Located on the south facade of Santa Maria Novella, Florence, this armillary sphere was designed and erected by Ignazio Danti in 1572 enabling him, with other instruments, to measure the exact length of the year by determining the true equinox. At the vernal equinox in March 1574 Danti observed that it was eleven days early according to the Julian calendar. This was one of the factors that led to the introduction of the Gregorian calendar in 1584.