September 04, 2007
Astronomers often present striking star trail images that give the illusion that the cosmos revolves around Earth. But if you look close enough, you can see a distinct line of "stars" that don't move. These are geostationary satellites orbiting Earth at an altitude of 35,786 km (23,000 mi). Here, the orbital period is 24 hours, so a satellite appears to hover over one spot on Earth's equator. Geostationary orbit is prime celestial "real estate" for both communications and Earth observations since ground stations (like your satellite TV dish) don't have to track back and forth across the sky. There are approximately 238 geostationary satellites orbiting Earth. The image above shows 34, with visual magnitudes ranging from +10 to +14. The view spans 232.5 to 266.5 degrees east longitude, along the celestial equator -- this is about 9.4% of the geostationary arc. It was taken at Kitt Peak, Arizona (31.95 degrees north latitude, 111.5 degrees west longitude), on March 19, 2007, between 2:30 and 11:00 UT. Note that the bright upper star trail at top is the bright star Spica, the lower reddish one is Alpha Hydra.
Credit: Bill Livingston, National Solar Observatory, Tucson, Arizona. Additional support: Pete Marenfeld, National Optical Astronomy Observatories, Tucson, Arizona.