Astronomical clock

An astronomical clock measures time while displaying information such as the positions of the Sun, Moon, or planets, or the dates of celestial events.

Clock escapement: A mechanism that regulates a clock by releasing stored energy in controlled intervals. Escapements pace the gear trains that drive both timekeeping and astronomical displays.
Prague Astronomical Clock: A medieval astronomical clock installed on Prague’s Old Town Hall in 1410. Its calendar, zodiac, and moving figures make it a celebrated multi-display example.
Astrolabe: An instrument that models the celestial sphere and solves problems involving the positions of celestial bodies. Astrolabes supplied astronomical schemes and visual conventions later adapted for clock dials.
Civic time: Time organized and announced by public institutions for a community. Public astronomical clocks joined local schedules to visible measures of celestial time.
Gear train: A set of meshing gears that transmits motion and changes speed or direction. Different gear ratios reproduce the unequal cycles of the Sun, Moon, and planets.
Strasbourg Astronomical Clock: A monumental astronomical clock in Strasbourg Cathedral, with its current mechanism completed in 1843. Its automata and planetary displays show a later, highly complex form.
Medieval universities: Institutions of higher learning that developed across Europe from the eleventh century onward. Their teaching of astronomy helped sustain demand for instruments representing celestial cycles.
Public clock: A clock installed in a shared space to make time publicly visible. Astronomical clocks expanded the functions of public clocks beyond telling the hour.
Lunar phase: The changing fraction of the Moon’s sunlit hemisphere visible from Earth. A Moon dial can show the phase through a geared rotating disc.
Rostock Astronomical Clock: A fifteenth-century astronomical clock in St. Mary’s Church in Rostock, Germany. Its calendar extends across centuries, linking clockwork to long-range chronology.

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