Meredian Lines and Easter

Bianchini's meridian line with solar disc at solar noon

Geoff Manaugh on meredian lines found in Catholic cathedrals.  The intended use:

“The way meridian lines operate is both surprisingly complex and quite easy to grasp. As the sun tracks from north to south on its annual migration between the summer and winter solstices, its image on the cathedral floor also shifts, moving slowly along the meridian line. Halfway between the solstices, of course, are the spring and autumn equinoxes. Once the position of the solar circle indicates the spring equinox, believers must simply wait for the next full moon; the first Sunday after that full moon will be the proper date of Easter.”

The eventual actual use:

“This [the fact that cathedrals are sinking into the ground over time] is true to the extent that the meridian line installed inside architect Filippo Brunelleschi’s Cathedral of Santa Maria del Fiore in Florence eventually became used as nothing more than a diagnostic tool for determining how much the cathedral itself had shifted. No longer useful for astronomical observation at all, the meridian became something more like an emergency light on the building’s creaking dashboard.”

Read more on astronomy and Catholic cathedrals: