An astronomical clock is a clock with special mechanisms and dials to display astronomical information, such as the relative positions of the sun, moon, zodiacal constellations, and sometimes 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 phase of the moon, 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 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, shown rotating around the earth once a day around a 24 hour analog dial. This view accords both with daily experience and with the philosophical world view of pre-Copernican Europe.
Although not a clock in the traditional sense, the 2nd century BC Antikythera mechanism of ancient Greece was used to calculate the positions of the sun, moon, and stars at any given point by use of complex mechanical gears. As Cicero later wrote in the 1st century BC, Posidonius' orrery achieved virtually the same thing..
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 earliest known endless power-transmitting chain drive for his clock-tower and armillary sphere to function (for more info see water clock). Contemporary Muslim astronomers and engineers also constructed a variety of highly accurate astronomical clocks for use in their observatories, such as the timekeeping astrolabe by Abd al-Rahman al-Sufi (Azophi) in the 10th century, the castle clock (a water-powered astronomical clock) by Al-Jazari in 1206, and 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. Medieval researcher Lynn White Jr. wrote:
Most of the first clocks were not so much 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 geared astrolabes.The astronomical clocks developed by Richard of Wallingford in St Alban's during the 1330s, and by Giovanni de Dondi 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.
Both these clocks, and others like them, were probably less accurate than their designers would have wished. The gear ratios may have been exquisitely calculated, but their manufacture was somewhat beyond the mechanical abilities of the time, and they never worked reliably. Furthermore, in contrast to the intricate advanced wheelwork, the timekeeping mechanism in nearly all these clocks until the 16th century was the simple verge and foliot escapement, which had errors of at least half an hour a day. The first astronomical clock to be powered by springs, allowing much greater accuracy, was invented by Taqi al-Din in 1559.
Astronomical clocks were built as demonstration or exhibition pieces, to impress as much as to educate or inform. The challenge of building these masterpieces meant that clockmakers would continue to produce them, to demonstrate their technical skill and their patrons' wealth. The philosophical message of an ordered, heavenly-ordained universe, which accorded with the Gothic era view of the world, helps explain their popularity.
The growing interest in astronomy during the 18th century revived interest in astronomical clocks, less for the philosophical message, more for the accurate astronomical information that pendulum-regulated clocks could display.
Here are some notable examples of astronomical clocks.
It was possible to re-program the length of day and night everyday in order to account for the changing lengths of day and night throughout the year, and it also featured five robotic musicians who automatically play music when moved by levers operated by a hidden camshaft attached to a water wheel. Other components of the castle clock included a main reservoir with a float, a float chamber and flow regulator, plate and valve trough, two pulleys, crescent disc displaying the zodiac, and two falcon automata dropping balls into vases.
The Strasbourg Cathedral has housed three different astronomical clocks since the 14th century. The first clock was built between 1352 and 1354 and stopped working sometime at the beginning of the 16th century. A second clock was then built by Herlin, Conrad Dasypodius, the Habrecht brothers, and others, between 1547 and 1574. This clock stopped working in 1788 or 1789 (as it apparently stopped working gradually, each component being disconnected one after the other). After a lapse of 50 years, a new clock was built by Jean-Baptiste Schwilgué (1776-1856) and about 30 workers. This clock is housed in the case of the 2nd clock. It shows many astronomical and calendrical functions (including what is thought to be the first complete mechanization of the part of the computus needed to compute Easter) as well as several automata.
During World War II the clock was nearly destroyed by Nazi fire. The townspeople are credited with heroic efforts in saving most of the parts. It was gradually renovated till 1948. In 1979 the clock was once more cleaned and renovated. According to local legend the city will suffer if the clock is neglected and its good operation is placed in jeopardy.
Dating originally from 1420, the clock was remodelled approximately once every century. When the retreating Nazi German army passed through Olomouc in the final days of the war in May 1945 they opened fire on the old astronomical clock, leaving only a few pieces (that can now be seen in the local museum). As a result of the serious damage the clock was reconstructed in the style of socialist-realism in the first years of communist rule in Czechoslovakia (late 1940s-early 1950s). The religious and royal figures were replaced with athletes, workers, farmers, scientists and other members of the proletariat, while the glockenspiel was altered to play three pieces of traditional local music.
The lower dial represents the earthly sphere and indicates minute, hour, day, month, year and phase of the moon. The upper dial represents the heavenly sphere and shows a star map, the sun, earth and planets against a background of the twelve houses of the zodiac. The third and highest level is where the saints and apostles once paraded during the daily musical display (at 12 noon). Their role is now performed by faded-looking volleyball players, auto mechanics and factory workers.
The intricate background mosaic covers the clock's entire height of 14 metres and has representations of the twelve seasons and two traditional festival; the ride of the kings and the procession of maidens.
The Olomouc astronomical clock was featured in the opening scenes of the film "The Joke" based on the book by Milan Kundera.
He further improved the observational clock, using only one dial to represent the hours, minutes and seconds, describing it as "a mechanical clock with a dial showing the hours, minutes and seconds and we divided every minute into five seconds."
The astronomical clock in Lund Cathedral in Sweden, Horologium mirabile Lundense was made at the end of the 14th century. After it had been in storage since 1837, it was restored and put back in place in 1923. When it plays, one can hear In dulci jubilo from the smallest organ in the church, while six wooden figures, representing the three magi and their servants, pass by Mary and Jesus.
Arguably the most complicated of its kind ever constructed, the last of a total of four astronomical clocks designed and made by Norwegian Rasmus Sørnes (1893-1967), is characterized by its superior complexity compactly housed in a casing with the modest measurements of 0.70 x 0.60 x 2.10 m. Features include locations of the sun and moon in the zodiac, Julian calendar, Gregorian calendar, sidereal time, GMT, local time with day-light saving time and leap year, solar and lunar cycle corrections, eclipses, local sunset and sunrise, moonphase, tides, sunspot cycles and a planetarium including Pluto's 248 year orbit and the 25 800 year period of the polar ecliptics (precession of the earth's axis). All wheels are in brass and gold plated. Dials are silver plated.
Sørnes also made the necessary tools and based his work on his own observations of the firmament. This remarkable timepiece will probably be the last one ever to be designed and made by hand by one single person as true craftsmanship and a work of art. The result, outstanding for its performance and accuracy, remains a symbol of the transition from the mechanical age, Sørnes' electromechanical pendulum system pointing forward into the age of digital clocks. Having been exhibited at the Time Museum in Rockford, Illinois, and at The Chicago Museum of Science and Industry, the clock was sold in 2002 and its current location is not known. The Rasmus Sørnes Astronomical Clock no.3, the precursor to the Chicago Clock, his tools, patents, drawings, telescope and other items, are exhibited at the Borgarsyssel Museum in Sarpsborg, Norway.
There are many examples of astronomical table clocks, due to their popularity as showpieces. To become a master clockmaker in 17th century Augsburg candidates had to design and build a 'masterpiece' clock, an astronomical table top clock of formidable complexity. Examples can be found in museums, such as London's British Museum.
Currently Edmund Scientific among other retailers offer a mechanical Tellurium clock, perhaps the first mechanical astronomical clock to be mass marketed.
and many others.
Although each astronomical clock is different, they share some common features.
Most astronomical clocks have a 24 hour analog dial around the outside edge, numbered from I to XII then from I to XII again. The current time is indicated by a golden ball or a picture of the sun at the end of a pointer. Local noon is usually at the top of the dial, and midnight at the bottom. Minute hands are rarely used.
The sun indicator or hand gives an approximate indication of both the sun's azimuth and altitude. For azimuth (bearing from North), the top of the dial indicates South, and the two VI points of the dial East and West. For altitude, the top is the zenith and the two VI and VI points define the horizon. (This is for the astronomical clocks designed for use in the northern hemisphere.) This interpretation is most accurate at the equinoxes, of course.
If XII is not at the top of the dial, or if the numbers are Arabic rather than Roman, then the time may be shown in Italian hours (also called Bohemian, or Old Czech, hours). In this system, 0 o'clock occurs at sunset, and counting continues through the night and into the next afternoon, reaching 24 an hour before sunset.
In the photograph of the Prague clock shown above, the time indicated by the sun hand is about noon (XII in Roman numerals), or about the 17th hour (Italian time in Arabic numerals).
The year is usually represented by the 12 signs of the zodiac, arranged either as a concentric circle inside the 24 hour dial, or drawn onto a displaced smaller circle, which is a projection of the ecliptic, the path of the sun and planets through the sky, and the plane of the earth's orbit.
The ecliptic plane is projected onto the face of the clock, and, because of the earth's tilted angle of rotation relative to its orbital plane, it is displaced from the center and appears to be distorted. The projection point for the stereographic projection is the North pole; on astrolabes the South pole is more common.
The ecliptic dial makes one complete revolution in 23 hours 56 minutes (a sidereal day), and will therefore gradually get out of phase with the hour hand, drifting slowly further apart during the year.
To find the date, find the place where the hour hand or sun disk intersects the ecliptic dial: this indicates the current star sign, the sun's current location on the ecliptic. The intersection point slowly moves round the ecliptic dial during the year, as the sun moves out of one constellation into another.
In the photograph of the Prague clock shown above, the sun's disk has recently moved into Aries (the stylized ram's horns), having left Pisces. The date is therefore late March or early April.
If the zodiac signs run around inside the hour hands, either this ring rotates to align itself with the hour hand, or there's another hand, revolving once per year, which points to the sun's current zodiac sign.
In the photograph of the Brescia clock above, the triangle, square, and star in the center of the dial show these aspects (the third, fourth, and sixth phases) of (presumably) the moon.