Autumnal Equinox - September 22 / Today in Science / Science NetLinks: "Autumnal Equinox
The autumnal equinox is observed as a day-long affair, but the actual event is only a single point in time, usually observed on the 22nd or 23rd of September in the northern hemisphere. What happens is that earth reaches a point in its rotation so that the equator lines up perfectly parallel with that of the sun, putting the two on an equal “plane” for a short period of time. This point in time is called “solar noon.”"
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