The astronomical clock in the Old-Town Square of Prague
The Prague Astronomical Clock or Prague Orloj (Czech: Pražskř orloj, pron. [prash-skee or-loi]) is a medieval astronomical clock located in Prague, the capital of the Czech Republic. The Orloj is mounted on the southern wall of Old Town City Hall in the Old Town Square and is a popular tourist attraction.
The Orloj is composed of 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; and a calendar dial with medallions representing the months.
The oldest part of the Orloj, the mechanical clock and astronomical dial, dates back to 1410 when it was made by clockmaker Mikulas of Kadan and Jan Sindel, the latter a professor of mathematics and astronomy at Charles University. The Prague Orloj was the third clock of its kind, the first having been constructed in Padua in 1344.
Later, presumably around 1490, the calendar dial was added and clock facade decorated with gothic sculptures.
In 1552 it was repaired by Jan Taborsky, clockmaster of Orloj, who also wrote a report on the clock where he mentioned Hanus as maker of the clock. This was a mistake, and was corrected during the 20th century.
The Orloj stopped working many times in the centuries after 1552, and was repaired many times. In the 17th century moving statues were added, and figures of the Apostles were added after major repair in 1865-1866.
The Orloj suffered heavy damage in last days of WW II when Germans directed artillery fire at the Old-Town Hall. The hall completely burned along with wooden sculptures of the Orloj and the calendar dial face of great artistic value made by Josef Manes. The authentic machinery was repaired and the Orloj started working again in 1948, but only after significant effort.
There exists a good deal of misinformation relating to the construction of the Orloj. For a long time it was believed that the Orloj was constructed in 1490 by clockmaster Jan Ruze (also called Hanus) and his assistant Jakub Cech. Another fictitious story involves the clockmaker Hanus being blinded on the order of the Prague Councillors to prevent him from constructing another similar clock.
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 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.
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 numbers at the outer ege 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.
Inside the large black outer circle lies another movable circle marked with the signs of the zodiac and indicating the movement of the stars. The small golden star shows the position of the vernal equinox, and sidereal time can be read on the scale with golden Roman numbers.
Old Czech Time Scale
At the outer edge of the clock, golden Swbach numerals are set on a black background. These numbers indicate Old Czech Time (or Italian hours), measured starting with 1 at sunset. This ring moves 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:
- The position of the golden hand over the Roman numerals on the background indicates the time in local Prague time.
- The position of the sun over the curved golden lines indicates the time in unequal hours.
- The position of the golden hand over the outer ring indicates the time in Old Czech Time.
Additionally, the distance of the Sun from the center of the dial shows the time of sunrise and sunset.
The movement of the Moon on the ecliptic is shown similarly to that of the Sun, although the speed is much faster. The half-silvered sphere of the moon also shows the Lunar phase.