The clock itself
The Prague Astronomical Clock (Orloj), mounted on the south wall of the Old Town Hall, is one of the oldest astronomical clocks in the world still in operation, dating to 1410 with later additions. Beyond simply telling the time, its main dial tracks the position of the sun and moon against the zodiac, and a lower calendar dial (added in the 19th century) marks the days of the year.
The hourly show
Every hour on the hour, from around 9am to 11pm, a small mechanical show plays out above the dial — wooden figures representing the Twelve Apostles appear in the two little windows while, below them, animated figures on either side of the clock (a skeleton representing Death among them) go through their motions. It's brief and, if you've heard it hyped up, can feel modest in person — the setting is what makes it worth seeing, not spectacle.
The square around it
Old Town Square itself is worth slowing down for beyond the clock. The twin Gothic spires on the east side belong to the Church of Our Lady before Týn; the large bronze memorial near the centre honours Jan Hus, the religious reformer burned at the stake in 1415, unveiled on the 500th anniversary of his death. The pastel baroque and Gothic facades ringing the square are worth a slow loop on their own.
Getting the good photo
The square is one of the most photographed spots in the city, which means it's also one of the most crowded by mid-morning. Arrive close to opening hours, or come back in the evening when the clock and church facades are lit and the day-trip crowds have thinned.