A team of researchers from the Network Applications Technologies Department at CESNET has created a fully kinematic digital twin of the Old Town Astronomical Clock in Prague, the oldest still-functioning astronomical clock in the world. The goal of this two-year project was to capture all parts of the historic mechanism in digital form, thereby allowing both the general public and experts to gain insight into its inner workings, which are normally inaccessible. The project is also significant because researchers created a comprehensive, previously non-existent documentation of this 600-year-old monument for future generations. Prague City Tourism and st.dio also collaborated on the project.
Digitising a complex living mechanism
Creating a digital model of the astronomical clock was an exceptionally challenging process – the mechanism cannot be disassembled, is not accessible from all sides, and, moreover, is in constant operation. Nevertheless, CESNET experts managed to capture more than 750 major components and over 1,000 parts in total, including the smallest elements. The model also reflects the clock’s pre-restoration state in the 19th century, bringing the total number of represented parts to around 1,700. During several visits, they took approximately 4,647 photographs using, among other things, a special periscopic lens – Laowa Periprobe 24 mm T/14 – and had to precisely measure certain dimensions. The digitization of the apostles alone required more than 2,640 photographs at a resolution of 45 Mpx.
The researchers also had access to historical drawings by Romuald Božek dating from before 1864, which depicted only part of the machine, as well as approximately 250 drawings by Václav Heisler from the 1970s. However, they do not capture nearly all the details and contain inaccuracies. At the same time, it was necessary to consider the deviations of the actual handmade machine from the dimensions in the drawings. The astronomical clock keeper and academic sculptor Petr Skála also contributed to the model’s accuracy.
From model to real-time experience
To create the 3D model and present it, the researchers used Reality Capture, Fusion 360, and Blender, as well as the web technologies TypeScript, Three.js, and React.
The final model is available online at https://orloj.cesnet.cz and at the exhibition kiosk in the Knights’ Hall of the Old Town Hall. The application displays the movement of the astrolabe, the clock mechanism, and the apostles in real time and runs in 4K resolution at 60 frames per second. It allows you to observe the movement of the astrolabe and the clock mechanism not only in real time, but also in fast motion. It is also possible to set it to a specific date and time.
The digitization project goes beyond the scope of the model itself. It tests methods for digitizing historical clock mechanisms and technical heritage, with the data and expertise gained serving as a basis for documentation and further research.
The Old Town Astronomical Clock Digitization Project (TQ030002900) was supported by the Technology Agency of the Czech Republic as part of the SIGMA program.
