Satellites in LEO need to move fast, taking between 90 and 225 minutes to complete one orbit of Earth. The upper limit is about 1,250 miles (2,000 km), just inside the protective shell of Earth's Van Allen radiation belts. To minimize drag on spacecraft from the upper atmosphere and allow for high-resolution close-up views of our planet, 190 miles (300 km) is the lower limit. These are the bright satellites you'll see zipping across the night sky. Satellites that need to closely observe Earth's surface and weather, or that are too heavy to lift into high orbits, are placed in low Earth orbit (LEO).
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Those satellites pass over the same spot on Earth every 12 hours and are not observable to the naked eye, but they can zip through your telescope's field. This includes the 31 Global Positioning System (GPS) satellites, and the satellites in the navigation networks of Russia (GLONASS), Europe (Galileo), China (BeiDou), India (NavIC) and Japan (QZSS). Spacecraft that need to move around the Earth at a modest rate orbit closer in, around 12,600 miles (20,200 km) above sea level. If you need imaging gear, consider our best cameras for astrophotography and best lenses for astrophotography. If you're looking for a telescope or binoculars to observe satellites, check our guide for the best binoculars deals and the best telescope deals now. When searching the app store, you'll find many apps designed for that purpose, and not skywatching.
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These are the satellites that you aim your satellite TV dish at.
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At their extremely distant orbits, the Earth's shadow covers only a small patch of the sky, so they can appear all night long. In the night sky, geosynchronous satellites are concentrated in a strip running several degrees below the celestial equator. While observing with your telescope, you might spot one moving very slowly against the background stars. At that range, this type of satellite will be dim. To do this, they need to orbit above Earth's equator, at 22,236 miles (35,786 km) above sea level. Telecommunication and some surveillance satellites have geosynchronous orbits designed to keep them in view from the same spot on Earth at all times.
If you see an object with flashing lights, it's an aircraft.
Other than crewed vehicles, satellites do not have lights - and no artificial light in orbit would be bright enough to spot from the ground anyway. Most satellites shine with a steady light, but objects that are tumbling can glow and darken as they travel. The brightness of a satellite depends on its distance from you and on how much sunlight it deflects toward you, which is a function of both its flat surface area and reflectivity. Satellites can also pop into view when they exit the shadow. When a passing satellite seems to disappear, it has entered Earth's shadow - a circular patch of the sky where the sunlight can't reach it. That's why most satellites are seen an hour or two before dawn and after dusk. Orbiting satellites are visible to us because they are high enough overhead to be illuminated by sunlight, even when the sun is below the horizon for ground observers.
(Image credit: Alan Dyer/Stocktrek Images via Getty Images) The ISS' orbit is approximately 250 miles (402 km) high, so it is one of the objects that completes one orbit of the Earth every 92.5 minutes (or 16 per day).
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