muffled detonation booms and you quicken your pace. Is this the start of the revolution? Not yet. It’s the little cannon built by watchmaker Rousseau telling you the time: it’s noon, precisely.

There’s a phrase still used in French today – midi pétante (noon sharp) – that includes a hidden historical reference (pétante literally means “exploding”). It originates from a unique horological invention: a cannon clock, that can still be seen behind the flower gardens of the Palais-Royal in Paris. The palace, built by Richelieu in 1628, was gifted to King Louis XIII in 1636. It became the residence of the regent Anne of Austria, before passing into the hands of the Orléans family. In 1780, Louis Philippe d’Orléans, known as “Philippe Egalité,” a cousin of Louis XVI, launched a real estate operation to clear his debts. He added three wings to the palace that included shopping arcades on the ground floor with apartments above.

At the time, Parisians set their watches by sundials installed on the city walls, which showed the true time, solar noon, when the sun reached its zenith. But the new wings of the Palace cast a shadow over the capital’s most frequently consulted sundial (now relocated to Rue des Bons-Enfants).

A watchmaker newly established in the palace’s Beaujolais gallery had a brilliant idea: if Parisians could no longer read the time, they would hear it! In 1786, the watchmaker and mathematician known as Monsieur Rousseau installed a small bronze cannon in front of his shop, on the Paris meridian, topped with a magnifying glass. From May to October, when the sun was directly overhead, the rays focused by the magnifying glass ignited a fuse that detonated the powder. The explosion could be heard for hundreds of metres around. Parisians could then set their watches at noon sharp!

The cannon quickly became famous in the capital, inspiring poet Jacques Delille to comment: “One’s morals may go wrong, but at least one’s watch may be put right” – a reference to the infamous precincts of the Palais Royal, whose cafes drew the crowds and whose arcades concealed amorous encounters. In 1799, the cannon was moved further south, to the middle of the parterre where it still stands today.

In 1826, the time indicated by the Palais Royal cannon ceased to be the official time. True solar time (as read on sundials) was replaced by the mean solar time of Paris (clock time). In 1891, all of France adopted the time of the Paris meridian. Twenty years later, in 1911, everything changed again. France adopted Greenwich Mean Time and the little cannon was banned (you can read more about that on page 110).

Abandoned to the elements and forgotten throughout most of the 20th century, the cannon was restored in 1990 and, for a while, its voice was heard again. But in 1991 it was forced to fall silent once more with the implementation of the Vigipirate plan during the Gulf War, for fear Parisians might think a terrorist attack was happening every day at noon! But the real attack occurred in 1998 when the cannon was stolen. The culprits were never found, so it was replaced with a replica in 2004.

Despite being obsolete for a century, the cannon was reactivated in 2011 by the Ministry of Culture, which decided to pay tribute to its legacy while abandoning the solar ignition. Every Wednesday, a pyrotechnician checks his watch, then, at noon sharp, triggers the cannon in front of delighted tourists and history buffs. All that remains of its past glory is a plaque bearing the Latin motto that was engraved on its base: Horas non numero nisi serenas – “I count only the happy hours.”