Hooke’s joint and the Avignon sundial

Extract

In the summer at Avignon a little Disneyland train conveys tourists from the Palais des Papes to the summit of a little hill behind the cathedral called the Rocher des Doms. In the garden on this hill there is a most remarkable sundial; the visitor himself provides the gnomon, and a linear scale on the flagstone pavement shows him where to stand at different dates. His shadow then falls towards one or other of a ring of fixed pyramidal markers which indicate the hours of the day.