Astronomical clock
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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.
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[edit] Definition
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.
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.
[edit] History
In the 11th century, the Song Dynasty Chinese horologist and mechanical engineer 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 for his clock-tower and armillary sphere to function (for more info see water clock).
The early development of 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 for the first time 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.
The astronomical clocks developed by Richard of Wallingford in St Alban's during the 1330s, and by Giovanni de'Dondi in Padua during the 1350s, 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-sided construction showing the positions of the known planets as well. 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 the realities of friction and limitations of manufacture would have prevented them being accurate and reliable.
The challenge of building astronomical clocks meant that clockmakers would continue to produce them, to demonstrate their technical skill and their customers' wealth. Astronomical clocks were built as demonstration or exhibition pieces, to impress as much as to educate or inform. The philosophical message of an ordered, heavenly-ordained universe was part of the appeal.
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.
[edit] Examples
Here are some notable examples of astronomical clocks.
[edit] Strasbourg
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.
More on this clock can be found here.
[edit] Prague
- See that article for an annotated diagram of its functions.
One of the most famous of this type of clock is the Old-Town Hall clock in Prague, Czech Republic. It is also known as the Prague Orloj. The central portion was completed in 1410. The four figures are set in motion at the hour, with Death (represented by a skeleton) striking the time. On the hour there is a presentation of statues of the Apostles at the doorways above the clock, with all twelve presented at noon. In 1870 a calendar display was added below the clock.
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.
[edit] Olomouc
Olomouc, the former capital of Moravia in the eastern part of the Czech Republic, also has an impressive exterior astronomical clock on the main town square.
[edit] Su Sung's Cosmic Engine
The Science Museum (London) has a scale model of the 'Cosmic Engine', which Su Sung constructed in China in 1092. This great astronomical clock was about ten meters high (about 30 feet) and was indirectly powered by falling water and mercury.
[edit] Lund
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.
[edit] Copenhagen
The Copenhagen city hall has a complete astronomical clock, set in an interior glass cabinet. The clock was designed over a period of 50 years by amateur astronomer and professional clockmaker Jens Olsen. Some of the components (such as the computus) were inspired by the Strasbourg clock, which was studied by Olsen. It was assembled from 1948 to 1955. Between 1995 and 1997 the clock underwent a complete restoration.
[edit] The Rasmus Sørnes Clock
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.
[edit] Others
Many European countries have examples of astronomical clocks. You can see clocks of varying complexity at Wells Cathedral, Exeter, Ottery St Mary, Wimborne, Hampton Court, Sion, Winterthur, Cremona, Split, Mantua, Strasburg, Brescia, Stendal, Roskilde, Münster, and many others.
The Gros Horloge in Rouen is a famous astronomical clock (14th century), located in the Gros Horloge street.
The Zytglogge in Bern is a famous astronomical clock from the 15. century situated in the Capital of Switzerland.
The Cathedrale Saint Jean in Lyon also has an astronomical clock from the 14th century.
In Esslingen, Germany, at the headquarters of Festo, Professor Hans Scheurenbrand has constructed the Harmonices Mundi (named after Kepler's book), which consists of a technologically sophisticated astronomical clock, a world time clock, and a 74 bell glockenspiel.
[edit] Table clocks
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.
The Palace of Versailles near Paris has a sumptuous rococo table top astronomical clock which took 12 years for a clockmaker and an engineer to build. It was presented to Louis XV in 1754.
[edit] Watches
More recently, independent clockmaker Christiaan van der Klaauw created a wristwatch astrolabe, the "Astrolabium" in addition to the "Planetarium 2000", the "Eclipse 2001" and the "Real Moon." Ulysse Nardin also sells several astronomical wristwatches, the "Astrolabium," "Planetarium", and the "Tellurium J. Kepler."
[edit] Computer clock
On the internet can since 2005 be found a normal clock running to the sun called a tempometer.
[edit] Generic description
Although each astronomical clock is different, they share some common features.
[edit] Time of day
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).
[edit] Calendar and zodiac
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.
[edit] Moon
A dial or ring indicating the numbers 1 to 29 or 30 indicates the moon's age: a new moon is 0, waxes and become full around day 15, and then wanes up to 29 or 30. The phase is sometimes shown by a rotating globe or black hemisphere, or a window that reveals part of a wavy black shape beneath.
[edit] Hour lines
Unequal hours were the result of dividing up the period of daylight into 12 equal hours, and night time into another 12. In Europe, there's more daylight in the summer, and less night, so each of the 12 daylight hours is longer than a night hour. Similarly in winter, daylight hours are shorter, and night hours are longer. These unequal hours are shown by the curved lines radiating from the center. The longer daylight hours in summer can usually be seen at the outer edge of the dial, and the time in unequal hours is read by noting the intersection of the sun hand with the appropriate curved line.
[edit] Aspects
Astrologers placed importance on how the sun, moon, and planets were arranged and aligned in the sky. If certain planets appeared at the points of a triangle, hexagon, or square, or if they were opposite or next to each other, the appropriate aspect was used to determine the event's significance. On some clocks you can see the common aspects - triangle, square, and hexagon - drawn inside the central disk, with each line marked by the symbol for that aspect, and you may also see the signs for conjunction and opposition. On an astrolabe, the corners of the different aspects could be lined up on any of the planets. On a clock, though, the disk containing the aspect lines can't be rotated at will, so they usually show only the aspects of the sun or moon.
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.
[edit] Dragon hand: eclipse prediction and lunar nodes
The moon's orbit isn't in the same plane as the earth's orbit around the sun, but crosses it in two places. The moon crosses the ecliptic plane twice a month, once when it goes up above the plane, and again 15 or so days later when it goes back down below the ecliptic. These two locations are the ascending and descending lunar nodes. Solar and lunar eclipses will occur only when the moon is positioned near one of these nodes, because at other times the moon is either too high or too low for an eclipse to be noticed from earth. Some astronomical clocks keep track of the position of the lunar nodes with a long pointer that crosses the dial. This so-called dragon hand makes one complete rotation every 19 years. When the dragon hand and the new moon coincide, the moon is on the same plane as the earth and sun, and so there is every chance that an eclipse will be visible from somewhere on earth.
[edit] References
- John North. God's Clockmaker, Richard of Wallingford and the invention of time. Hambledon and London, 2005.
- Tor Sørnes. The Clockmaker Rasmus Sørnes. Borgarsyssel Museum, Sarpsborg Norway, 2003 Norwegian edition, and 2006 English edition.
- King, Henry Geared to the Stars: the evolution of planetariums, orreries, and astronomical clocks, University of Toronto Press, 1978
[edit] External links
[edit] See also
