SUNDIALS

History
It is thought that there may have been a mention of a sundial in Isaiah XXXVIII 8, which would be about 700 B.C. However, the earliest certain evidence we have is of Egyptian L-shaped sundials, and then the hemicycle of the Chaldean astronomer Berosus, which is dated at approximately 300 B.C. These early dials would divide each period to daylight into 12 equal parts. These equal parts were called "temporary" hours. Since the duration of daylight varies from day to day, the temporary hours of one day would differ from those of another.

Sundials were then markedly improved by, first, the Greeks using their increased knowledge of geometry, and then, the Romans. It was in the 13th Century that the science of gnomics was vastly improved by the Arabians. One of their writers, Abdul Hassan, made the great step of reckoning by "equal hours" in summer and winter, rather than by the temporary hours of the Berosian Dial. It was this change that marked the introduction of striking clocks in the earlier part of the 14th Century, and, necessarily, the introduction of a new generation of equal reckoning sundials to general use in the 14th and 15th Centuries. Although portable dials had been continuously improved since their introduction by the Romans, nothing could stop the mechanical watches and clocks superseding sundials in the 18th Century.

General Principles
It must be remembered that sundials were invented and developed by scientists who believed that it was the Earth that was fixed, and the Sun and Stars revolved around the Earth's axis uniformly, from East to West, once a day. When perceived this way, it appears that the Sun lags a little behind the Stars, making its day some four minutes longer. So that at the end of the year it finds itself again in the same position, having made a complete revolution of the heavens relatively to the stars, from East to West. The diurnal motion of the stars is strictly uniform, but the daily retardation of the sun is not always exactly four minutes. Because this "four minutes" is constantly altering, time, as measured by the Sun's motion (and consequently by a sundial), does not move at a strictly uniform pace.

Setting your Sundial
Explanation
This slight irregularity of the Sun's "motion" could not be followed by the mechanical clocks and watches except by extreme complication. Time measured by these devices is such that the length of a clock day will be the average of all the solar days in the year. Four times a year the clock time (called "mean time") and the sundial time (called "apparent time") agree exactly. The time as measured by your sundial, now going a little slower, and now a little faster, will sometimes be ahead and sometimes behind (the greatest difference will be 16 minutes for a few days in November). The best time to set your sundial is on one of the four days when "mean time" and "apparent time" agree exactly; April 15th, June 15th, September 1st and December 24th.

Procedure
Site your sundial in an absolutely level position, and ensure the gnomon points to True North. On one of the four days, at 12 noon, check that the time shown by the sundial agrees exactly with that told by your watch. One has to remember, of course, that the sundial will show the same time Summer and Winter. So when British Summer Time is in force one hour will have to be added to the number on which the shadow falls. Consequently there is probably only one day of the year when your sundial will appear to be telling the correct time. Do remember that, if Ptolemy was standing beside you, he would insist that it was the fault of your inaccurate watch!