Precisely at 1 p.m. (13:00 hours) every day, barring Sundays, Good Friday and Christmas Day, a howitzer gun is fired on the ramparts of Edinburgh Castle. It is loud enough to be heard over two miles away and sends flocks of birds wheeling into the sky, as well as causing shoppers on Princes Street to jump. Edinburgh's professional rugby team, The Gunners, take their nickname from this daily ritual. But why is it done?
It all goes back to the days of sail, when navigation relied on sextant, charts and a sailor's mathematical skills. Key to making everything work was knowing the precise time, so that when a noon sextant reading was taken you knew it really was noon and not, say, 12:27. It was therefore absolutely vital that ship's chronometers were accurately set. To help in this regard a Scot, Captain Wauchope of the Royal Navy, invented the "time ball", a sphere that fell down a vertical mast at a precise, fixed time, which is normally noon or 1 p.m. (presumably because it is such a singular event). Ships in harbour could then set their chronometers by the time ball. A time ball was set up on top of Nelson's Monument, the building shaped like an inverted telescope on Calton Hill, in 1852 for use by ships in the Forth and the Port of Leith.
There were however two practical problems with this solution. First you had to be looking in the right direction at the right time; in other words you pretty much had to know it was one o'clock in order to be watching for the time ball to fall. Second, you had to be able to see it, and on days of mist, fog or driving rain that was impossible.
So, in June 1861 a new solution was introduced. This was the famous one o'clock gun. It wasn't the first (Glasgow, Greenock and Newcastle all beat Edinburgh to this idea) but all those earlier guns have now ceased to operate, leaving Edinburgh's One O'Clock Gun as the only military time gun in the UK.
The system for firing the One O'Clock Gun was devised by Charles Piazza Smyth, the Astronomer Royal for Scotland at the time. The electrical signal that controlled the time ball was also connected to the firing mechanism of the gun by a 4200 foot (1280 metre) steel cable, which was laid in just two days by sailors from Leith. So although you might think that the falling time ball causes the gun to fire (or vice-versa), both are actually triggered by a third device, nowadays the mean time clock at Blackford Hill Observatory. Completing the system was the Edinburgh Time Gun Map, which showed how many seconds it took for the sound of the gun to reach various points in the city. The time from the firing of the gun to it being heard in Leith harbour, which was of course the whole point, is 11 seconds. That such a brief period of time was taken into account gives some idea of just how accurate the whole system had to be.
The gun used today is of course not the original. That was a 64-pound muzzle loading cannon which required four gunners to prepare it for action. The present gun is a 105mm light artillery gun which has been in use since 2001, when it replaced the famous World War II 25-pounder that had been used since 1953. The location of the gun has also changed from the original "Half Moon" Battery to the present "Mills Mount" Battery, which has four guns in total, used also for royal salutes and at midnight on Hogmanay to signal the New Year.
There is now a permanent exhibition about the One O'Clock Gun in Edinburgh Castle.