The Astronomical Dial face of the famous Astronomical Clock of Prague (Pražský orloj) in Praha, Česká Republika.
The medieval astronomical clock that is on the southern wall of the the Old Town City Hall in the Old Town Square and is composed of three main components: the astronomical dial, a clockwork hourly show of figures of the Apostles and other moving sculptures, and this calendar dial whose medallions represent the months.
The Prague Astronomical Clock is one of the oldest European clocks of its kind and is unique in being the oldest where the original clockwork has been in operation from the beginning to the present time.
The astronomical dial is the oldest part of the clock, dating to 1410, when it was made by clockmaker Mikuláš of Kadaň and Jan Šindel, the latter a professor of mathematics and astronomy at Charles University.
The astronomical dial is flanked by sculptures representing the four things that were most despised at the time: Vanity, Greed, Death and the Heathen. These sculptures are animated: Death pulls the bell cord, the Heathen shakes his head, Vanity admires himself in his mirror and Greed watches his bag of gold.
The Astronomical dial of the clock is a form of mechanical astrolabe and has a background that represents the Earth and sky, surrounded by the 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 above the horizon. The red and black areas indicate portions of the sky below the horizon.
During the daytime, the sun is over the blue and at night it sits over the black. At dawn and dusk, the sun is over the red 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).
The Roman numbers represent the timescale of a normal 24 hour day in local Prague time and the golden hand indicates the time.
The movable circle marked with the signs of the zodiac is the Zodiac ring which indicates the location of the sun on the ecliptic. The sun is currently seen in Libra and moves through the signs in anticlockwise order so it will enter Scorpio next.
The small golden star shows the position of the vernal equinox, and sidereal time can be read on the scale with golden Roman numerals.
At the outer edge of the clock, golden Schwabacher numerals indicate Old Czech Time (also known as Italian hours), with 24 indicating the time of sunset. This ring moves back and forth during the year to coincide with the time of sunset. The position of the golden hand over the outer ring indicates the hours passed after sunset in Old Czech Time.
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.
For a helpful animation of the clock in action (sped up so that it is easier to understand its motion) visit: www.praguealacarte.com/orloj/orlojLoader.html
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