Meridian Line
This is the meridian line within the church of Santa Maria degli Angeli e dei Martiri in Rome. A meridian line is an instrument used to measure the date and the the changing of the seasons (i.e. the solstices and the equinoxes). At Solar noon, an image of the sun is projected along the scale built into the floor, showing the date. The maximum and minimum projections of the sun along this scale [reasonably] precisely show when the solstices occur. Halfway between these two points are the equinoxes.