Orloj.
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Parts of this clock date back to 1410. It tracks movements of sun, moon, seasons, and more; it takes the tour guides quite a while to explain it all. At least I think it does- I'm not sure 'cause I don't go with tour groups. I just know that they're still talking by the time I feel compelled to move away.
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At the top of every hour the orloj becomes the focus of the tourist hordes, as the figures on the sides move, the doors near the top open to reveal a parade of saints, and finally the golden rooster crows.