Prague’s iconic astronomical clock gets digital twin
A digital replica of Prague’s famed astronomical clock is now on display in the Knights’ Hall of the Old Town Hall. Thanks to the virtual model, visitors can gain a clearer understanding of how the world’s oldest functioning mechanism actually works.
Every full hour, the astronomical clock – the Orloj – draws crowds who gather to watch it come to life. The “Walk of the Apostles” is accompanied by other animated sculptures, most famously a skeleton representing Death who strikes the hour. Yet the moving figures are only one part of what makes the Orloj remarkable. The clock also shows the positions of the Sun and Moon, along with other astronomical details, while its calendar dial represents the months. But how does the intricate system actually work, especially given that it dates back to 1410?
Architect Jiří Mašek, who designed the exhibition, explains that the new digital model of the Orloj allows the public, for the first time, to see not only the famous façade but also the machinery that drives the entire mechanism. Visitors can learn what each component of the clockwork does and, crucially, observe the whole system in motion. He describes the advantages of the digital model:
“It’s a digital twin, so we aren’t limited to the present time. I can, for instance, change the year and speed up time to make it easier to observe how the machine and the astrolabe work in accelerated time, which would otherwise be difficult to imagine.”
The mentioned astrolabe is an early scientific instrument that serves as a star chart and physical model of the visible half-dome of the sky. It was used to estimate time.
Mašek adds more on what the model can do:
“The digital model also allows us to view the last historical state of the clock before the major reconstruction in 1864. The Orloj has remained roughly in its current form since then. We were able to recreate this thanks largely to Mr Skála, the ‘Orloj keeper’, who has access to all surviving sources and historical drawings.”
The role of the “Orloj keeper” must be one of the most unique job titles in the world. Thanks to him, the clock continues to function and is taken care of.
Digitising the Orloj took two years, often carried out at night when the clock could be stopped and dismantled. To capture the details of every component, the team used a technique known as photogrammetry, explains Jiří Kubišta of CESNET (Czech Education and Scientific NETwork), who worked on the digitisation process:
“With this method, we photographed each Apostle more than 200–250 times from multiple angles—below, above, and all around. Using photogrammetry, these images are then combined into a highly detailed 3D model.”
The project cost nearly 6.5 million Czech crowns. In addition to forming part of the permanent exhibition at the Old Town Hall, the 3D model will also aid further historical research on the Orloj, says Jana Adamcová, Vice-Chair of Prague City Tourism:
“The technical documentation of the Orloj serves not only to record its current state and its form in 1865, but also as an educational and research tool. Documentation as comprehensive as this has never before existed in the history of the Orloj.”
Visitors to the Old Town Hall, itself dating back to 1364, can see the new exhibition with either a tower ticket or a ticket to the building’s interior. The aim of the permanent exhibition is not only to introduce tourists to the history and technology of the Orloj, but also to make it more accessible to Czechs and Prague residents. To that end, the digital model is also available through a special online app, allowing users to explore the individual parts of the clock, complete with commentary.
