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A stamp honoring Leonardo Ximenes

Appreciating the Duomo of Florence and the Summer Solstice

One of mankind’s first methods of telling time was by using the sun dial. Although technology has left its mark well beyond this simple device, the precision of nature and man’s ingenuity is displayed each year at this time in the Duomo of Florence. Each June it hosts a unique demonstration – the passage of the midday sun through a gnomon set into the cathedral’s iconic dome. For reference, a gnomon is the name for the projecting piece on a sun dial that shows the time by the position of a shadow.

The device goes unnoticed by visitors for most of the year, but it takes on special significance between late May and early July each year. The gnomon is very hard to spot. It is located at a height of almost 300 feet inside the towering dome. For those with eyes like an eagle (or more appropriately for the Duomo, a peregrine falcon), look just below the windowed lantern on the southern wall and you will see what appears to be a small bronze shelf jutting outward. The device is what is known as a pinhole gnomon and instead of casting a shadow in the manner of most sun dials, it projects a circle of light – a miniature image of the sun. The significance of the light that it projects at midday during this time of the year becomes more relevant when you look at the floor. A brass meridian line runs across the floor of the Chapel of the Cross and is accompanied by an inlaid circle.

The projected image moves as the sun moves across the sky. For our readers who have astronomic tendencies, we know that it is really based upon the rotation of the Earth in relation to the sun. Each day for weeks preceding the solstice, the image can be seen sliding down the walls of the cathedral and across the floor, but it is only during the summer solstice that the alignment reaches perfection. At midday on June 21, the projected light completely fills the circle on the floor.

Since the gnomon in Florence is one of the highest placed in the world, the display is considered by most to be the very impressive, especially considering its setting inside the Duomo, but it is by no means unique. Both San Petronio in Bologna and Santa Maria degli Angeli e dei Martiri in Rome also contain the same type of optical display across imbedded meridian lines on the floor.

Perhaps it is more impressive if you consider that the gnomon in Florence was put in place more than 500 years ago. The Florentine mathematician Paolo Toscanelli first placed the device in the Duomo in 1475. It was not until 1756 that the Sicilian astronomer and Jesuit priest Leonardo Ximenes laid the meridian into the floor that we see today.

The purpose behind both the gnomon and the meridian in the cathedral were religious in nature. The design was used to more accurately measure the length of the year, which in turn determined that date on which Easter fell. It was a valuable tool in the days when the most accurate way of measuring time was by observing the sun. Even though the greatest ‘spectacle’ is observed on the summer solstice, the tracking through the gnomon also identifies the vernal equinox, which is vital in the determination of Easter, the first Sunday following the first full moon, following the vernal equinox.

Today, the gnomon no longer serves a scientific purpose, but remains a fascinating sight. The disc of light travels at about three inches per second across the backdrop of the cathedral for roughly 30 minutes around 1:00 pm, which because of daylight savings time is the true solar noon.

Every year the cathedral invites guests to watch, with commentary from a guide, on several dates when the spectacle is at its best. You can receive an invitation by sending an email with details are on the Duomo’s website. So if you don’t have plans for next June 21, mark it on your calendar and spend the midday in Florence at the Duomo.