A boy named Petr Skala slides a clock from the wall in his family’s home. He begins to take it apart. Petr wants to understand a profound mystery: How is it possible to measure time?

That was decades ago. Now Mr. Skala is 71 years old. He has become a clock master, and is busy taking down part of another clock—one much older than he is. The Orloj astronomical clock has stood in Prague, Czech Republic, for 600 years. The elderly clock needs repair—a job given to Mr. Skala.

The Orloj is much more than a typical clock. It does tell time. One hand measures Old Bohemian Time. In Old Bohemian Time, the day begins and ends at sunset. Another dial measures German Time, in which the day begins and ends at midnight. If you know how to read it, the Orloj can also tell you the phase of the moon, the number of daylight hours, and what time the North Star will rise. An added clock face underneath shows the calendar.

A crowd gathers at the foot of the clock tower each time the hour turns. Four figures beside the clock move. They represent what people thought of as dangers to the city of Prague at the time the clock was built. One figure clutches a bag of gold. He stands for avarice. Another admires himself in a mirror. He represents vanity. A skeleton figure represents death, and a Turkish figure indicates the invading Ottoman people. Above the clock, two blue doors open. Figures of six men file through on each side. These represent the 12 apostles. The Orloj has put on this show since 1410—with only a few interruptions. Imagine all the history it has seen!

How do you fix the oldest working clock in the world? If you asked Mr. Skala, he would probably say, “Very, very carefully.” Mr. Skala has until August to finish fixing the clock. He will replace metal gears added in previous renovations with a wooden system more like the original machinery. He will restore the paint’s old color. When he’s done, the clock will technically have new parts. But it will look and work like its old self again.