Jens Olsen's World Clock
A Danish locksmith spent decades calculating a clock he never saw finished. It started ticking in Copenhagen City Hall ten years after his death.
A clock that outlived its maker
Jens Olsen was a Danish locksmith and clockmaker who lived from 1872 to 1945. According to the Linda Hall Library, he saw the famous astronomical clock in Strasbourg as a young man and dreamed from then on of building one like it for Denmark.
That dream took most of his life. He was about 50 before he finished the mathematical calculations, and 71 before he found the money to start construction. He died on 17 November 1945, aged 73, with the clock far from complete.
His team carried on. Clock historian Denis Roegel, who has published a detailed technical study of the clock, names Svend Danielsen as a co-worker on it from 1944 to 1955. King Frederik IX started the finished clock at exactly 3 pm on Thursday, 15 December 1955, which the Museum of Copenhagen notes was 50 years after City Hall opened.
Telling time several ways at once
Most clocks show one kind of time. Olsen's shows several. The Linda Hall Library lists Copenhagen time, solar time and the time around the world among its displays.
Roegel's survey fills in the detail. A large dial shows ordinary civil time, and a smaller one shows sidereal time, the time kept by the stars rather than the sun, with its own seconds dial. A so-called triple dial shows the equation of time, the difference between clock time and sundial time, alongside mean and true local time.
Another dial, which Roegel calls the synchronoscope, gives the time anywhere in the world. A separate dial shows the times of sunrise and sunset in Copenhagen.
All of this sits on the front of the clock, which Roegel describes as three main parts. Below the time dials, a set of number windows spells out the current year, along with the day, date and month, much like a calendar that turns its own pages.
See also: what sidereal time measures on OnlineClock.app.
Sky, calendar and Easter
The upper part of the clock is about the heavens. Roegel describes a dial showing the configuration of the sky, another tracing the motion of the sun and moon along the ecliptic, and a planetary dial showing the planets' average motions out as far as Neptune. The Linda Hall Library adds that the clock predicts future solar and lunar eclipses.
That planetary dial has a famous gap. It follows only eight planets and leaves out Pluto, which was discovered in 1930. The Linda Hall Library suggests Olsen either finished his planetary calculations before then or somehow anticipated Pluto's later demotion.
Lower down is a perpetual calendar. Twelve small openings show the months with their weekdays, moveable feasts and the cycle of the moon's phases, while a group of dials handles the computus, the old method for working out the date of Easter. Another dial counts Julian days and years within the 7,980-year Julian period, which began in 4713 BC.
Some holidays, such as Easter, fall on a different date every year because they depend on the moon. According to the City of Copenhagen, at midnight every New Year the clock spends six minutes working out the coming year's public holidays and full moons, a calculation people would otherwise do with tables.
See also: today's Julian day number on OnlineClock.app.
The slowest gear in the world
The clock's best-known part is a gear that takes 25,753 years to turn once. Its job, the Linda Hall Library explains, is to shift the star map slowly as Earth's axis wobbles, which moves the celestial north pole around a circle over almost 26,000 years.
The Museum of Copenhagen calls it the slowest-turning gear in the world. The Linda Hall Library notes dryly that in its first 60 years it did not move very far.
That is a deliberate piece of engineering humility. The gear is not there to be watched moving; it is there so that the star map stays correct over spans of time far longer than the building, the city or perhaps the calendar it follows.
Inside the machine
Not every part moves all the time. Roegel describes intermittent drives: the equation-of-time work is triggered once an hour, while the computus is set in motion once a year. Everything, including the eclipse work, is driven from the main dial, which holds the escapement, through 16 arbors carrying 18 sets of rockers.
The materials have a story too. The City of Copenhagen says the brass came mainly from kitchen utensils melted down from Danish homes in the 1940s. The gears are gilded with about four kilograms of gold, and all the metal dials are coated with rhodium, one of the rarest precious metals.
Sources disagree on the size of the mechanism. The City of Copenhagen gives 14,448 parts, while the Linda Hall Library gives 15,448. Neither explains how it counted, so the safest summary is about fifteen thousand.
The city and the Museum of Copenhagen both call it the world's most accurate mechanical clock, beaten only by atomic clocks. That is the owners' own claim rather than an independently published measurement.
Winding, care and restoration
The main clock needs winding only once a week. Roegel reports that it is rewound on Wednesday mornings, reset to the right time weekly and its drift written down in a notebook. The computus mechanism is wound just once a year.
The finished clock is not quite what was first drawn. Roegel reproduces a planned layout from 1947 with dials in different positions, and notes that the main dial was meant to show 24 hours rather than 12.
The clock sits in a transparent case designed by architect Gunnar Biilmann Petersen, on a granite base sunk into the ground, and visitors can see most of the mechanism from the front and the back. Clockmaker Søren Andersen restored it for the first time between 1995 and 1997, and a team including Andersen restored it again in 2019 and 2020.
Seeing the clock today
The clock stands on the right as you come through the main entrance of Copenhagen City Hall. An exhibition about it is run by the Museum of Copenhagen.
Entry is free through the main entrance. At the time of writing the museum listed hours of 9 am to 4 pm on weekdays and 9:30 am to 12:45 pm on Saturdays, but check before you go, since City Hall is a working building. It was not always free: Roegel notes that visitors originally paid one krone.
Questions people ask
Who built Jens Olsen's World Clock?
Danish clockmaker Jens Olsen designed it and made the calculations, but he died in 1945 while it was being built. His co-workers finished it, and King Frederik IX started it on 15 December 1955.
What is the slowest gear in the world?
The gear in Jens Olsen's World Clock that follows the slow wobble of Earth's axis. It turns once every 25,753 years, gradually moving the clock's star map.
Is it free to see the World Clock?
Yes. The clock is inside Copenhagen City Hall, and entry through the main entrance is free during opening hours.
How many parts does the clock have?
Sources differ. The City of Copenhagen says 14,448 and the Linda Hall Library says 15,448, so roughly fifteen thousand.