The first gnomon in the Florentine cathedral was installed in 1475 by Paolo Dal Pozzo Toscanelli (1397-1482), a famous astronomer, friend of Filippo Brunelleschi (1377-1446) and correspondent of Christopher Columbus. At midday on the Summer solstice (June 21) a beam of sunlight, passing through a hole in a small bronze shelf inserted in the wall at the base of the southern window in the lantern, lit up an inlaid marble disk in the floor of the Chapel of the Cross. The position of this disk was corrected on June 12, 1510, when a new marble disk was installed around the existing one, with a diameter corresponding to the stenopaic image of the Sun. The first observations probably served to determine the duration of the year, allowing Toscanelli to correct the Alfonsine and Toledan tables. In the spring of 1755 the grand-ducal astronomer Leonardo Ximenes (1716-1786) constructed a new meridian line for the purpose of measuring variation in the obliquity of the ecliptic.