The Prague Astrolabe Clock

[Prague City Hall astrolabe clock]

The Prague astrolabe clock was originally constructed in about 1410 but has undergone numerous restorations and modifications. The astrolabe dial on the clock is somewhat unusual in that it uses a southern projection for the horizon. The horizon arc curves in the opposite direction than on a normal planispheric astrolabe which allows the Sun to be shown above or below the horizon in a more intuitive way. The clock is geared so the rete, which is divided by the zodiac in five degree sections, rotates in a sidereal day. A hand with a Sun image rotates in a tropical day. The time, position of the Sun in the zodiac, sidereral time and the Sun's altitude above the horizon can be seen at a glance. Another hand shows the phase and position of the moon. A close examination of the picture shows us that the photograph was taken near the beginning of Virgo at about 6:55 AM. Noting the position of the moon and with the help of The Electric Astrolabe, we can surmise that the photo was taken on August 23, 1981.

The original clock probably had only an astrolabe dial with a concentric ring that was adjusted to Bohemian hours each day at sunset (i.e. 24 equal hours with the day beginning at sunset). In about 1490, modifications were made to add a manually adjusted calendar. The calendar was given its own drive mechanism in 1566, and the limb was divided into the normal two 12 hour sections. A new striking mechanism and the moon hand was added in the mid-17th century. Moving figures of the Apostles were added sometime after 1659.

A major renovation was undertaken in 1865, when a new striking mechanism was installed, the calendar was repainted and a new escapement was added, replacing the old foliot. The clock was seriously damaged during street fighting on May 8, 1945, but has been carefully restored. The diameter of the dial is 3.1 m and the ecliptic diameter is 2.8 m.

The clock's gearing is described briefly in King, Henry C., Geared to the Stars, University of Toronto Press, 1978, pg. 55. has an animated description of the clock dials.