Principal timepiece of the Leiden Observatory, Andreas Hohwü, Amsterdam, 1861
Inv V03461

Timepieces have always been important for observatories. Astronomers want to know exactly when something occurs in the sky. Therefore the clocks must run as accurately as possible. Christiaan Huygens is the inventor of the first usable astronomical timepiece: the pendulum clock. The time the pendulum of a pendulum clock takes to swing back and forth depends on the length of the pendulum. The longer the pendulum, the slower it moves to and fro.Unfortunately pendulum clocks are sensitive to temperature. When it is warmer the pendulum expands and hence the clock slows down. This is fatal for precise measurements. This clock has a mercury compensation pendulum. As a weight for the pendulum a column of mercury in glass has been used. When the temperature rises, the pendulum becomes longer. But the mercury expands above, and hence the swing time remains the same.