Plane dial

A third common group preserved from antiquity are plane dials designed for horizontal or vertical receiving surfaces. Dials of this type are characterized by their bat wing shape, where the upper curve is the track of the tip of the gnomon’s shadow for the day of the summer solstice, the horizontal straight line below is the shadow track for the equinox, and the lowermost curve is the shadow track for the winter solstice. Between these curves run eleven more or less vertical hour lines. The hours indicated by this and most other types of sundial were seasonal rather than equinoctial, meaning that their length changed in the course of the year.

The configuration of horizontal plane dials is further explained in a video provided by the Museo Galileo.

Commentary. Philipp Nothaft (May-June 2019)