Sitting on the clock tower in Piazza dei Signori, the astronomical clock of Padua was built in 1344 and is one of the oldest clocks in the world still in working order. The designer, Jacopo de'Dondi, became so famous for this work that his family name was later changed to "Dondi dell'Orologio," or "Dondi of the Clock."

The magnificent timepiece has a 24-hour dial, so the hour hand makes a full rotation only once a day, not twice, moving at half the usual speed and starting at the right ("zero hour") rather than at the top. The clock also strikes the hours on a bell from 1 to 24. The dial shows the day of the month, the current phase of the Moon, the motion of the planets, and the position of the Sun in the Zodiac. 

After the original clock was destroyed at the end of the 14th century, a replacement was built in 1423 as a faithful copy of the original, with one exception: It was missing one of Zodiac signs, Libra. There are various stories about the origin of this curious lack, the most famous saying that the builder deliberately omitted the symbol, which is an emblem of justice, because he was not paid the amount of money he was promised. Another version of the story says that the symbol was also missing from the original clock as a protest by the designer against the rule of the Carraresi family.

In reality, this missing symbol comes from the use of the pre-Roman Zodiacal system, which united the constellations of Scorpio and Libra. The two halves of Libra were seen as the Scorpio's claws and later became the scales, but even today the name of the two most luminous stars in Libra are the Arabic names for "northern claw" and "southern claw." Anyway, some people say that the Libra sign is actually hidden somewhere in Piazza dei Signori—try looking for it!