On the second floor of the tower is a unique room, previously used for determining noon. The whole room turned into a camera obscura, due to ray of sunlight coming through a small hole up in the wall. The light falling on a string stretched on the floor determined high noon. The string is stretched across the room, during the summer solstice beam of light falls on its southern end, at the winter solstice on the northern end.
Noon measured in Meridian hall was from 1842 accompanied by a waving of a flag from the tower and from 1891 also by a shot from a cannon. Last midday signals were made in 1928. Replicas of the signal flags are now on display at the top of the tower.
On the walls of the Meridian hall are original astronomical instruments, two wall quadrants that were constructed by mechanic Jan Klein. These were used for measurings of the angular distance between celestial objects and their height above the horizon. Quadrants are firmly set into the walls on the southern and northern sides of the room.