Known as the Orloj, the astronomical clock was designed and built by the Imperial clockmaker Mikuláš of Kadaň and Charles University professor Jan Šindel. Consisting of three main components, the astronomical dial and the mechanical clock itself were first noted in a document on 9 October 1410. The third component – a calendar dial – was added around 1490 when the façade was decorated with additional gothic sculptures.
One of the most famous features of the clock is the hourly “Walk of the Apostles” in which carved figures of the Apostles appear in procession at two windows above the main clock face. These were not added to the Orloj until a major repair on the clock in 1787-1791, although some of the other moving allegorical statues such as Death had been added beforehand. Further statues were added in later years, including the golden crowing rooster that didn’t appear until 1866.
The clock is a masterpiece of medieval engineering, and also serves as evidence of the European view of the universe at the time since the Earth appears at the centre. Against this background are the four key moving parts of the astronomical dial: the zodiacal ring, the Old Czech time scale, and two clock hands representing the Sun and the Moon and their position on the eliptic. The half-black, half-silver Moon even contains a mechanism to show the lunar phases.
The Orloj suffered serious damage in the Second World War when German forces attempted to suppress the Prague Uprising of May 1945. Restoration successfully returned the clock to working order in 1948, since when it has been renovated another two times.