A clockwork cosmos
Inside Strasbourg’s Cathédrale Notre-Dame stands an 18-meter-high astronomical clock, a scientific instrument built into a Renaissance-era cabinet. While the current mechanism dates to 1842, it is the third in a series of clocks built on this site, and it preserves the ambition of its 16th-century predecessor. That earlier clock, a collaboration between mathematicians, artists, and Swiss clockmakers, was a mechanical model of the known universe.
The project for the second clock began around 1547, and it was completed by 1574 under the direction of mathematician Conrad Dasypodius. Although it was built on a geocentric Ptolemaic model, its astrolabe and planetary dials tracked the mean positions of the sun, moon, and the then-known planets with precision. One dial was designed to display all eclipses, both solar and lunar, over a 32-year interval. Another feature was a celestial sphere depicting 1,020 stars from Ptolemy's star catalog, painted by artist Tobias Stimmer. This clock functioned for 214 years before stopping around 1788.
The perpetual computus
The current clock was built between 1838 and 1843 by Jean-Baptiste Schwilgué, a self-taught clockmaker who had dreamed of repairing it since he was a boy. Rather than a repair, he constructed an entirely new mechanism inside the original 16th-century housing. Schwilgué’s primary goal was to solve a complex calculation: the perpetual calculation of the date of Easter.
The date is difficult to mechanize, as it depends on a complex formula involving both the solar and lunar cycles. Schwilgué invented and built a mechanical "computus," an analog computer that correctly calculates the date of Easter according to the Gregorian calendar. This device was one of the first of its kind. Schwilgué also updated the astronomical displays to a Copernican heliocentric model. A celestial globe tracks the positions of more than 5,000 stars. Every day at solar noon (around 12:30 PM local time), the clock's automata come to life. Figures representing the ages of life parade past Death, and the 12 apostles march before a figure of Christ, accompanied by the crowing of a large rooster.