Image linked from Wikipedia
This clock, if you know how to read it, tells you the time, the date, the position of the sun in the sky, the position and phase of the moon, the current time of sunrise and sunset. Quite impressive for a clock installed in 1410!
Now you can enjoy the confusing complexity right here on your desktop:
This clock shows similar information, it tells you the time, the date, the position of the sun in the sky, the current time of sunrise and sunset and the precise time difference between when the sun is highest in the sky and 12 noon. It is a challenge to read, but there are some hints on the website (click on [show/hide] in the help box). This pdf may help too...
Unlike the Prague astronomical clock which is only accurate in Prague my one at richardwheeler.net works perfectly no matter where you are in the world by using HTML5 geolocation to find your location and then recalculating the clock display.
Better than that it can also show you what an astronomical clock would look like if you were at the same latitude and longitude, but on a different planet! This is a bit buggy (it doesn't accurately calculate the time of the year) but gives a little insight into how astronomical clocks would appear on other planets. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto.
In these extraterrestrial versions of the clock the hour hand still moves once around a 24 hr dial or twice around a 12 hr dial per day on that planet. The second hand still moves once per second, but the different length of the day means may move more or less than 1 step on the dial. This is particularly strange for Mercury where the year is 88 Earth days, but it takes 176 Earth days from sunrise to sunrise (a Mercury day); the day is longer than the year.
The dial with the zodiac symbols also has big changes between planets. This is because its eccentricity (the distance it is offset from the centre of the clock) depends on the axial tilt of that planet. Uranus is particularly strange because its axial tilt is near 90 degrees; it spins on its side.