The Orloj is one of the most famous and most recognizable old clocks in the world, and is located in the old town square in Prague, Czech Republic. Built in 1410, it is one of the first mechanical clocks ever built (technically considered the third) but is the oldest continually operating clock. Placed on the southern wall of the old town hall, the Orloj is an astronomical clock with a dial representing the movement of the sun and the moon as well as highlighting various astronomical details about the sky above. The dial is a form of what is known as a mechanical astrolabe, which is a device that based on the medieval understanding of astronomy. The clock displays the current state of the universe and is considered by many to be one of the first public planetariums. The background of the clock representing the Earth and the sky is surrounded with four moving components (the zodial ring, the outer rotating ring, and two icons representing the moon and the sun).
The oldest portion of the Orloj, the mechanical clock and astronomical dial, was made by clockmaker Mikulas of Kadan Mikuláš of Kadaň and mathematics and astronomy professor Jan Šindel. Large gold numerals indicating the local time (Central European Time) based on the 24-hour timescale surround the outer edge of the blue circle which represents the Earth.
The Orloj was badly damaged during the Prague Uprising when the German military fired on the south-west side of Old Town Prague. The attempt to destroy one of the centers of the resistance was unsuccessful but the hall and wooden sculptures of the face were burned as were other buildings in the town square. A great deal of care and attention were put into the restoration of the clock and the wooden Apostles, and the Orloj began to run again in 1948. The statues have been covered with nets to keep pigeons away since the last restoration and renovation in 2005. At this time the statues and the lower calendar ring were restored and repaired.
- The Calendar Dial is the newest portion of the clock.
- The clock is the most photographed monument in the Czech Republic.
- The Old Town Hall was founded by issue of King Jan Lucemburský on 18 September 1338.
- There was a celebration for the 600th anniversary that included a light show with video.
- The Old Town Hall was declared a cultural monument in 1962.
- Old Town Hall was paid for by a tax levied on wine.
- The 12 medallions with the zodiac below the clock were added by Josef Manes in 1865.