Prague Astronomical Clock (Prague Orloj)

The Prague Astronomical Clock or Prague Orloj (Pražský Orloj) is one of the city's major tourist attractions. It is mounted on the Old Town Hall (on its southern wall) in the Old Town Square. The Orloj is composed of three main components. The first is astronomical dial, representing the position of the Sun and Moon in the sky and displaying various astronomical details then "The Walk of the Apostles", a clockwork hourly show of figures of the wooden Apostles and other moving sculptures; and finally a calendar dial with medallions representing the months.

The astronomical dial is a form of mechanical astrolabe, a device used in medieval astronomy. It has a background that represents the standing Earth and sky, and surrounding it operates four main moving components: the zodiacal ring, an outer rotating ring, an icon representing the Sun, and an icon representing the Moon. The blue circle directly in the center of the background 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 at night it sits over the black. During dawn or dusk, the mechanical sun is positioned over the red part of the background. 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. Inside the large black outer circle there is another movable circle – The Zodiac ring. It indicates 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 is represented by golden Swbach numbers set on a black background at the outer edge of the clock. This numbers measure time starting with 1 at sunset. This ring moves during the year to coincide with the time of sunset.

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. Later, presumably around 1490, the calendar dial was added and clock facade decorated with gothic sculptures. The Orloj stopped working many times throughout 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.