Every hour from 9 am till 9 pm the figures of twelve apostles appear at the doorways above the clock. At the same time the four figures flanking the clock are set in motion, these represent four things that were despised at the time of the clock's making. The first is Vanity, represented by a figure admiring himself in a mirror. Next, a stereotypical Jew holding a bag of gold represents greed or usury. Across the clock stands Death, a skeleton that strikes the time upon the hour. Finally, the infidel Turk wears the Turban.
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.
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 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. 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.
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 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.
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