Wikipedia: The Cathedral has a heliometer (solar "observatory") of 1690, one of a number built in Italy in the 17th and 18th centuries. The device itself is quite simple: a tiny hole in one of the minor domes acts as Pinhole camera, projecting an image of the sun onto the floor at solar noon (12:00 in winter, 13:00 in summer). There is a bronze line, la Meridiana on the floor, running precisely N/S. The ends of the line mark the positions as at the summer and winter solstices; signs of the zodiac show the various other dates throughout the year. The purpose of the instrument was to standardise the measurement of time and the calendar. The convention in Sicily had been that the (24 hour) day was measured from the moment of sun-rise, which of course meant that no two locations had the same time and, more importantly, did not have the same time as in St. Peter's Basilica in Rome. It was also important to know when the Vernal Equinox occurred, to provide the correct date for Easter.
Not far from the Palermo Cathedral is small Norman-era chapel with Arabic domes and a Romanesque cloister.
In the Romanesque cloister of the Norman chapel.
