Klementinum | Tours of the Baroque Library and Astronomical Tower

This guide to the Klementinum's astronomical tower in Prague explains how, from 1842, a beam of sunlight sliding across the floor and touching a north-south string marked solar noon to the second, signaled to the city by a flag waved from the gallery. It follows how, starting in 1891, a cannon in Letna Park across the river was aimed along that same meridian to fire the instant the flag moved, correcting Prague's pocket watches for some thirty-five years. Look up at the balcony where that flag once snapped.