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Sun time and clock time

This is one of a series of articles written for "Clocks" magazine by the late Noel Ta'bois, and reproduced with permission here as a memorial to him.

This article originally appeared in Clocks in October 1985

Sun time embraces apparent, true, real, local, and meridian solar time; clock time embrace mean time; standard time, and summer or daylight saving time. It is highly desirable that the dialist fully understands all these terms.

A sundial shows time by the sun which is called solar time (Latin: sol, sun), as opposed to time by the stars - the astronomers' all-important sidereal time (Latin: sidus, star). The speed of the sun's apparent movement across the sky is not uniform and follows a yearly pattern of variation. Mean time, which is uniform, averages the variations. Both the sundial and the clock show solar time; the sundial, apparent time, and the clock, mean time. The difference between the two is called the Equation of Time.

Equation here means 'correction to compensate for a discrepancy' and a simple graphical representation is shown on this page. The initial letters of the months along the bottom indicate the date, and the minutes on the left are marked plus or minus to show whether they have to be added to or subtracted from apparent time to give mean time.

The curve on the graph is more often drawn the other way up, with the valley in February and the mountain in November. The plus then signifies that the sun is fast, and the minus that it is slow, compared with mean time. This alternative method of presentation makes for confusion so one must have a care not to add when one should subtract, and vice versa.

When the sun is on the Greenwich meridian (Latin: medius, middle; dies, day) it is noon, the middle of the day. The time shown by a sundial on the Greenwich meridian differs from mean time only by the Equation of Time. But suppose the sundial were in say, the Orkney Islands. When the sun is on the meridian there, it is noon apparent solar time, but since the Orkney Islands lie on longitude three degrees west, noon will occur there 12 minutes later than noon at Greenwich because the sun moves one degree in four minutes (3 x 4 = 12). East of Greenwich meridian the sundial would be fast by four minutes for every degree. Thus apparent time varies from mean time not only by the Equation of Time but also with longitude.

When man's maximum speed of travel was limited by the horse, both apparent time and mean time were local time, local because they were affected by the longitude of the location. The Equation of Time correlated the two. With the coming of the railways and the telegraph, local mean time became a nuisance, so mean time at the time meridian was used over the whole time zone. In a small country like Britain one time zone covers the entire country. In order now to apply the Equation of Time one needs to know 'the local apparent solar time of the zone', as I have seen it stated. What a mouthful! I know of no simple alternative to this phrase so I will call it 'meridian time'.

In old books mean time, which is clock time, is always referred to as true time; sun time is apparent time. Nowadays it is usual to refer to apparent time as true (or real) time; mean time or clock time is something artificially made by man. It's all very confusing! Furthermore, when reading about local apparent time one needs to know whether or not it refers to local apparent time on the time meridian where, as I indicated in the previous paragraph, it has special significance. Therefore I try to decide which the author has in mind - this can be very difficult! - and then mentally substitute either 'local' or 'meridian' for 'apparent'. The significance of what I refer to as meridian time is very poorly covered in textbooks.

Another source of confusion is that one often reads: 'the Equation of Time is the difference between time shown by a sundial and time shown by a clock'. This statement is only correct either if the sundial is on the time meridian (and most are not) or if the hour lines on the dial plate have already been corrected for longitude, which is rare.

I can now summarise by giving you the rules for checking a watch by a sundial.

  • Correct for longitude. For every degree of longitude that the dial is west of the time meridian add four minutes to local solar time (the sundial reading) to give meridian time. For every degree east, subtract four minutes.
  • To get mean time from meridian time add the Equation of Time. Note that I say just 'add' not 'add or subtract'; the sign is incorporated in the Equation of Time figure.
  • When summer time is in use, add one hour.

This is all you need to know to put your watch right by a sundial, assuming of course that the calculation of its hour lines and its orientation are right. Some sundials are wrong. You can check whether they are at fault by starting with a watch known to be showing the correct time and working backwards.

When you have grasped all this you will be as amused as I am to observe people looking at sundials and consulting their watches. Are they using the sundials to check their watches or vice versa? I suspect the latter for they frequently make remarks which cast doubts on the accuracy of the sundials. Little do they know that if they make the corrections that you can now make, it is highly probable that it is for their watches which will be found to be at fault! For maximum entertainment stand close to a sundial situated as near as possible to Land's End, at the end of July when the difference between local solar time and BST is nearly one and a half hours. Another good spot is on the east coast of Norfolk, say in Yarmouth, at the beginning of November (Guy Fawkes day) when the difference is about 22 minutes.

I have given you all the essentials of sun time and clock time and I have not once needed to mention latitude; it does not come into the argument. How latitude does interest the diallist I will explain next month.