Astronomical clock

An astronomical clock, horologium, or orloj is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the Sun, Moon, zodiacal constellations, and major planets.

The term is loosely used to refer to any clock that shows, in addition to the time of day, astronomical information. This could include the location of the Sun and Moon in the sky, the age and Lunar phases, the position of the Sun on the ecliptic and the current zodiac sign, the sidereal time, and other astronomical data such as the Moon's nodes for indicating eclipses), or a rotating star map. The term should not be confused with an astronomical regulator, a high precision but otherwise ordinary pendulum clock used in observatories.

Astronomical clocks usually represent the Solar System using the geocentric model. The center of the dial is often marked with a disc or sphere representing the Earth, located at the center of the Solar System. The Sun is often represented by a golden sphere (as it initially appeared in the Antikythera mechanism, back in the 2nd century BC), shown rotating around the Earth once a day around a 24-hour analog dial. This view accorded both with the daily experience and with the philosophical world view of pre-Copernican Europe.

The Antikythera mechanism is the oldest known analog computer and a precursor to astronomical clocks. A complex arrangement of multiple gears and gear trains could perform functions such as determining the position of the sun, moon and planets, predict eclipses and other astronomical phenomena and tracking the dates of Olympic Games. Research in 2011 and 2012 led an expert group of researchers^[who?] to posit that European astronomical clocks are descended from the technology of the Antikythera mechanism.

In the 11th century, the Song dynasty Chinese horologist, mechanical engineer, and astronomer Su Song created a water-driven astronomical clock for his clock-tower of Kaifeng City. Su Song is noted for having incorporated an escapement mechanism and the earliest known endless power-transmitting chain drive for his clock-tower and armillary sphere to function. Contemporary Muslim astronomers and engineers also constructed a variety of highly accurate astronomical clocks for use in their observatories, such as the astrolabic clock by Ibn al-Shatir in the early 14th century.

The early development of mechanical clocks in Europe is not fully understood, but there is general agreement that by 1300–1330 there existed mechanical clocks (powered by weights rather than by water and using an escapement) which were intended for two main purposes: for signalling and notification (e.g. the timing of services and public events), and for modelling the solar system. The latter is an inevitable development because the astrolabe was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system. American historian Lynn White Jr. of Princeton University wrote:

Most of the first clocks were not so many chronometers as exhibitions of the pattern of the cosmos … Clearly, the origins of the mechanical clock lie in a complex realm of monumental planetaria, equatoria, and astrolabes.

The astronomical clocks developed by the English mathematician and cleric Richard of Wallingford in St Albans during the 1330s, and by medieval Italian physician and astronomer Giovanni Dondi dell'Orologio in Padua between 1348 and 1364 are masterpieces of their type. They no longer exist, but detailed descriptions of their design and construction survive, and modern reproductions have been made. Wallingford's clock may have shown the sun, moon (age, phase, and node), stars and planets, and had, in addition, a wheel of fortune and an indicator of the state of the tide at London Bridge. De Dondi's clock was a seven-faced construction with 107 moving parts, showing the positions of the sun, moon, and five planets, as well as religious feast days.