page_title: Prague astronomical clock
section_title: Moon
hierarchical_section_title: Prague astronomical clock / Astronomical dial / Moon
caption_attribution_description: Čeština: Schema vnitřního stroje měsíce Pražského orloje
context_page_description: The Prague Astronomical Clock, or Prague Orloj, is a medieval astronomical clock located in Prague, the capital of the Czech Republic.
  The clock was first installed in 1410, making it the third-oldest astronomical clock in the world and the oldest clock still operating.
context_section_description: The movement of the Moon on the ecliptic is shown similarly to that of the Sun, although the speed is much faster, due to the Moon's orbit around the Earth. The Moon's arm is on the 379-tooth gear inside the clock machine.
  
  The half-silvered, half-black sphere of the moon also shows the Lunar phase. The Moon has a 57-tooth gear inside its sphere, and is slowly rotated by a screw-thread attached to a weight, advancing two teeth per day. This movement, powered only by gravity, makes the Orloj unique in the world among astronomical clocks showing the phases of the Moon. The mechanism was created by an unknown maker, probably in the mid-17th century. Unlike the original device, the construction of which was described in a report from 1570, this mechanism produces much smaller deviation from the actual lunar phase of about one day in five years.
caption: The unique mechanism inside the Moon
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