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Amazing NASA Video Shows Fall Equinox from Space
Credit: NASA Earth Observatory.

Today is the first day of fall, also known as the fall equinox. and a new amazing animation released by NASA today shows just how the equinoxes and solstices look from the perspective of space.

Around 6 a.m. local time each day, the sun, Earth, and any satellite that follows a geosynchronous orbit form a right angle. This alignment offers the satellite a nadir (or straight-down) view of the Earth's terminator, the line where the shadows of nightfall meet the sunlight of dusk and dawn. The shape of this line between night and day varies with Earth's seasons, during which the length of the day changes.

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The Spinning Enhanced Visible and Infrared Imager (SEVIRI) on EUMETSAT's Meteosat-9 captured these four views of the day-night terminator on the solstice or equinox that marks the beginning of each season: the winter solstice on Dec. 21, 2010; the spring equinox on March 20, 2011; the summer solstice on June 21, 2011, and the fall equinox on Sept. 20, 2011. Each image was taken at 6:12 a.m. local time.

On March 20 and Sept. 20, the terminator is a straight north-south line, and the sun is said to sit directly above the equator. The term "equinox" that is used to mark the beginning of spring and fall refers to the equal lengths of day and night on these dates.

This year, the fall equinox (and so the beginning of fall) fell at exactly 9:05 a.m. UTC (5:05 a.m. EDT) Sept. 23. [Related: ((CONLINK|30803|'Wannabe' La NiÃ