On October 4, 1957, the world’s first artificial satellite, Sputnik, was sent into earth orbit from a launch site in the Soviet Union. A few months later, the United States successfully launched Explorer 1, and another satellite now revolved about the earth. Today, there are thousands of satellites in orbit; and every so often, you can see one passing overhead. It looks like a moving star, or like a light from a high-flying jet, but the satellite moves along at a pretty good clip, crossing the sky in only a matter of minutes, and yet you can't hear any sound coming from it. These satellites reflect sunlight down to the darkened earth, and so are visible for a couple of hours after sunset or a couple of hours before sunrise, a time when we are in earth's shadow, but the satellite is just outside it. Satellites typically travel from west to east, except for those in polar orbits which move along a north-south path.