spans sunset on one day to sunset the next, or sunrise on one to sunrise the next. As an example, for a city 45°N and 123°W (Portland, Oregon), the 2006 autumnal equilux was on September 25 when sunrise was at 7:01 am and sunset was at 7:02 pm. The 2006 autumnal equinox was on September 22 at 9:03 pm (all times in Pacific Daylight Time). On both hemispheres, the autumnal equilux lags behind the equinox, and the vernal equilux is ahead of the equinox.[1]
Definition of EquinoxIn astronomy, equinox can have two meanings:
- The moment when the Sun is positioned directly over the Earth's equator and, by extension, the apparent position of the Sun at that moment—see below.
An equinox in astronomy is that moment in time (not a whole day) when the center of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year.
Sources:
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Details about the Length of Day and Night at the Equinoxes
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Calculation of Length of Day (Formulas and Graphs)
Equinox. (2008, February 20). In Wikipedia, The Free Encyclopedia. Retrieved 06:12, February 21, 2008, from http://en.wikipedia.org/w/index.php?title=Equinox&oldid=192747467
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