The Sun as a timekeeper: A vertical sundial
 

The simplest form of vertical sundial makes use of a plane which passes through the east–west line as well as, of course, being vertical. The length of the style must point to the north celestial pole. Time

Figure1. A vertical sundial.
lines should be drawn either side of the style, the angles γ of a line from the base of the style relative to the noon-line being given by the formula
tan γ = cos ∅ tan h
where ∅ and h are the same as for the horizontal dial.
In order to be able to record the early morning and late evening shadows of summer days when the Sun rises and sets north of the east–west line, the sundial may be continued on its reverse face if the style perforates the plane and is continued for projection of shadows on the north face (see figure 1).