Astronomy Principles and Practice Fourth Edition.pdf

406 Practical projects
Figure 24.2. 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 φ tan h
where φ 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 24.2).
24.2.3 A noon-marker
The simplest noon-marker can be achieved by using a vertical plane which is oriented also in the east–
west direction. By fixing a disc containing a small central hole on an arm which projects at right angles
from the plane, during the hours close to mid-day a patch of light surrounded by a shadow will appear
on the plane. It is an easy matter to mark on the plane the centre of the bright patch, corresponding to
an image of the Sun.
If during the course of a year, at each noon the Sun’s position is noted, as expected the mark
will rise and fall according to the noon altitude during the winter and summer. In addition, however,
the marks move first eastwards and then westwards, repeating the excursions twice in the year, so
producing a figure-of-eight pattern—the analemma. These departures from a vertical line correspond
to the equation of time. On those days when the equation of time is zero, the Sun’s image at local noon