The Prague Astronomical Clock, or Prague Orloj was first installed in 1410, making it the third-oldest astronomical clock in the world and the oldest one still working.
There are many legends surrounding this Clock, the most famous of which is about the master clockmaker Hanus himself It is said that the Old Town Councilors had his eyes burnt out with a hot poker, so that he would not be able to build another such instrument elsewhere, which could overshadow the beauty and the fame of the Prague Clock. Master Hanus then allegedly asked his apprentice to take him to the clock, which he deliberately damaged so seriously, that nobody could repair it. Those who tried either died in doing so, or have gone mad. In reality, the Clock was not very reliable and often did not work, in spite of extensive repairs. A further legend gives the Skeleton magical power of foretelling the future and says that if the clock is left damaged for a long time, hard times will result for the Czech nation.
The clock mechanism itself 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—notably a figure of Death (represented by a skeleton) striking the time; and a calendar dial with medallions representing the months.
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 zikimsonik 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 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.
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 Aquarius into Pisces.
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 with numerals. Zodiac is on the 366 teeth gear inside the machine. This gear is connect to the sun gear and the moon gear by 24 teeth 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:
- 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 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. Sun and hand of the sun are at the 365 teeth 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 half-silvered sphere of the moon also shows the Lunar phase. Moon is on the 379 teeth gear inside the machine.