Sundials through the Ages

     The first thing to understand about a sundial is that even the best of them are only accurate to clock time four days each year. That’s not a defect, just an artifact from Earths elliptical orbit around the Sun. A clock tracks the average 24 hour length of a day. A sundial directly reads the position of the Sun in the sky. The two can be off by 16 minutes in the Fall and Winter and 6 minutes during the Spring and Summer. 

     There is a fix for that - a way to read a sundial very close to clock time. A good sundial will have an analemma, a figure-8 shaped curve which shows the correction factor for each day. By reading the dial directly, then adding or subtracting the correction factor you can get remarkably accurate time. The analemma is created by making a mark each day at precisely the same clock time, usually Solar noon. That’s a lot of work for most, but even once per week will plot out an accurate curve. 

     The next important thing to understand is that the larger a dial is, the more precise it can be. A small, horizontal garden dial looks nice but is difficult to read to within a few minutes of clock time. A vertical dial does not take up much real estate and can be as large as the wall it occupies. Additionally, a vertical dial can be a calendar as well as a clock. Hours are marked by the position of the shadow, and days by its length.