Sundials and Calendar ReformA form of sundial was the earliest means of checking the accuracy of calendars. Chinese astronomers would check the positions of the solstices and equinoxes by observation of a meridian line sundial. This consisted simply of a line which was drawn in the local meridian (North-South line) with an obelisk set at the South end of the line. Positions were marked along the line to indicate where the tip of the obelisk's shadow should be at the equinoxes and the solstices. By this means, astronomers could determine where adjustments were required to the calendar to keep it in step with the motion of the sun.
A slightly different form of meridian line was developed in Italy during the fifteenth century. This time, instead of using the shadow of an obelisk, the sun was allowed to shine through the roof of a building onto a meridian line on the floor inside, and the point of light was used as the indicator. Such a meridian was set up in Florence Cathedral in about 1460 when Paolo Toscanella pierced the Brunelleschi dome and marked a meridian on the floor of the nave. This meridian was only marked out for the period immediately around the summer solstice (by a graduated marble slab) and so could only be used for checking whether the solstice coincided with the correct date in June.
It was partly study of the movement of the sun on the Florentine meridian line which indicated the urgent need for calendar reform in the sixteenth century. Ignazio Danti made use of the line when carrying out his calculations, but found it too inaccurate for his purposes. He established a further two meridians in the church of Santa Maria Novella and, in 1576, oversaw the construction of the meridian indicator in the Cathedral of San Petronio in Bologna. This indicator was replaced in the following century by the famous astronomer, Giovanni Domenico Cassini, who made use of it in continuing to monitor the accuracy of the new Gregorian calendar.