PRAGUE — With its soaring Gothic cathedrals and ancient alleyways lined with baroque architectural marvels spread beneath a majestic castle that overlooks the city, Prague can seem a place frozen in time.

But for more than 600 years, one clock has been measuring the passing hours, noting when day turns to night as well as the position of the sun, moon and stars.

Until last week.

Prague’s famed Astronomical Clock, the Orloj, in the city’s Old Town, has been stopped for repairs.

It is not the first time it has been stopped, but every intervention is fraught, given the clock’s age and fragility. And this one is the most ambitious in years. So for Prague’s clock master, Petr Skala, it is a little like being a surgeon at work on a very old patient.