Jens Olsen World Clock

(And how at least some of it works)

Introduction

I confess that despite my interest in horology, it wasn’t until visiting Copenhagen that I heard of this clock. Its online presence is far less than would be expected, a quick search would likely land you on its Atlas Obscura page or its brief Wikipedia page. Similarly there are scarce publications on it, the main being a technical description by Otto Mortensen from 1957 which is no longer in print. But everywhere you will find the same statement: this is the most accurate mechanical clock in the world. But that isn’t even its most impressive accolade, for only a small fraction of the clock is actually necessary to achieve that. The true impressiveness of this contraption lies in the breadth of the functions that it fulfills. When you view this clock you are offered a menu of displays that at first can be overwhelming. Would you like your time Central European? Perhaps sidereal? Solar, local, Gregorian, Julian, they are all there. Planetary motions, lunar perturbations, even motions with periods of 26,000 years whose inclusion were not necessary except to fit into the effort of the project.

But what I find most compelling is the transparency of the design. Not only is the clockwork physically exposed and visible inside its glass case, but the design has been modularized in such a way that discrete information is passed mechanically from one module to another. Shafts between modules have periods that make intuitive sense, such as 1 hour or 1 sidereal day, and go directly from the module that generates that speed to the module(s) that need that speed for their display.

History

As the name suggests the clock is the brainchild of Danish clockmaker Jens Olsen, locksmith by trade. He spent decades designing the astronomical calculations and soliciting funds to build it, and the project became a source of national pride during World War II. Unfortunately, he ultimately died in 1945 before most of construction was complete. The work was carried on and completed by his friend and peer Otto Mortensen, who wrote a book on the clock. In 1955, the clock was set in motion by Jens Olsen’s granddaughter and King Frederick IX and has continued ticking since.

Design

Overall Concept

The clock sits in its own dedicated room in Copenhagen City Hall inside a glass case fed by a filtered, temperature regulated, and dehumidified positive pressure air system. The case is divided into three vertical sections: The left (as viewed from the front) generally relating to solar time and world time, the center showing mean time and sidereal time, and the right showing astronomical information. Generally each module corresponds to a single display, but there are a few modules with a cluster of displays and one with no display at all. At risk of my fingers going numb typing this these are the modules and the information they display.

Left Section of Case

  • Triple Dial Module

    • Equation of Time (Local & General)

    • Local Time

    • Solar Time

  • Synchroscope Module

  • Sunrise/Sunset Module

    • Sunrise (Mean and Solar Time)

    • Sunset (Mean and Solar Time)

  • Gregorian Calendar Module

    • Year, Month, Day of Month, Day of Week

Center Section of Case

  • Mean Time (Central European Time)

  • Sidereal Time Module

  • Main Calendar Module

    • Dominical Letter

    • Epact

    • Solar Cycle

    • Cycle of Indication

    • Lunar Cycle

    • Days of Month, Moon, Holidays

  • Equation Module (Behind Calendar)

    • [No displays]

Inter-Module Communication

The modules can largely be examined in isolation due to their nature but there are interdependencies between them that can be considered as signals of sorts, generated in one module and transmitted mechanically to another. The signals are generally one of three types:

  1. Constant rotations are transferred via rotating shafts that interface with both the providing and receiving module via a set of equal bevel gears. Rotation rates are intentionally chosen to have meaningful values.

  2. Linear motions are transferred via thin steel ribbons being pulled. As the ribbons cannot be pushed the receiving module must have a weight to ensure tension is always maintained. Above the clockwork there is a set of bell-cranks such that these ribbons generally go upwards from one module, across above the works, and down to the other module.

  3. Impulse motions are transferred via control rods. Similar to the linear ribbons, these generally go upwards to the bell-crank system and across and down to the other module. These correspond to “trigger” signals to initialize a calendar event like rolling to the next day.

Right Section of Case

  • Planisphere Module

  • Sun & Moon Module

    • Sun Ecliptic Longitude

    • Moon Ecliptic Longitude

    • Lunar Node Ecliptic Longitude

    • Lunar Perigee Ecliptic Longitude

  • Planet Position Module

    • Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

  • Julian Calendar Module

    • Julian Year

    • Julian Day

If we map these all out we can construct a block diagram that states what each module communicates out and receives in. For clarity I’ve rearranged the modules and removed the bell-crank relay system, lest this graphic turn into spaghetti. Note that the equation module (which had no display) features prominently.

Modules

Mean Time Module

Escapement

The escapement used is a Denison-style double three-legged gravity escapement. This choice represents a prioritization of timekeeping accuracy above power use and ease of manufacturing, in the video notice that every motion of the escapement pulls along with it a pair of fly vanes which, when the escapement is arrested, slows to a stop on a friction clutch. The result is that the energy of the escapement is being constantly lost to air resistance and the friction of the clutch. The

Pendulum

Despite the thermally regulated environment in which the clock has been placed, the pendulum is designed to be temperature compensated. The rod is composed of Invar, which has an exceptionally low thermal expansion coefficient. Even so, Otto Mortensen mentions that there is a small ebony tube to counteract any expansion (although I was unable to locate it). The pendulum is suspended on a suspension spring of two metal strips. I was unable to locate a material specified but believe it to be Elinvar based on Mortensen’s familiarity with Charles Édouard Guillaume. Guillaume received the Nobel Prize in 1920 for developing a series of nickel-steel alloys, among them Invar (of exceptionally low thermal expansion coefficient, Elinvar (of exceptionally low elasticity variation with temperature), and platinite (which nobody uses for anything).

Gearing

The gearing of the mean time module is entirely conventional, consisting of a train of wheels that translate from the escape wheel to the seconds wheel (with hand), then to the minute wheel (with hand), then to the hour wheel (with hand), then to the power source.