The Cathedral has a meridian, which may be considered as an earlier type of heliometer (solar "observatory"), one of a number constructed in Italian churches, mainly in the 17th and 18th centuries. This one was built in 1801 by the famous astronomer Giuseppe Piazzi, the director of the Observatory of Palermo who discovered the first minor planet or asteroid, Ceres. 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 ). There is a bronze line, "la meridiana" on the floor, running precisely N/S. At solar noon (circa 12:00 in winter, 13:00 in summer), the sun image passes through this line. At different times of the year the passage occurs at different point of the line. The ends of the line mark the positions at the summer and winter solstices; signs of the zodiac show various dates throughout the year.