Just a stone's throw from Prague's Charles Bridge, on the Old Town side of the Vltava, stands the Clementinum, after Prague Castle the biggest complex of historical buildings in the city. Named after a church on the site dedicated to St Clement, the Clementinum was established by the Jesuits in the middle of the 16th century. After centuries as a college, it is now home to the Czech National Library and the main Prague weather station.
When I visited the Clementinum's beautiful reading room there were many young people studying; I asked Hana Vajnerova of the library's public relations department whether the quiet reading room was open to all students.
"Anybody, not only students but anybody who has reached the age of 18."
How long have people been reading and studying in this hall here?
"The Jesuit Order was dissolved in 1773 and then all of the Clementinum's buildings were turned into educational institutions - secondary schools, universities. Maybe two centuries."
As we walked from the reading room past St Clement's church to the Clementinum's famous Baroque Great Library and Astronomical Tower, my guide pointed out a statue dedicated to Josef Stepling, a mathematician greatly influenced by the Enlightenment. The statue stands in front of the Clementinum meteorological station, which was set up by Stepling and has been keeping continuous weather records since 1775.
"He was the first director of this observatory, it means the astronomical department. He was very important for developing astronomical studies. The Empress Maria Theresa ordered this monument to commemorate Josef Stepling."
In the entrance to the nearby Chapel of Mirrors there is a bust of Mozart, which Hana Vajnerova told me was regarded as one of the ugliest in the world. Mozart is said to have been greatly impressed by the beauty of the Clementinum's Baroque Great Library. I followed the great composer's footsteps to a room just before its entrance which contains some of the oldest manuscripts in the library's collection, and two extremely interesting clocks standing side by side. The clocks reflect contemporary uncertainty about whether the astronomer Copernicus was right, and the Sun really was at the centre of the solar system.
"The clock on the left is based on a planetary system according to the theories of Tycho Brahe - the Earth is the centre of the universe. The other clock is made according to Nicholas Copernicus, and it's a heliocentric system of planets."
So that's the correct clock with the Sun at the centre of the solar system.
"Yes, and if the clocks are turned on the planets are adjusted according to their exact position at the time. They move and show the position and movements of the planets during the whole year."
The Great Library itself - designed in the 1720s - has to be seen to be believed: cherubs top bookshelves which look like they haven't been touched for centuries and there are a series of intriguing looking globes of various sizes in the centre of the room. Hana Vajnerova was keen to point out one particular bookcase, on the balcony at the opposite end of the hall from the main doors.
"What's important from the point of view of Czech history is over there - the bookcase on the balcony which has a plaque reading 'bibliotheca nationalis', it means national library. It's actually the beginning of the tradition of the Czech National Library."
From what year was that?
"It's from the times of Raphael Ungar, which means the end of the 18th century. He selected books written in Czech at that time and placed them in that bookcase. Most books here were in Latin, in German and in some other languages."
So that bookcase has great significance for Czech history?
"Yes, it has. And Ungar introduced a cataloguing system which is used up to the present."
From the Great Library we followed a creaky wooden staircase up into the Astronomical Tower.
"Here we are in the so-called Meridian Room. It has its name from the string you can see on the floor: it's the local Prague meridian. The sunlight comes in through the small hole by my right hand, and you can see the spot of sunlight over there on the opposite side, on the wall. The spot moves and moves and when it's cut by the string in two halves exactly, it's the exact astronomical noon."
That tiny spot of sunlight was of enormous significance. When it signalled noon, a flag was flown from the balcony that runs around the top of the tower. Until 1918 the waving of a flag was followed by the firing of a cannon from near Prague Castle's Old Castle Steps, so the citizens of the city could hear that it was midday. But they were not the only people who benefited from the Clementinum clock.
"The Clementinum gave the time to railways in the western half of the Austrio-Hungarian Empire. The eastern part was set by the observatory in Budapest. And here are copies of the noon flags. White and red are Czech colours, the white flag was used during war times. Black and yellow are the imperial Hapsburg, or Austrio-Hungarian colours. This practice was abandoned in 1926 when the signal began to be given by radio, by Radio Journal, a station which still exists. But signal for the radio signal came from the Clementinum."