Prague's Clementinum, just a stone's throw from the Charles Bridge on the Old Town side of the Vltava River is, after Prague Castle, the second biggest complex of historical buildings in the Czech capital. Established by the Jesuits in the middle of the 16th century, the Clementinum has in recent centuries been home to the Czech National Library and the main Prague weather station. It is also closely associated with time-keeping in the Czech lands, as Radio Prague's Ian Willoughby found out when he visited its Meridian Room.
"It has its name from the string you can see on the floor: it's the local Prague meridian."
Hana Vajnerova of the Clementinum's public relations department points out the taut wire that was used as a meridian by astronomers of centuries past. A tiny hole in the Astronomical Tower's outer wall was key to time-keeping in the Prague of their day.
"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."
When the sunlight reached the centre of the meridian, a flag was flown from the top of the Astronomical Tower: the citizens of the city could see that it was midday, the other clocks in Prague were all set according to it, and all that thanks to a tiny ray of sunlight. But it wasn't just the people of Prague who benefited from the Clementinum clock.
"The Clementinum gave the time to railways in the western half of the Austro-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 Austro-Hungarian colours."
For a time after the flag was waved at noon, a cannon was fired from near Prague Castle, so citizens could also hear that it was midday. The cannon was abandoned in 1918, but flags were used for a few years longer, says Hana Vajnerova.
"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."
After centuries of time-keeping service, the Clementinum's fascinating Astronomical Tower was faithfully restored in 1990.
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