Every day at noon the gnomon rod casts its shadow on the centre line of the meridian band. When the day of spring or autumn equinox arrives the two orb balls cast a double shadow onto the meridian band. Away from the equinox, only one orb shadow will show.
Our armillary sundial casts distinctive shadows at noon on the day of autumn equinox, as seen in the photo. Its unique design allows the noon shadow of the gnomon rod to fall on the broad meridian band at its centre line. The two orb balls also cast their noon shadow onto the meridian band. Through the year only one shadow of a ball will show but at spring and autumn equinox you can see two.
The upper orb of the armillary sphere lies on the ring that represents the tropic of Cancer, while the lower orb is on the ring of Capricorn. The rings are parallel to the band that represents the equator. At the equinox, the sun is over the equator, so the shadows are opposite the balls.
As winter approaches the sun goes lower in the sky until one of the shadows marks winter Solstice. At the highest point of summer the other shadow marks summer solstice. To be accurate the sundial must be level and on a true north line.