Earth Satellites

Objects that revolve around the Earth are called Earth satellites. The Moon is the Earth's only NATURAL satellite, moving in an almost circular orbit with a period of about 27.3 days. Since the mid-20th century, many countries -- including India -- have launched a large number of ARTIFICIAL satellites, each placed in an orbit suited to its intended practical use: navigation, surveillance, communication, deep-space observation, or weather monitoring.

Two broad classes of artificial satellite illustrate very different orbital choices. COMMUNICATION satellites are placed in the EQUATORIAL plane with the same sense of rotation and exactly the same period (24 hours, one day) as the Earth's own rotation -- so, as seen from any fixed point on the Earth's surface, such a satellite appears to hang perfectly stationary in the sky. This is why they are called geostationary (or geosynchronous) satellites, and they are used extensively for television transmission, telephony, and radio-wave signal relay -- India's own INSAT series of satellites being a prominent example. POLAR satellites, by contrast, are placed in much LOWER orbits (altitudes of roughly 500-800 km), with correspondingly short periods of about 85 minutes, letting a single satellite complete around 16 orbits per day. They travel in a north-south path over the Earth's poles while the Earth itself rotates east-west be …