Designed by Pompeo Schiantarelli and over 27 metres long, it consists of a brass
strip arranged between marble panels, into which are set beautiful painted medallions
that depict the twelve signs of the zodiac.
At midday, local time, the sunlight enters the hole of the gnomon - placed high up in
the room's South-West corner - and its rays strike the meridian line of the floor,
running up or down its length according to the season.