At the base of the Old Town Hall tower is the most popular feature of Old Town Square. The first Prague Astronomical Clock by Mikuláš z Kadaně became operational in 1410. Eighty years later, it was replaced with the current masterpiece created by clockmaker Jan Růže (alias Hanuš). The calendar on the bottom displays the day, month and year. In the middle is the astronomical dial. It tracks hours and minutes in Babylonian time, Old Czech time and civil time. The sun and moon arms show their position in the sky and the zodiac. To see the third feature, stand here on the hour between 9 am and 9 pm. That is when the 12 Apostles rotate from the upper windows. The accuracy of Prague Orloj has been maintained for centuries by an assigned clock master and astronomer. Some men have served this role for decades.