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The Greenwich time ball is a historic timekeeping mechanism located at the Royal Observatory in Greenwich, London. It consists of a large metal ball that is dropped at a precise time each day to mark the exact time for ships and citizens in the area.

The purpose of the time ball is to enable mariners and the public to synchronize their timepieces, especially in an era when accurate timekeeping was crucial for navigation and trade. Originally installed in 1833, it was one of the earliest time signals used globally.

The time ball is mounted on a mast at the top of the Octagon Room of the Royal Observatory. It is dropped at 1 p.m. (13:00) every day, with the principle based on astronomical observations. The ball takes about five seconds to descend from its starting position to its resting position, indicating precisely when it is one o'clock. The drop can be seen by anyone who has a clear line of sight to the observatory, including ships on the River Thames.

Although the time ball is no longer essential for modern timekeeping purposes, it remains a popular attraction for visitors to Greenwich. It serves as a symbol of the history and importance of Greenwich Mean Time (GMT), which was initially used as the world's time standard until being replaced by Coordinated Universal Time (UTC) in 1972.