Communications satellites are usually put into a synchronous (or geosynchronous) orbit, circling the earth once every 24 hours and so appearing to hover over a single spot on the surface. This type of synchronized movement is important, since you have to know where to aim your satellite dish. In the computer field, synchronous usually refers to the use of a simple timing signal that permits very rapid exchange of data between computers. The kind of mysterious coincidence sometimes called synchronicity—such as the appearance of two different comic-strip characters named Dennis the Menace in the U.S. and Britain within three days of each other in 1951—has fascinated people for centuries.