Equinox from Space
September 22, 2009
The image above shows a view of the Earth's terminator from approximately 22,300 miles (35,800 km) above the Earth's surface. It was acquired early yesterday morning, September 21, 2009 (at 0600Z) by the Meteosat-9 geostationary satellite, which orbits the Earth directly over the Equator, above the Prime Meridian (0 degrees longitude). Notice that the terminator, the band demarking the boundary between daylight and darkness, stretches from the North Pole to the South Pole. The only time this happens is when the Earth is at or near one of the two equinoxes; around September 22 and March 21. At these times, every location on the globe experiences nearly 12 hours of daylight and darkness (equinox).
Like all geostationary satellites, Meteosat-9 is designed to revolve in the same direction that the Earth rotates (west to east). At such a high orbital altitude (about 1/10 the distance to the Moon), one complete orbit takes 24 hours, which is of course the same amount of time as the Earth requires to rotate once on its axis.