The Cathedral of Florence, Italy, was designed in 1420 by a great architect named Brunelleschi.
Since the cathedral was to be built on a swampy site, Brunelleschi feared that it might someday begin to sink into the soft earth, and he wanted the people of Florence to always have advance warning.
So he designed the cathedral’s dome with a small opening in it, just big enough to let a beam of light into the church, and had a small metal plate set in the floor of the cathedral, directly under the dome.
The plate was placed so that every year on June 21, the beam of light entering through the hole in the dome would directly strike the metal plate. If the light ever failed to strike the plate on that day, it would mean that the church had shifted on its foundation!