A medieval mechanical astrolabe that tracks three different types of time

Countries & Cities · Published 15 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: The Prague Orloj is more than a clock; it is a mechanical planetarium. While most timepieces tell you the hour, this 15th-century masterpiece simultaneously tracks the position of the Sun and Moon, the phases of the lunar cycle, and the shifting

Created in 1410 by horologist Mikuláš of Kadaň and astronomer Jan Šindel, the Orloj is a mechanical astrolabe that functions as a primitive planetarium. It displays the universe's orientation relative to Earth using a geocentric model. The dial's background represents the standing Earth and the local sky, with a blue circle for the sky above the horizon and red and black areas for the sky below. During the day, the mechanical Sun sits over the blue section; at night, it moves over the black.

The clock is a masterpiece of complex gearing. The Sun is attached to a 366-tooth gear, while the Moon moves on a 379-tooth gear. A unique 17th-century mechanism uses a screw-thread and a weight to rotate the Moon's sphere via gravity alone, advancing two teeth per day. This allows the clock to display the lunar phase with a deviation of only about one day every five years. It also features a 365-tooth gear for the zodiac, connected to the Sun and Moon gears by a 24-tooth gear.

The Orloj provides three distinct timekeeping systems. First, it shows local Prague time (Central European Time) via golden Roman numerals. Second, it displays "unequal hours," where the time between sunrise and sunset is divided into 12 parts, varying by season. Third, it shows "Old Czech Time" (Italian hours), where the number 24 indicates sunset, which fluctuates between 16:00 in winter and 20:16 in summer. The outermost ring, measuring approximately 300cm in diameter, moves back and forth to stay aligned with the setting sun.

The clock has survived centuries of turmoil, including heavy damage during the 1945 Prague uprising when Nazi forces fired on the Old Town Square. While it was restored in 1948, a 2018 renovation sparked controversy. A heritage group alleged that the new artwork "radically changed" the appearance and genders of the figures originally illustrated by Josef Mánes in 1866, leading to a formal investigation by the National Heritage Inspectorate.

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