The Edinburgh gun that is only there for the fog

Here is the Edinburgh Castle detail I love, and almost everyone misses it because they are too busy jumping out of their skin. Everyone knows the gun that fires at one o'clock. Hardly anyone asks the obvious question underneath it: if the whole point is to tell the time, why do it with a cannon you can only hear, when the sky over the city already had a perfectly good clock you could see?

Because it did. A mile east of the castle, on top of the Nelson Monument on Calton Hill, a big zinc covered ball has been climbing its mast and dropping at exactly one o'clock since 1852. That was the real timekeeper, dreamed up by Charles Piazzi Smyth, the Astronomer Royal for Scotland, so the ships anchored in Leith and out in the Firth of Forth could set their chronometers by it and work out their longitude. The catch was our weather. On a proper haar of a day the ball just disappeared into the grey, so in 1861 they added the gun at the castle as a backup you could hear when you could not see, the two of them wired together across the rooftops. And even then the captains had to be clever, because sound is slow, so the bang took a good ten seconds to reach Leith and longer to reach a ship further out, and every navigator had to subtract that little delay before he trusted it.

I painted the castle on its black rock in watercolour and ink, and the whole time I kept thinking about that ball rising quietly on Calton Hill while everyone below waited for the bang, and about some captain out on the Forth, watch in hand, counting off ten seconds in his head. There's a poster with a letter home and a postcard if you'd like one.

Shop the poster here: Edinburgh Castle, Edinburgh · Travel Poster