The time ball was the 19th century's answer to keeping people's clocks and watches in sync, something that had become increasingly necessary as railroads required arrivals and departures to be at an exact time. A large, bright-colored ball was mounted on a pole on the top of a tall building, tower, or anything else that could be seen from a distance. Though the ball generally appeared solid to observers, it was often made of different pieces such as the "dozen thin semicircles made of sheet copper, half of which were crescents" used in the 1877 design of the Western Union time ball in New York City. This kept air resistance down when the ball moved.

The time was usually measured by an astronomical observatory and sent by telegraph if the ball was located elsewhere; the ball would be dropped down the pole so that it reached the bottom exactly at noon or some other agreed-upon hour, and anyone within view was able to set their timepieces. (After that it would be hoisted up again for the next use.)

One of the earliest time balls was set up at the Royal Observatory at Greenwich, England, in 1833; sailors on the Thames used it to set their clocks correctly (a necessity at the time for calculating longitude). The ball has dropped daily there since then at 1:00 p.m.; it has become a tourist attraction since it is no longer the sole way to keep people's watches in agreement. At least 150 public time balls are known to have existed around the world, most commonly in port cities; on the web I've found references to time balls in England, the United States, Australia, New Zealand, Hong Kong, Mauritius, St. Helena, the Cape of Good Hope, and India.

The U.S. Naval Observatory started its own time ball in 1845, but like the majority of time balls stopped running in the 1920s. The Naval Observatory organized a "Round-the-World Time Ball Drop" to bring in the year 2000, perhaps inspired by the yearly drop of a time ball on New Year's Eve in Times Square in New York. Numerous disused time balls were reactivated for this one event.

Sources:
Ierley, Merritt. Wondrous Contrivances: Technology at the Threshold. New York: Clarkson Potter Publishers, 2002.
http://greenwichmeantime.com/info/timeball.htm
http://www.rog.nmm.ac.uk/museum/
http://members.global2000.net/~jcsl/telegraph/phelps/ball.htm
http://www.doane.edu/crete/academic/science/phy/DoaneCollegeTimeBall.htm
http://www.tumblong.uts.edu.au/artefacts/greenwich/more.html

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