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The Astronomical Clock

Prague's Astronomical Clock (or Horolorgium, or Orloj)

This is an incredibly old and complex instrument. First, old: installed in 1410, this is the oldest astronomical clock still operating. Second, complex: I’ll admit, as I stood staring at in the morning, I had absolutely no idea what time it was. The constellation of interweaving dials and arms was completely incomprehensible for me at the time (morning, without coffee).

 The complexity is due to the fact that this horolorgium (or astronomical clock, or orloj) is measuring several different things at once and each of these measures are built into one instrument in which all information radiates from a single central axis. This single central axis point is important because it means there’s an elaborate coordinated system of dials and arms arrayed on top of each other, meaning the information from this system is also arrayed on top of itself, and so you basically have a massive mess which would really be more useful if it were just broken apart into its constituent parts (like this sentence). It’s like a designer’s worst nightmare and an engineer’s wet dream. 

 Anyway, the orloj (or astronomical clock, or horolorgium) measures a variety of different things, most of which – if I’m going to be honest - were not going to help the average 15th century Prague resident go about their day-to-day. These include: sunrise, sunset, the position of the sun with respect to solistice, equinox, the position of the moon with respect to the sky, the phase of the moon, the Zodiac month, and several different measures of time, which are weird and fascinating:

  • European central time (boring!)
  • Bohemian time – A measure of time made up of 24 equal hours but where the day begins at sunset. An interesting consequence of this is that your day is not quite 24 hours during the fall and winter because sunset occurs earlier each day, and a little in excess of 24 hours during the spring and summer because sunset occurs later each day. 
  • Star Time – Measures time according to the period required for a star to transit over a single point on earth and (i.e., return to the same location in the sky for an earth-bound observer). Weirdly, this is very slightly faster (by about four minutes) than the time it takes the sun to transit over a single point because the earth is both spinning on its axis and orbiting the sun at the same time, which affects the transit time for the sun. 
  • Babylonian time – Babylonian time divides the time between sunrise and sunset into 12 equal hours which were based on the position of the sun in the sky. This means that there is no fixed unit of time, seeing as those would vary considerably depending on your position on earth, and the time of year. Neither would it be possible to understand your time of day relative to someone else without a lot of math. One fortunate consequence of this is that large conference calls would be next to impossible. 

Sources for the above can be found at:

Astronomical Clock (top)


View from the top of the clock tower, looking at Church of Our Lady before Týn

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Location: Prague, Czechia

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