It is not uncommon for members of the public to walk up to a sundial, compare the time it tells with that given by their watch, and turn away disgusted because the two disagree. Yet if the dial is a properly calculated for its geographic position and the watch is accurately set, then the chances are that both dial and watch are correct. This, in simple terms, is because there are two, three or even four kinds of correct time, running simultaneously and this must be taken into account.
The wristwatch worn by the member of the public, whether mechanical or electronic, will have been designed to mark out twenty-four equal hours every day, no matter what the season. What the watch shows is commonly known as "mean time". It would be logical then to think that if the wearer looked due south, when his or her watch showed twelve o'clock midday, that the sun would appear central in the sky. However, time based on the sun being overhead at midday ("local time") was abandoned in most countries in the 19th century when the railways were introduced.
In England for example, it was found impossible to write a train timetable using local time, because the gradual rotation of the earth means that the sun 'souths' or is effectively overhead in Bristol some ten minutes later than it is in London. Midday local time in Exeter, Plymouth and Penzance, further down the same railway line, are similarly later again. So which town's midday would be midday for the purposes of the timetable? It was duly decided that midday throughout Great Britain would be deemed to be when the sun "souths" at Greenwich, directly over the notional line of nought degrees longitude. A sundial in Bristol, set out for local time will therefore appear to be some ten minutes wrong when compared to a wristwatch set for Greenwich Mean Time. Yet both are performing accurately.
At the time of the First World War, many countries decided to add an hour to their standard time in the Spring and remove it again in the Autumn, so that the maximum number of hours of daylight could be exploited for war work. Many countries continue to do this today for the public benefit. A sundial calculated to show accurate local time, may therefore not only disagree with mean time, but further appear to be a whole hour adrift because of "Daylight Saving" or "Summer Time". And yet in its own terms, it will still be performing accurately.
And there is more. The shadows on sundials move because of the apparent procession of the sun each day from east to west. It might be thought therefore that like the mechanism of the observer's wristwatch, sundials would divide every hour in every day throughout the year into equal parts. But they do not. This is because the earth travels round the sun in an ellipse and not in a perfect circle. Furthermore, the axis of the earth, around which it spins, is not upright in relation to the sun, but tilted. To make matters worse, as the earth turns, it also wobbles slightly. The gravitational pull of some of the planets add to the mix and the net result is that solar days and hours vary in length according to the seasons, as does the apparent height of the sun in the sky.
Thus the time shown on a supremely accurate sundial may vary from that shown on a supremely accurate watch because of its geographic position, because of political decisions relating to daylight saving and because of the elliptical passage of the earth around the sun. While all this sounds very complicated, it need not be, because a sundial can be set out to take account of its geographic position and have dual dials for summer and winter time. As for the passage of the earth in space, the differences on a daily basis can be engraved on the dial, usually in easily read tabular form. An historic printed form of "Equation table " is shown above.
This being so, the watch owner can simply check the sundial, add or subtract the number of minutes the table recommends and suddenly sundial and watch will agree. In short, sundials indicate "Natural Time", while watches show "Mean Time", an artificial time, invented for human convenience. Simple addition or subtraction allows the reconciliation of the two. Even the most modest garden sundial should be capable of indicating the time within a margin of two or three minutes, providing it is properly made and properly understood.