Prague's Astronomical Clock is one of the oldest and most elaborate clocks ever built. It has two main dials (clock faces), one directly over the other, and numerous wooden figures which are animated when the clock chimes the hour.
The main dial of the clock looks nothing like a modern clock. It is packed with information, giving not only the time but the positions of the Sun and Moon as well. The clock was originally intended purely to provide astronomical information. The rest of the clock was added much later. The dial features the following:
- Around the outside of the dial are two circles of numbers. The inner circle is in Roman numerals; the hours as we know them are marked around the circle. All 24 hours of the day are marked, with 12 noon at the top and 12 midnight at the bottom. An hour hand points to the current time. There is no minute hand - people didn't worry about such small units of time in 1410.
- The outer circle is inscribed with Arabic numerals (1, 2, 3, etc up as far as 24). These show 'Old Bohemian Time'. This is the number of hours since sunset. Since the time of sunset varies during the year, this circle of numbers rotates relative to the fixed Roman numerals, a fact which is not obvious when one is looking at the clock.
- Mounted on the hour hand is a smaller dial that displays the 12 signs of the zodiac. A golden Sun and a silver Moon show the positions of the Sun and Moon against the stars. The Moon is painted half black. It rotates so that it always shows the correct phase of the Moon.
- The main dial is painted in blue, red and black, representing day, twilight and night. The position of the Sun against these painted areas shows the current state. Also on the main dial are lines dividing the day into the hours of 'Babylonian Time'. These lines divide the daylight into 12 hours. Because of this, the hours themselves vary in length with the seasons.
The clock is an amazing feat of engineering even by todays standards. A grand celebration is nothing if not appropriate for the 600th anniversary of such a technological marvel.