A sundial indicates time by measuring the angle of the sun in the
sky, which moves by 15 degrees each hour. Normally we
measure time relative to midday, the time when the sun is highest in
the sky. This sundial is different, as it measures time relative to
sunset and indicates how many hours of daylight remain in the day.
The sundial is mounted vertically on a wall facing west. A gnomon,
projecting horizontally from the top centre point of the sundial,
casts a shadow on the wall. During the course of the afternoon the
tip of the gnomon’s shadow will move from the bottom of the wall up to
the top until, at sunset, it is level with the gnomon. The left hand
curve of the sundial’s markings plots the path of the Sun’s shadow
during the summer solstice, the middle line is the path during the
equinox, and the curve on the right shows its path at the winter
solstice. The hour lines are angled to account for the different
length of day between summer and winter.
This way of measuring time is related to ‘Italian Hours’ whereby hours
are measured relative to the last sunset. This was commonly used in
parts of Europe between the 14th and 18th centuries. You always knew
how much time you had left to get your day’s work done before sunset.