(Gibbs 1976: 198)

The gnomon shadow's passage over a demarcation on the dial does not represent a set amount of time, but the completion of a fraction of the total daylight in a given day. Because there is more daylight during some parts of the year than others, the actual period of time between demarcations varies across seasons. In order to read a sundial with a greater degree of precision, one must be familiar with the differing durations of daytime over the course of a tropical year.