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Prague Astronomy Clock

Does Anybody Really Know What Time It Is?

On the south side of the Old Town Hall (the left side if you are looking at the front of the hall from Old Town Square ) is one of the most unique sights Prague has to offer, the Prague Astronomical Clock (Pražský Orloj). Astronomical clocks do more than tell you what time of day it is (which, for most of human history was impressive enough); they tell you what time of year it is, which phase the moon is in and the location of the sun and moon in the sky. Pretty impressive considering the clock in question was built over six hundred years ago. This clock has three different parts: the figures on the top, the astronomical clock and the calendar.

Astronomy Clock, Prague
Photo courtesy Kallerna

When the clock strikes the hour, the figures on the top part of the clock come to life: on one side, death beckons a Turkish entertainer, on the other side are figures representing vanity and greed, and above them all, the twelve Apostles show themselves in the windows.

The bottom part is a calendar. The outer ring has every day of the year listed. Today is the day indicated by the pointer at the top. This was added to the clock in 1870.

By far the most interesting part of the clock is the astronomical dial, located in the center. Built in 1410, it is oldest functioning astronomical clock in the world.

Astronomy Clock Face, Prague
Photo courtesy Alfvan Beem

The clock face shows Earth at the center with a representation of the sky surrounding it. The light blue is daylight and the black part is night. The clock faces south, so on the left side of the face (east) is Ortis (sunrise) and Avrora (daybreak); on the west is Occasvs (sunset) and Crervscvlvm (twilight). The first gold ring around earth represents the Tropic of Capricorn, the middle ring is the equator and the ring just above the roman numerals is the Tropic of Cancer. Surrounding all this are arabic numbers (the ones we use), roman numerals (the ones Super Bowls and motion pictures copywriters use) and numbers in Schwabacher or gothic typeface, which is the one font you think of when you think of Germany. Overlaying this is a ring showing the signs of the zodiac. There are three hands, one with a star, one with a moon and one with a sun AND a gold hand on the end. (I’m pretty sure Dumbledore has a similar click in his office.) Using all of this, the clock tells you what time of day it is (three different ways), what time of year it is and what phase the moon is in, as well as many other things that we take for granted today.

The pointer hand tells the time of day. The roman numerals indicate local time or Central European Time and the gothic numbers indicate the time elapsed since sunset (called Old Czech time).

The sun hand marks the sign of the zodiac in the ring, which moves around once per sidereal day (23 hours, 56 minutes, 4 seconds), so that the sun slowly moves through signs of the zodiac. The sun also indicates the length of the day. When sun is at the Tropic of Cancer (outermost) ring, it is at its highest in the sky and the day is the longest. This is the summer solstice, the first day of summer. When the sun crosses the equator on its way to the inner ring, it is at the autumnal equinox. When the sun is at the Tropic of Capricorn (innermost) ring, it is the shortest day of the year, the winter solstice. The position of the sun on the arabic numbers indicates Babylonian time, where the day (sunrise to sunset) is divided into twelve equal parts. These parts get smaller as the sun gets closer to the Tropic of Capricorn and the days get shorter.

The moon hand points to where the moon is in the sky. If it is in the black region at the bottom of the clock face, it has not risen yet. Inside the moon hand is a sphere with half painted black. This represents the phases of the moon: all black means new moon and all silver means full moon.

The star hand indicates sidereal time using roman numerals. Sidereal days are shorter than regular days by about four minutes.

Whew! All that from a clock that was built over six hundred years ago.

Here’s a video from the 600th anniversary celebration – it’s fascinating:

Know Before You Go
It is free and you can see it anytime of the day or night. It is neat to see the display when the hours are chimed, but it is not necessary. The area around the clock can be busy in the middle of the day, but that won’t obscure your view.

Bottom Line
This clock is worth seeing in person.



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