The One O'Clock Gun is a time signal, and is fired every day at precisely 13:00, excepting Sunday, Good Friday and Christmas Day. The gun was established in 1861 as a time signal for ships in the Firth of Forth, and complemented the time ball, which was installed on the Nelson Monument in 1852, but which was useless during foggy weather. The gun could easily be heard by ships in Leith Harbour, 2 miles away.
The original gun was an 18-pound muzzle-loading cannon, which needed four men to load, and was fired from the Half Moon Battery. This was replaced in 1913 by a 32-pound breech-loader, and in May 1952 by a 25-pound Howitzer. The present One O'Clock Gun is an L118 Light Gun, brought into service on 30 November 2001.
The gun is now fired from Mill's Mount Battery, on the north face of the castle, by the District Gunner from 105th Regiment Royal Artillery. Although the gun is no longer required for its original purpose, the ceremony has become a popular tourist attraction.