In the list of sundials, the clock face declination is usually not provided in degrees but by means of a compass card, depending on the direction the line perpendicular to the clock face is pointing. The gnomonic projection As previously mentioned, the sundial is able to provide a lot of information, however, most of them require an object which casts a point shadow. A flat, horizontal clock face is displayed in fig. Z is the starting point of the shadow rod Z-G which is parallel to the earth s axis. In this example, we want to use it as an object which casts a point shadow.
G is the center of the displayed visible part of the celestial globe.
In the geocentric world system, all celestial bodies move along this conceived sphere. The hour angle provides the true solar time . K. It is called gnomic projection. This applies to all Central Europe. These conical section curves are called date or declination lines.