The astronomical gnomon (sundial) at St. Sulpice Church. It is designed to follow the variations in the height of the sun and moon.

Because such a pole cast a longer shadow in the north of Egypt than in the south, Ptolemy was able to deduce that the
earth was round and to calculate the approximate dimensions of the globe way back in the 2nd century.

This setting has notoriously been put into the limelight in 2004 with the publication of the NOVEL "The Da Vinci Code" by Dan Brown.