Astronomical Clock: How Prague's Orloj Maps the Sky
Prague's Orloj has shown the Sun, Moon and zodiac over Old Town Square since 1410. Here is how to read it, and how it survived six centuries.
What makes a clock astronomical
Most clocks answer one question: what time is it? An astronomical clock answers several at once. Alongside the hour, it shows where the Sun and Moon sit in the sky, which sign of the zodiac the Sun is passing through, and what phase the Moon is in.
Such clocks come from an age when the sky was the calendar. Sunrise, the full Moon and the turning seasons governed farming, church festivals and much of daily life, so a machine that tracked them was useful as well as impressive.
Among the most famous still at work is the Orloj on the Old Town Hall in Prague. Crowds gather under it on the hour, and it makes a good model for understanding how the whole family of astronomical clocks works.
Who built the Prague Orloj, and when
For a long time, the clock was credited to a master named Hanuš (Jan Růže) around 1490. Physicists at Charles University in Prague now treat that as a legend. Research published in the 1990s dates the oldest parts, the clockwork and the astronomical dial, to 1410.
The 1410 builders were the clockmaker Mikuláš of Kadaň and Jan Šindel, a professor of mathematics and astronomy at Charles University. A written record from 1459 already describes the astrolabe in working order, according to the city's own history of the clock.
A calendar dial was added in a second phase, probably around 1490, when Gothic sculpture was also added to the facade. Prague's city tourism site credits Master Hanuš with that calendar plate, which may be how his name became attached to the whole clock.
Folklore has a darker version of his story, retold by Hyperallergic: the city blinded its clockmaker so he could never build a rival. No record supports it, but guides still tell it.
Reading the astronomical dial
The upper dial is built like an astrolabe, a medieval instrument that flattens the sky onto a disc. Its background is painted in zones. Blue marks the daytime sky above the horizon, a dark circle at the bottom stands for astronomical night, and bands labeled AVRORA and CREPVSCVLVM mark dawn and dusk.
Three pointers move over that background: one for the Sun, one for the Moon and one for the zodiac ring. According to Charles University, they are driven by three wheels of equal size with 365, 366 and 379 teeth, all turned by a single pinion. The difference in tooth count lets each pointer keep its own pace.
The zodiac wheel turns once per sidereal day, the time it takes the stars to return to the same place. The Sun wheel turns once per mean solar day. The Moon wheel follows the Moon's slower average motion, while a small ball, half silver and half black, rotates once a lunar month to show the Moon's phase.
The dial also counts hours in more than one way. Hyperallergic lists Old Bohemian time, which started the day at sunset; Babylonian hours, which divided daylight itself; Central European time, read by the Sun hand; and star time.
See also: how star time differs from solar time on OnlineClock.app.
See also: the Moon's current phase, live on OnlineClock.app.
The calendar dial and the moving figures
Below the astronomical dial is a calendar disc that turns once a year. During repairs in 1865–1866 a new disc painted by the Czech artist Josef Mánes was installed, and his twelve month scenes and twelve zodiac medallions are counted among his finest work. The disc on the tower today is a replacement; a new calendar plate was fitted in the 2018 restoration.
Some of the stonework is far older than any moving part. Charles University says the figures flanking the astronomical dial, and the mask and figures on the arch above it, were carved in the early 15th century by a masons' workshop led by Peter Parler.
The moving figures are what most visitors come for. A skeleton representing Death tolls a bell and turns an hourglass, while a Turk shakes his head and figures of Vanity and the Miser join in. Dates for them vary: Prague's city history links a Death figure to the 1490 phase, while Charles University and Radio Prague place the wooden moving statues in the 17th century.
The Twelve Apostles, who file past two small windows above the dial when the hour strikes, are also disputed. Charles University says they arrived only after the 1865–1866 repairs, while Radio Prague dates them to 1787–1791. Either way, today's figures are not the first set: the apostles burned in 1945 and were recarved by the sculptor Vojtěch Sucharda.
War damage and the 2018 restoration
In the last days of the Second World War, in May 1945, German shelling and fire badly damaged the Old Town Hall. The city archives inside were destroyed and the clock was hit hard. Volunteers repaired the original clockwork, and the clock ran again in 1948 with Sucharda's new apostles. In 1976 the sculptures were replaced with copies, and the originals went to the Prague City Museum.
The most recent major restoration took more than 18 months. The Czech tourist board says it removed damage from the 1945 fire and from later poor repairs. The mechanism was returned to its 1860s form, and the tower clock regained gilded hands and numbers closer to its Baroque look.
Inside, the works are now driven by hemp ropes wound on wooden drums, a faithful copy of the 19th-century arrangement. The clock was set going again on 28 September 2018, St. Wenceslas Day, with the apostles back on the hour.
Smaller details changed too. The apostles now appear behind a new blue stained-glass door that replaced a metal one from the 1970s, and the Madonna and Child statue on the corner of the building is an exact copy of the original. The relaunch coincided with the Old Town Hall's 680th anniversary.
Despite all the repairs, much of the old machine survives. Hyperallergic, citing Deutsche Welle, reports that about three-quarters of the clock's components are original.
Why astronomical clocks still fascinate
Strip away the statues and the Orloj is a mechanical model of the sky. Its makers did not see timekeeping and astronomy as separate subjects: the hour on the dial and the position of the Sun were two readings of the same machine.
That link has never really gone away. Astronomers still distinguish solar time from star time, and the Moon still runs through the cycle that the Orloj's little silver-and-black ball has been tracking for centuries.
Questions people ask
How old is the Prague astronomical clock?
Its oldest parts, the clockwork and astronomical dial, date to 1410. The calendar dial was added later, probably around 1490.
Who built the Prague Orloj?
Clockmaker Mikuláš of Kadaň and Jan Šindel, a mathematics and astronomy professor at Charles University. The older story crediting a master named Hanuš with the whole clock is now treated as a legend.
When do the apostles appear?
The Twelve Apostles pass two small windows above the dial when the clock strikes the hour during the day. Prague's tourism site gives the show as running from 9 a.m. to 11 p.m.
What does the zodiac ring on an astronomical clock show?
It shows which zodiac sign the Sun is in. On the Orloj, the zodiac wheel turns once per sidereal day, following the apparent motion of the stars.
Keep exploring
Sources
- The Prague Astronomical Clock (Orloj) - Charles University, Institute of Theoretical Physics
- Historie Pražského orloje - Prague City Tourism (prague.eu)
- Astronomical Clock - Prague City Tourism (prague.eu)
- Astronomical clock restored - VisitCzechia
- Three apostles being removed from Prague's Astronomical Clock - Radio Prague International
- The History of One of the Oldest Astronomical Clocks in the World - Hyperallergic