A cross on the facade of the Basilica of Santa Maria Novella will indicate the day of the equinox

Equinox: when the length of the day is equal to that of the night… (from the Latin aequinoctium, comp. of aequus “equal” and nox noctis “night”)

This natural cycle of the Earth is renewed twice a year, when the hours of light and darkness are perfectly equal. The Latin term reflects precisely this condition of balance and refers to the moments that mark the passage between the seasons: from winter to spring and from summer to autumn.

In Florence there is a special place to observe this phenomenon: the facade of the church of Santa Maria Novella. This is thanks to Egnazio Danti, a Dominican friar, cosmographer and mathematician at the court of the Grand Duke Cosimo I de’ Medici who was commissioned (it was 1572) to build astronomical instruments to precisely measure the equinox and study the movements of the Sun and the stars.

On the right side of the facade, Danti installed an astronomical quadrant, of which only the marble base remains today. Equipped with sundials, it used the shadow cast by an iron rod (the gnomone) on a reticle to indicate the time. The quadrant also allowed the inclination of the Earth’s axis to be calculated and the position of the Sun to be measured during the year, making a fundamental contribution to calendar studies.

On the left side of the facade, Danti instead built an equinoctial armilla, an instrument used to indicate the moment of the equinox. The armilla is made up of two bronze circles of 1.30 cm in diameter perfectly perpendicular to each other.

The vertical circle (also called the meridian circle) has the task of identifying every day when the sun reaches its maximum height, that is, solar noon. At that moment, the circle will project the shadow of a long rectangle on the facade.

When the horizontal (or equatorial) circle also projects a shadow with the shape of an elongated rectangle, at solar noon, no shadows in the shape of elongated circles will be seen on the facade, a shadow in the shape of a cross will be seen: the day of the equinox has arrived.

Contrary to popular belief, the equinox does not always fall on the same day but varies every year.

This year, the autumn equinox will be on September 22nd, from 12:45 pm in the basilica it is possible to observe the passage of sunlight and the sundials of Santa Maria Novella together.