Places We Love

Copenhagen’s Round Tower, the Landmark Where Stars Align


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By Sidsel Overgaard

Visitors to Copenhagen know Rundtårn (the Round Tower) as the place to go for dazzling vistas over verdigris rooftops. But the original purpose of the 17th-century structure was a loftier one: to provide a connection to the stars.

Denmark’s King Christian IV commissioned the Round Tower in the mid-1600s in the hope of continuing the tradition of Danish astronomy put in place by Tycho Brahe. Attached to a church and to the University of Copenhagen’s library, the observatory enabled some 200 years of scientific discovery before being turned over to amateur astronomers, who continue to stargaze there today.

One of the most interesting stories about the observatory is one of the least well known. For about 100 years, starting in the mid-1700s, the Round Tower also served a controversial function as the city’s primary timekeeper.

The “controversial” part of the story centers on Professor Christian Horrebow. Horrebow ran the observatory during Denmark’s brief rule by the German doctor Johann Friedrich Struensee (for further details see the film A Royal Affair). The modernization-minded Struensee directed Horrebow to make sure every clock in the city was telling the same time.

It may be hard to imagine today, but during Horrebow’s lifetime accurate clocks were still a newfangled concept. There was no such thing as standard time. Horrebow’s suggestion for coordinating the city´s few public and private clocks seemed simple: using astronomical measurements, a flag would be hoisted above the Round Tower at exactly noon once or twice a week. All clocks could then be set accordingly.

It didn’t take long for the city’s clockmakers to voice their frustration with the system, but Horrebow was prepared. In a written response he explained that there were two ways of telling the time, “apparent solar time” and “mean time.”

To understand apparent solar time, think sundial. It’s the time according to the sun’s actual position, and due to Earth’s irregular orbit it varies constantly. Mean time, on the other hand, is an averaging out of the time to make it more regular. It’s useful if you’re working with something mechanical, like a clock. Apparent solar time and mean time match up only four days during the year. Horrebow’s suggestion to the clockmakers? Try putting two minute hands on each clock—one for each version of the time.

The tug-of-war between Horrebow’s dedication to astronomical integrity and the rest of the population’s desire for a straightforward answer to “What time is it?” went on for a decade. But as you can deduce, the clockmakers won. That explains why there’s one minute hand on your watch, and not two.