Prague's astronomical clock: the medieval sky above the square

On the southern façade of the Old Town Hall in Prague, every hour, a small crowd looks up at the Orloj. A skeleton pulls a cord, the apostles parade past two little windows, a cockerel crows. It is a show that has been repeated for centuries and that hides, behind its theatrical appearance, one of the oldest astronomical machines still working in the world. The clock is at once a scientific instrument, a work of art and an allegory of passing time.

The origins: 1410

The oldest part of the mechanism and the astronomical dial date from 1410. Historical research attributes the construction to the clockmaker Mikuláš of Kadaň, with the collaboration of Jan Šindel, astronomer and master at the University of Prague, who was probably responsible for the mathematical design. The circular calendar below was added later, towards the end of the fifteenth century, together with part of the Gothic decoration of the façade.

In the following centuries the clock was repeatedly repaired, stopped and set going again. The statues of the apostles were introduced only in the nineteenth century. In May 1945, during the Prague uprising, the Town Hall was badly damaged by fire and the Orloj too suffered serious harm; after the war it was restored and the apostles were carved anew by Vojtěch Sucharda.

Reading the astronomical dial

The great dial is built on the principle of the astrolabe. The background represents the Earth at the centre and the visible sky, with blue zones for day and dark zones for night. An eccentric ring carries the signs of the zodiac and shows the Sun's position on the ecliptic; other hands show the Sun and the Moon, the latter with its phases. Roman numerals give the common hour, while curved Arabic figures mark the unequal hours, which divided daylight into twelve parts.

An outer ring with Gothic numerals shows the hours according to the old Bohemian custom, which counted from sunset. Three different ways of measuring time thus coexist in a single image, evidence of an age in which the sky was still humanity's main clock and in which astronomy and astrology shared the same instruments.

The figures and the legend of master Hanuš

Beside the dial stand four allegorical figures: Vanity with a mirror, Greed with a purse, skeletal Death with an hourglass and a bell, and a Turk shaking his head. Their presence reminds the onlooker that wealth and beauty are fleeting and that death spares no one. It is a thoroughly medieval message, conveyed in a language everyone could understand.

The most famous legend tells that the city councillors had the builder, called master Hanuš, blinded so that he could not make a similar work for other cities; in revenge the master is said to have stopped the mechanism. Historians have shown that the attribution to Hanuš, which goes back to a sixteenth-century chronicle, is mistaken. Another popular belief holds that if the clock stands still for long, misfortune will befall the city.

The Orloj thus remains a place where science and imagination meet in full view of everyone. Those who linger a few minutes longer than the brief show of the figures can discover, in its lines and rings, an entire conception of the cosmos: the motionless Earth at the centre, the Sun and the Moon travelling among the constellations, the hours changing their length with the seasons.