Here is a great picture of Rebecca and Filippo with the cannon on top of Hill Janiculum.
In 1847, when watches and clocks were not widely available, Pope Pius IX decided to have a cannon at Castel Sant'Angelo fire daily at noon so that all the belltowers in town could ring the bells of their churches at once.
Since then, every day at noon, this cannon, now hosted on Hill Janiculum, fires in the direction of River Tiber.
