Prague Astronomical Clock (1410): The Medieval Orloj

The Prague Astronomical Clock, known in Czech as the Orloj, was installed on the tower of the Old Town Hall in Prague in 1410. Built by the clockmaker Mikulas of Kadan with the astronomer Jan Sindel, it is the oldest astronomical clock in the world that is still in operation. Its painted dials, moving figures and hourly procession of apostles draw large crowds to Old Town Square, but its real marvel is a medieval mechanical model of the sky.

Background: clocks and astronomy in medieval Prague

Mechanical clocks driven by falling weights and regulated by a simple escapement appeared in Europe in the late thirteenth century. By the fourteenth century, wealthy cities competed to install public clocks, and the most ambitious added astronomical dials that showed the motions of the Sun, Moon and stars. Earlier, in Song China, engineers had built water-driven astronomical clock towers such as Su Song’s clock of 1088.

Prague around 1400 was one of the great cities of central Europe. Emperor Charles IV had made it his capital, founded a university there in 1348 and built the Charles Bridge. The Old Town council, proud of its wealth and status, commissioned a clock to match.

Who built it

The earliest record of the clock is a document of 9 October 1410, in which the Old Town council recognised the clockmaker Mikulas of Kadan as the maker of the clock and confirmed his payment and duties. The astronomical design is generally attributed to Jan Sindel, a professor of mathematics and astronomy who was rector of the university in 1410.

For centuries, however, Prague tradition gave the credit to “Master Hanus”. This was Jan Ruze, a clockmaker who repaired and extended the clock around 1490. A sixteenth-century report by the clockmaker Jan Taborsky, who looked after the clock from the 1550s, helped spread the attribution to Hanus. Only in the twentieth century did historians establish the role of Mikulas and Sindel.

How the astronomical dial works

The astronomical dial is a mechanical astrolabe, a medieval instrument that shows the sky using a geometric method called stereographic projection. The Earth is at the centre. The blue upper part represents the sky above the horizon by day; the black lower circle represents the night. Between them, bands labelled in Latin AURORA (dawn), ORTUS (sunrise), OCCASUS (sunset) and CREPUSCULUM (twilight) mark the times when the Sun is just below or just above the horizon.

Several pointers move over this background:

• A golden Sun travels around the dial once a day, and its position against the background shows whether it is day, night or twilight.
• A smaller, eccentric ring carries the signs of the zodiac, showing where the Sun and Moon are among the constellations through the year.
• A sphere for the Moon, half silver and half dark, turns to show its current phase.
• A small star on the hand marks sidereal time, the time measured against the stars rather than the Sun.

The dial also shows several ways of counting hours. Roman numerals on the inner ring give ordinary local time. The outer ring, with gold numerals 1 to 24 in Gothic script, shows Old Czech time, in which the day began at sunset; this ring turns through the year so that 24 always falls at sunset. Curved lines inside the blue area mark “unequal hours”, in which the daylight was divided into twelve parts whose length changed with the seasons.

Later additions

Around 1490, in the time of Jan Ruze, a calendar dial was added below the astronomical dial and the clock front was decorated with Gothic stonework. The calendar dial shows the months with scenes of country life and signs of the zodiac. The present paintings follow designs by the Czech painter Josef Manes, completed in 1866; his original is kept in the Prague City Museum and a copy is on the clock.

Moving statues were added beside the dials in the seventeenth century. Next to the astronomical dial stand four figures representing vices and fears: Vanity with a mirror, Greed with a money bag, Death as a skeleton who rings a bell and turns an hourglass, and a Turk, a figure representing pleasure. Beside the calendar dial stand a philosopher, an angel, an astronomer and a chronicler.

During a major repair in 1865-1866 the procession of the twelve apostles was added. On the hour, two small windows open above the dial and the apostles pass one by one, while Death rings his bell, and the show ends with the crowing of a golden cockerel.

Damage and restoration

The clock was neglected for long periods and repaired many times, notably in the sixteenth century, in the late eighteenth century and in 1865-1866. Its worst crisis came in May 1945. During the Prague uprising against the German occupation, German fire set the Old Town Hall ablaze. The wooden apostles were destroyed and the dials badly damaged.

Clockmakers repaired the mechanism and the clock ran again in 1948, with new apostle figures carved by the sculptor Vojtech Sucharda. The most recent major restoration took place in 2018, when the clock was dismantled for several months, its mechanism overhauled and the dials returned to a more historically accurate appearance.

Legends and facts

The most famous legend says that after Master Hanus completed the clock, the councillors had him blinded so that he could never build a finer one for another city. In revenge the blind master is said to have reached into the mechanism and stopped it. The story has no historical basis; it grew up around the false attribution of the clock to Hanus.

Another legend warns that if the clock is neglected, Prague will suffer misfortune. People linked periods of disrepair with times of trouble, which may have helped persuade the city to keep paying for repairs.

The clock in its time and today

The clock was installed at a turbulent moment. In the same years the preacher Jan Hus was drawing crowds in Prague with his calls for church reform, and his execution in 1415 set off the Hussite Wars. The Old Town Hall, whose tower carries the clock, stood at the centre of the city’s political life throughout those conflicts.

Today the Orloj is one of the most visited sights in central Europe. Crowds gather beneath it every hour to watch the short show, which lasts less than a minute. Much of the old gearwork survives, including large toothed wheels that turn the Sun, Moon and zodiac pointers at their different speeds, although parts of the movement have been replaced or rebuilt during repairs over the centuries. A clockmaker looks after it, and the town hall tower above can be climbed for views over Old Town Square.

Timeline

• 1410, 9 October: the Old Town council records the clock made by Mikulas of Kadan.
• c. 1490: Jan Ruze (Master Hanus) repairs the clock; the calendar dial and Gothic decoration are added.
• 1552: Jan Taborsky begins long service as the clock’s keeper and repairer.
• 17th century: moving figures are installed beside the dials.
• 1865-1866: a major repair adds the procession of the apostles; Josef Manes paints the new calendar dial.
• 1945, May: fire during the Prague uprising badly damages the clock.
• 1948: the restored clock runs again with new apostles.
• 2018: a major restoration of the mechanism and dials is completed.

Why it matters

The Orloj is a rare surviving example of the medieval effort to model the heavens in brass and iron, long before precise timekeeping became possible with inventions such as the pendulum clock. Part of the UNESCO-listed historic centre of Prague, it links the city’s medieval scholarship with its modern identity.