Why is a ball dropped in Times Square to mark the new year? Why don't we watch a giant clock instead? Because of "public time." On New Year's Eve we celebrate according to a common standard of time. The oldest mechanical symbol of such public time is the time ball, which appeared about 1833. Spring-driven clocks weren't always reliable so we needed to synchronize our clocks. This was very important to ship navigators, so observatories and ports began using time balls, placed where they could be seen by ships waiting to depart. At noon, keepers of these balls would drop them down the pole and navigators would adjust their clocks. In New York, a ball was first placed atop the Custom House. The ball in Times Square was installed in 1907 and has marked special occasions.
Source: The Christian Science Monitor's Dec. 29, 1999, issue