This is the 7th book in Fred Sawyer’s Sciatherics series. It is a translation of Joseph Drecker’s 1925 classic Die Theorie der Sonnenuhren. This book is not a general introduction to modern gnomonics. Rather, it is the culmination of centuries of research primarily on those ancient sundials that attributed 12 (variable length seasonal) hours of sunlight to each day of the year. The system of seasonal hours was in use for well over 1,000 years and yet in that time there was never any clear understanding of the true mathematical nature of the hour lines or curves. Additional topics include ancient portable dials, Italian and Babylonian hours, Planetary hours, equal hour gnomonic sundials, Foster/Lambert dials, and bifilar sundials.