A marvellous man and his legendary clockwork universe

Few pieces of engineering are more satisfying than a clock – one of humankind’s first attempts to model the universe. Most early clocks do little more than mark the hours but there is one remarkable machine, on the southern side of the Old Town Hall in Prague, that attempts to model the universe.

The original part of the clock was built in 1410 by the Imperial clockmaker Mikulas of Kadan, under advice from the rector of the Charles University, Jan Sindel, whose varied career had seen him work in Nuremburg as a physician, teach mathematics at Vienna and medicine and astronomy in Prague, as well as becoming the personal doctor of the Emperor Sigismund, rector of the University and a close friend of Pope Pius II.

The dial took the form of a mechanical astrolabe engraved with an almanac of information for those educated enough to read it, and whose eyesight could stand the test. The face represents the Earth with morning and evening horizons in red (dawn and dusk) and black (night) beneath a blue sky. Across this sky a mechanical sun arches on the ecliptic each day while another hand marks out the movement, and phases, of the Moon as the zodiacal houses revolve on a ring behind them.

The dial allows the observer to read the time in four different ways. The original clock showed the ‘unequal’ hours, whereby the day and night were divided into 12 hours each and hence changed in length over the year with the length of the day. This had been the system in the Roman world and for much of the Middle Ages, but the Orloj also had a novel innovation – Old Czech Time – introduced from Italy in the mid-14th century, which divided the day into 24 equal hours starting at sunset. The distance of the sun from the centre of the dial reveals times of sunrise and sunset on any day.

From the mid-16th century, ‘German’ time was added, with its 12-hour ‘am’ and 12-hour ‘pm’, and sidereal time (based on the Earth’s rotation relative to the fixed stars) was included in 1865.

The original sun mechanism was accompanied by a moon, a half silvered, half black sphere capable of showing phases – the only astronomical clock in the world to do so. This phase mechanism seems to have been a 16th-century addition by Jan Taborsky and was replaced in the following century with the current mechanism, which is accurate to one day in five years.

One might think the clock itself was impressive enough, but by the late 15th century a second dial was added, containing a complex calendar, and the whole machine was decorated with Gothic sculpture. The calendar showed on its circumference which saint each day was dedicated to, with a complex coded system for calculating the day in the month taken from the first Sunday after New Year. This was useful in an age where dates were often given relative to a saint’s day.

This work was probably carried out by Jan Hanus, who was, for many years, credited with creating the original machine.

In the mid-17th century, wooden figures were added. Three, representing Vanity, Lust, Avarice await the fourth, Death’s, ringing of the hour bell. As the skeleton of Death chimes the hour, the three other figures solemnly shake their heads, perhaps unwilling to accept the passing of time and the hurrying-on of their fate.

A century later, a happier procession of Apostles was introduced, who appear in a doorway above the clock and process across each hour of the day. Another 100 years on and a crowing golden cockerel was added.

The clock still runs today, although how many tourists can read its complex faces is a matter of debate. The mechanism, much of it the original work of Mikulas of Kadan, is kept in pristine order by the city, perhaps fearful of a legend that should the clock be neglected, the skeleton will shake its head in disappointment, and the city must act to avoid its doom – a warning stern enough to spur on any engineer.