scotianostra:

The One O’Clock Gun is fired every day (except Sunday) at precisely 13:00, allowing citizens and visitors to check their clocks and watches.

The origin of the tradition lies in the days when sailing ships in the Firth of Forth were able to check and reset their chronometers in the days before acurate timepieces were available.

In 1861 Captain Wauchope, a Scottish Naval Officer in the Royal Navy invented the time ball, still seen today on top of Nelson’s Monument , Calton Hill. At one O’clock the ball drops giving the signal to sailors, but this meant that someone would have to be looking out for it and it often couldn’t be seen in foggy weather. So, in the same year the gun was fired simultaneously to the time ball dropping. Originally an 18-pound muzzle loading cannon which needed four men to load and fire was fired from the Half Moon Battery. The gun could be easily heard by ships in Leith Harbour (2 miles away) The cannon was replace with a 25 pound Howitzer in 1953 and has is now fired form Mill’s Mount Battery on the North face of the Castle and has been fired every day except a Sunday since then.