The Pantheon was a huge sundial that marked the
passing of the seasons. In winter, the circle of light enters from the
oculus and illuminates the upper part of the dome.
On the days of the Equinox, the circle of light illuminates the cornice under the dome: a sign that Spring is coming.
other secrets and beauties of the Pantheon in our blog with a free
downloadable pdf, taken from the book «Pantheon. Architecture &
Light» by Marina de Franceschini.
The turning oint was in 1991, when the Italian astronomer Aldo Tavolaro was
the first to give a new interpretation of the dome linked to Archaeoastronomy. He
discovered that in the days of the Equinox (March 21st and September 23rd) at the
local noon the circle of light created by the oculus illuminated the lintel or cornice: «The image of the Sun is placed on the lintel for a few moments, just as the
Sun around that date crosses the celestial equator».
He also discovered that the Pantheon
functioned as a sort of seasonal sundial:
«It is possible to determine the dates of
the year by observing the position of the
spot of light at astronomical noon [...] If it
is winter, the oval of light never falls
below the lintel; if it is summer it
descends until it illuminates the floor of