Universal - the annular sundial.

Sundials are available in diverse forms and designs. The annular sundial is the universal design: It has the great advantage that it can be used anywhere in the world without the assistance of a compass and a spirit level. Annular sundials exploit the fact that the angle of the sun to the equatorial plane (so-called magnetic deviation) assumes various values which are typical of the relevant day during the course of a solar year. The shadow-caster - in the case of annular sundials, this is a small punched mask - is movable and is adjusted to the calendrical date; in this manner, the angle of the punched mask to the inner edge of the numerical ring corresponds to the magnetic deviation on the corresponding day; the correct (astronomical) time can be read off from the inner edge of the ring. (Sundials display the so-called »true local time« - in order to calculate Central European time, 4 minutes are deducted for each degree the measurement site is removed from the 15th degree of longitude in a westerly direction.)