A sundial is a piece of geometry. The gnomon is a rod parallel to the Earth's axis, and the shadow it casts falls along a line whose angle is the hour angle. Set the hour lines once, for your latitude and your season, and the shadow reads the hour for ever — as long as somebody remembers to wind the clock that the shadow points to.
It is tempting to divide the dial into twelve equal sectors of thirty degrees. That is wrong, and it is wrong in a way that shows up twice a year. The sun crosses the meridian at the same rate every day — fifteen degrees an hour — but the azimuth of the sun does not move at a constant rate, and near sunrise and sunset it moves faster than in the middle of the day.
So the hour lines bunch towards the noon line and spread towards the horizon. It is the same reason that the sun rises at a different quarter on different sides of the year, and the reason an analemma — the figure-of-eight traced by the shadow of a fixed-time mark over a year — is lopsided rather than symmetrical.
If you fix a time of day — say nine in the morning — and mark the tip of the shadow every day for a year, the marks do not trace a circle. They trace a figure of eight, and the loop is the equation of time: the difference between the sundial and the clock, which is nearly a quarter of an hour at the extremes and vanishes four times a year.
The figure is drawn here from the same equations as the shadow, at a fixed nine o'clock, so the tilt of the loop is a direct consequence of the obliquity of the ecliptic and the eccentricity of the Earth's orbit.