The best known time ball these days is the one that drops in New York City's Times Square to indicate the passage of another year. We're all familiar with how that ball works: when it hits the bottom, the clock has hit midnight, marking the division between old and new year. But past time balls dropped far more often: designed to help boats at sea calculate their longitude (which required accurate timekeeping), the balls typically dropped every day at 1pm (though in some places around the world, the time differed).
In a reversal of the modern New York City ball, early time balls started counting the time (say, 1pm) when they started dropping, not when they finished. To get sailors ready, the ball would only be raised a few minutes before the day's drop.
The earliest such ball was installed in 1829 in southern England, and others followed along the coastal UK shortly thereafter -- including the one shown at left, at the Greenwich Observatory. The balls initially got their time right by astronomical observations of the sun; the reason for the traditional 1pm drop was that astronomers were busy measuring the sun's position at noon and needed some time before dropping the ball. Later balls used telegraph signals to get Greenwich Mean Time, thus obviating the need for local sun observation. Time balls finally fell out of common use in the 1920s when radio began broadcasting time signals, though many time balls still remain in the UK and Australia.
For more awesome Time Ball info, check out the Deal Timeball Tower Museum located in Deal, Kent.