Digitization of the Old Town Astronomical Clock. We reveal the secrets of one of Prague's most famous monuments

The Old Town Astronomical Clock, which has been setting the pace in Prague for centuries, now also exists in digital form. As part of a digitisation project, the Network Applications Technology Department created a detailed 3D kinematic model of the clock's components, offering the public an insight into how this unique machine works.

Digitising a machine with thousands of parts was a major challenge
The clock mechanism consists of more than 750 main parts, and if even the smallest parts are included, the total number exceeds 1,700. As it was not possible to disassemble the clock, another approach had to be found. Our colleagues from the Network Applications Technology Department documented the individual parts using a combination of methods. These included photogrammetry with a periscopic lens, which converts a series of photographs into a three-dimensional model; manual modelling based on historical drawings; and procedural generation of repetitive parts. Some parts of the clock mechanism had to be scanned at night, when it was possible to stop the astronomical clock.

The accuracy of the model was ensured by Petr Skála, an academic sculptor and clockmaker who provided expert assistance during the scanning of the astronomical clock. The resulting 3D model enables you to observe the astrolabe and clockwork movements in real time or fast motion. You can set it to a specific date and time.

The organisations involved in the project

This result is the work of teams of experts from various fields, including the CESNET association, Prague City Tourism and st.dio. We digitised the clock mechanism itself, including Božek's chronometer, astrolabe, apostles, winding system and calendar plate. We created a fully kinematic 3D model that captures the movement of all parts of the astronomical clock in real time, and we provided the necessary network and storage resources to enable remote access.

Prague City Tourism, a municipal organisation, conducted archival research of historical sources, provided expert supervision by an astronomical clock specialist, and prepared an exhibition in the Knights' Hall at the Old Town Hall. st.dio designed and implemented the visualisation part in collaboration with us – an interactive web application and an information kiosk that allows visitors to view a digital model of the astronomical clock directly in the Knights' Hall exhibition.

Virtual presentation and exhibition

This medieval technical monument is the oldest functioning astronomical clock still powered by its original clockwork mechanism. It was created in the early 15th century by clockmaker Mikuláš of Kadaň in collaboration with astronomer Jan Šindel. Although it is one of Prague's most visited gems, few people understand how it actually works. The digital model therefore offers a unique opportunity to find out.

The Old Town Astronomical Clock digitization project extends far beyond the model itself. Thanks to it, the team has verified procedures and technologies that can be used to digitize other historical clock mechanisms and other technical monuments. The data and experience gained will also serve as valuable input for technical documentation and further research.

This project also attracted attention at the international conference Digital Heritage 2025 in Siena, Italy, where our colleague Jiří Kubišta presented it as an example of Czech work in the field of cultural heritage digitization.

*The Old Town Astronomical Clock Digitization Project (TQ030002900) was supported by the Technology Agency of the Czech Republic as part of the SIGMA program.

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Techniques used in digitizing the astronomical clock in a nutshell
Photogrammetry with a periscopic lens
Photogrammetry converts a series of photographs into a three-dimensional model. A periscopic lens makes it possible to capture even hard-to-reach places deep inside the machine and also captures the texture of the object, i.e., its color. In order to successfully convert the photographs into a 3D model, it was necessary to stop the moving machine for a short time.

Manual modeling based on historical plans
Some parts could not be accurately photographed, so the creators supplemented them with manual modeling based on historical drawings. Romuald Božek described the astronomical clock in detail as early as the 19th century, and Václav Heisler then created detailed technical plans during its repairs in the 20th century. These documents were used to create the missing parts in the 3D model. However, there were a number of inaccuracies in the historical drawings. It was necessary to supplement them with precise measurements of the machine, historical photographs, literature, and valuable information about its construction from the Prague clockmaker, academic sculptor Petr Skála.

Procedural generation
This involves the automatic creation of model parts according to specified parameters, such as size or shape. This method was suitable for repetitive elements such as gears.

Terminology: Do you know the individual parts of the astronomical clock?

Apostles
Twelve wooden figures appear every hour in the window niches above the dial.
They symbolize the faith and craftsmanship that have accompanied the Prague Astronomical Clock for centuries.

Astrolabe
It shows the movement of the Sun and Moon across the sky, the signs of the zodiac, and the time in several different systems.
It is the heart of the astronomical clock – combining the functions of a clock, calendar, and astronomical instrument in one.

Božek's Chronometer
A mechanism constructed by Czech clockmaker Romuald Božek during the so-called great repair of the astronomical clock in the 19th century. It ensures more accurate operation of the clockwork and stabilizes its mechanism.

Calendar dial
The lower circle of the astronomical clock shows the months, public holidays and signs of the zodiac, accompanied by allegorical representations of the seasons and rural activities. Its current appearance was designed by the painter Josef Mánes in 1866. This model depicts the original, which is kept in the Prague Museum; a copy is displayed in the Old Town Hall.