Monday, September 22, 2008

Happy Autumnal Equinox

If I've got this right then today at 11:44 AM EST (i.e. right now!) is the Autumnal Equinox. Have a very happy one!

Most people think that the Spring and Fall equinoxes are the days when there are equal amounts of daylight and night. If that's true then why is the Autumnal Equinox at 11:44 in the morning?

Here's the answer from Wikipedia. It's the sort of thing I would have expected to find on Bad Astronomy but, if it's there, I couldn't find it.
An equinox in astronomy is the moment in time (not a whole day) when the centre of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year.

More technically, at an equinox, the Sun is at one of two opposite points on the celestial sphere where the celestial equator (i.e., declination 0) and ecliptic intersect. These points of intersection are called equinoctial points—the vernal point and the autumnal point. By extension, the term equinox may be used to denote an equinoctial point.

[Image Credit: eSky]

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