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Prague Astronomical Clock
Astronomical Clock Building
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The Prague astronomical clock, or Prague orloj (Czech: Pražský orloj [praʃskiː orloj]), is a medieval astronomical clock located in Prague, the capital of the Czech Republic. The clock was first installed in 1410, making it the third-oldest astronomical clock in the world and the oldest one still operating.

One of the most famous astronomical clocks 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 1490 a calendar display was added below the clock along with decorative Gothic sculptures.

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 until 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.

Astronomical ClockThe Orloj is mounted on the southern wall of Old Town Hall in the Old Town Square. The clock mechanism itself has three main components: the astronomical dial, representing the position of the Sun and Moon in the sky and displaying various astronomical details; "The Walk of the Apostles", a clockwork hourly show of figures of the Apostles and other moving sculptures—notably a figure of Death (represented by a skeleton) striking the time; and a calendar dial with medallions representing the months. According to local legend, the city will suffer if the clock is neglected and its good operation is placed in jeopardy; a ghost, mounted on the clock, was supposed to nod its head in confirmation. According to the legend, the only hope was represented by a boy born on New Year's night.

Prague Astronomical Clock Hands

Astronomical Dial

The astronomical dial is a form of mechanical astrolabe, a device used in medieval astronomy. Alternatively, one may consider the Orloj to be a primitive planetarium, displaying the current state of the universe.

The astronomical dial has a background that represents the standing Earth and sky, and surrounding it operate four main moving components: the zodiacal ring, an outer rotating ring, an icon representing the Sun, and an icon representing the Moon.


Stationary Background

The background represents the Earth and the local view of the sky. The blue circle directly in the center represents the Earth, and the upper blue is the portion of the sky which is above the horizon. The red and black areas indicate portions of the sky below the horizon. During the daytime, the Sun sits over the blue part of the background and at night it sits over the black. During dawn or dusk, the mechanical sun is positioned over the red part of the background.

Written on the eastern (left) part of the horizon is aurora (dawn in Latin) and ortus (rising). On the western (right) part is occasus (sunset), and crepusculum (twilight).

Golden Roman numerals at the outer edge of blue circle are the timescale of a normal 24-hour day and indicate time in local Prague time, or Central European Time. Curved golden lines dividing the blue part of dial into 12 parts are marks for unequal "hours". These hours are defined as 1/12 of the time between sunrise and sunset, and vary as the days grow longer or shorter during the year.


Astronomical Clock Zodiac

Zodiacal Ring

Inside the large black outer circle lies another movable circle marked with the signs of the zodiac which indicates the location of the Sun on the ecliptic. The signs are shown in anticlockwise order. In the photograph accompanying this section, the Sun is currently moving anticlockwise from Cancer into Leo.

The displacement of the zodiac circle results from the use of a stereographic projection of the ecliptic plane using the North pole as the basis of the projection. This is commonly seen in astronomical clocks of the period.

The small golden star shows the position of the vernal equinox, and sidereal time can be read on the scale with golden Roman numerals. The zodiac is on the 366-tooth gear inside the machine. This gear is connected to the sun gear and the moon gear by a 24-tooth gear.


Old Czech Time Scale

At the outer edge of the clock, golden Schwabacher numerals are set on a black background. These numbers indicate Old Czech Time (or Italian hours), with 24 indicating the time of sunset, which varies during the year from as early as 16:00 in winter to 20:16 in summer. This ring moves back and forth during the year to coincide with the time of sunset.



The golden Sun moves around the zodiacal circle, thus showing its position on the ecliptic. The sun is attached to an arm with a golden hand, and together they show the time in three different ways:

  1. The position of the golden hand over the Roman numerals on the background indicates the time in local Prague time.
  2. The position of the Sun over the curved golden lines indicates the time in unequal hours.
  3. The position of the golden hand over the outer ring indicates the hours passed after sunset in Old Czech Time.

Additionally, the distance of the Sun from the center of the dial shows the time of sunrise and sunset. The Sun and its hand are on the 365-tooth gear inside the machine.



The movement of the Moon on the ecliptic is shown similarly to that of the Sun, although the speed is much faster (due to the Moon's own orbit around the Earth). The Moon's arm is on the 379-tooth gear inside the clock machine.

The half-silvered, half-black sphere of the moon also shows the Lunar phase. The Moon has a 57-tooth gear inside its sphere, and is slowly rotated by a screw-thread attached to a weight (advancing 2 teeth per day). This movement, powered only by gravity, makes the Orloj unique in the world among astronomical clocks showing the phases of the moon. The mechanism was created by an unknown maker, probably in the mid-17th century. Unlike the original device (the construction of which was described in a report from 1570), this mechanism produces much smaller deviation from the actual lunar phase (about 1 day in 5 years).


Animated Figures

The four figures flanking the clock are set in motion on the hour, and represent four things that were despised at the time of the clock's making. From left to right in the photographs, the first is Vanity, represented by a figure admiring himself in a mirror. Next, the miser holding a bag of gold represents greed or usury. Across the clock stands Death, a skeleton that strikes the time upon the hour. Finally there is a figure representing lust and earthly pleasures. On the hour, the skeleton rings the bell and immediately all other figures shake their heads, side to side, signifying their unreadiness "to go."

There is also a presentation of statues of the Apostles at the doorways above the clock, with all twelve presented every hour.



The calendar plate below the clock was replaced by a copy in 1880. The original is stored in the Prague City Museum.


History of the Astronomical Clock

The oldest part of the Orloj, the mechanical clock and astronomical dial, dates back to 1410 when it was made by clockmaker Mikuláš of Kadaň and Jan Šindel, then later a professor of mathematics and astronomy at Charles University. The first recorded mention of the clock was on 9 October 1410. Later, presumably around 1490, the calendar dial was added and the clock facade was decorated with gothic sculptures.

Formerly, it was believed that the Orloj was constructed in 1490 by clockmaster Jan Růže (also called Hanuš); this is now known to be a historical mistake. A legend, recounted by Alois Jirásek, has it that the clockmaker Hanuš was blinded on the order of the Prague Councillors so that he could not repeat his work; in turn, he disabled the clock, and no one was able to repair it for the next 100 years.

In 1552 it was repaired by Jan Taborský (ca1500–1572), master clockmaker of Klokotská Hora, who also wrote a report of the clock where he mentioned Hanuš as the maker of this clock. This mistake, corrected by Zdeněk Horský, was due to an incorrect interpretation of records from the period. The mistaken assumption that Hanuš was the maker is probably connected with his reconstruction of the Old Town Hall in the years 1470–1473. The clock stopped working many times in the centuries after 1552, and was repaired many times.

In 1629 or 1659 wooden statues were added, and figures of the Apostles were added after a major repair in 1787–1791. During the next major repair in the years 1865–1866 the golden figure of a crowing rooster was added.

The Orloj suffered heavy damage on May 7 and especially May 8, 1945, during the Prague Uprising, when the Germans fired on the south-west side of the Old Town Square from several armoured vehicles in an unsuccessful attempt to destroy one of the centers of the uprising. The hall and nearby buildings burned along with the wooden sculptures on the clock and the calendar dial face made by Josef Mánes. After significant effort, the machinery was repaired, the wooden Apostles restored by Vojtěch Sucharda, and the Orloj started working again in 1948.

The Orloj was last renovated in autumn 2005, when the statues and the lower calendar ring were restored. The wooden statues were covered with a net to keep pigeons away.


600th Anniversary

On October 9, 2010, the Orloj's 600th anniversary was celebrated with a light show on the face of the clock tower. Two projectors were used to project several animated videos on the clock. The videos showed it being built, torn down, rebuilt, and peeled away to show its internal mechanisms and the famous animated figures, as well as various events in the clock's history. The video interacted with the tower's architecture, such as rain rolling off the arch, and showing the passage of time with moving shadows.


Old Town Hall (Astronomical Clock)

The Old Town Hall in Prague, the capital of the Czech Republic, is one of the city's most noteworthy monuments. It is located in Old Town Square.

In 1338 the councilors of the Old Town bought a magnificent patrician house by the family of Volflin and adapted it for their purposes. During centuries the original building of the Town Hall practically disappeared under the addition reconstructions of later years and one of the external remnat of the original structure today is the Gothic stone portal with mouldings in the western part of the building.

The burghers of the Old Town extended their original Town Hall towards the west by buying the adjoining house and they started the construction of a stone tower on a square plan. The tower, which was the highest in the city in the Middle Ages, was completed in 1364 and the following centuries hardly left any traces on the structure.

The Town Hall is one very unusual historical object, because it is made out of many different smaller houses. The expansion continued in 1458 when so called "Mikes' house" was added to the west side. The Council Chamber in the east wing was vaulted with the net vault, which was supported with 2 pillars, at the end of the 15th century.

The Gothic "Cock" house was bought in 1835 and the "Minute" house was sold to the town council for the extension of the Town Hall in 1896. Mikes house was rebuilt in Neo-Renaissance style in 1879–1880. The author of the project was Antonín Baum. This wing was destroyed in last days of the World War II during Prague uprising. Many architectural competitions were declared during the 20th century. They were supposed to find the right architectonic design for expanding and rebuilding the Old Town Hall. All of the competitions either did not have a winner or the winning projects were not built.

The architectural development of the Old Town Hall in the Middle Ages was far from completed after finishing of the tower. There was an interruption due to Hussite movement (1419–1434). In 1458 another house was bought on the west side. It made possible extensive adaptations of the interior of the object. New halls were established in the south wing, but only the council room on the upper floor has preserved in its original appearance.

Internal adaptations reflected in the external reconstruction work can be seen on the south facade to this day. The reconstruction of the entrance hall on the groundfloor of Volflin house terminated in the construction of a beatutiful new portal in the Late Gothic style, which for more than 100 years marked urban architecture in the Czech lands. The Gothic arch of the portal has archivolts rich in stone ornaments. Decorated brackets support the outer arch which is a typical late-Gothic ogee arch crowned by an imposing finial. The brackets on either side of the portal terminate in slender pinnacles. The structure dates from the close of the 15th century, but the wooden double door itself dates as late as from the year 1652.

The window on the left of the portal was completed a few years later and kept the architectural style. The builder gave up the traditional Gothic arch in favor of a rectangular window, adoring the thickness of the walls with panelled pilasters. A moulded stone cross divides the window into four lights, the upper two of which are decorated with the amorial bearing of the Old Town of Prague and the Czech lion. Between and slightly above them may be seen the symbol "W" representing the royal initial of the Bohemian king Vladislaus II of Hungary (1456–1516) from Jagiellon dynasty. The rich vegetable decoration made from stone adorn the top of the window.

The window in the south facade is only of a slightly more recent date—the twenties of the 16th century—and already bears traces of the Renaissance style. The central window itself is the only original part, the two smaller wings were added in 1731. It has a high moulded cornice with plastic ornamentation. Brackets support panelled pilasters terminating in capitals on which rests the architrave with the inscription "Praga caput regni" (Prague, the capital of the kingdom). The window is surmounted by a semicircular tympanum with the armorial bearings of the Old Town of Prague. Generally speaking the lateral windows are kept in the same style as the original Renaissance main window, but the canopies, in the Gothic style, above the pilaster are a disturbing element. Renaissance style may also be seen in another window placed closely above the Gothic portal of Volflin house from the half of the 16th century.

The far-reaching reconstruction of the Old Town Hall at the turn of the 15th and 16th century included the erection of the east wing adjoining to the north wall of the tower. A monumental building was constructed during the Late Gothic period. There was a council chamber with a magnificent net vault that gave the room a very impressing atmosphere of spaciousness.


Astronomical Clocks Technology

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 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 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 accords both with daily experience and with the philosophical world view of pre-Copernican Europe.

Research in 2011 and 2012 led an expert group of researchers 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 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 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. Albans 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.

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.


Astronomical Clock Orloj
Astronomical Clock Tower


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