The Orloj was built in 1410, which means it was already old when Columbus sailed to America. That makes it one of the oldest working astronomical clocks in the whole world. It is still ticking today, doing the same job it was built for centuries ago.
The clock has two main round dials. The upper one shows the position of the Sun and Moon in the sky, which is what 'astronomical' means - it is tracking the movements of things in space. The lower dial is a beautiful calendar wheel, painted with pictures of the twelve months of the year.
Every hour on the hour, wooden figures appear from small doors above the clock. Twelve Apostles (important figures from Christian tradition) parade past the windows one by one. A golden cockerel flaps its wings and crows at the end. Visitors below in the square have been watching this show for hundreds of years.
The clock face is decorated in deep blue, gold and red, and is covered in symbols that medieval craftsmen used to explain time. Reading it is like cracking a secret code. Astronomers and clock-makers still visit Prague just to study how it works.