Sundials

Royal Greenwich Observatory

Information Leaflet No. 23: 'Sundials'
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History of the Sundial:

From time immemorial Man must have realised that the changing length of the shadow of an object indicated the time of day - that the shadow shortened towards noon and lengthened again towards evening. No doubt the first crude sundial consisted of no more than a vertical stake in the ground. Eventually Man must have realised that the changing length of the shadow could be used, probably with stone markers, in the same manner as we use the hands of a clock today. This notable step forward in Man's attempt at measuring time was at least 3,500 years ago as the earliest known sundial, found in Egypt, dates from that time.

The day was then subdivided into 12 parts, which we refer to as 'temporary hours'. Of course the temporary hours would vary in length, being longest in summer and shortest in winter, although in the Mediterranean lands the difference is nothing like as noticeable as it would be in the British Isles. In the latter case an 'hour' in summer would have been twice as long as an 'hour' in winter!

It was not until the 13th century that an Arab named Abul-Hassan introduced the idea of making all hours of equal length, and it was not until the 15th century that these equal hours were in general use.

During the Renaissance period the development of sundials proceeded rapidly and many varied and ingenious designs were produced. In addition to having hour and minute marks for telling the time, other features were often incorporated. Thus the 'furniture' of a dial might also consist of markings to indicate the seasons, the calendar date, times of sunrise and sunset, the signs of the Zodiac and the dates of the Sun's entrance into each sign, the position of the Sun relative to the horizon (Azimuth and Altitude), and the points of the compass. A few dials were made which also carried a crude form of tide table indicating the time of high tide at certain named ports when the Moon was observed in a particular direction. However, sundials were gradually superseded by clocks, though it is recorded that the French railways regulated their clocks by sundials right up to the end of the nineteenth century.

Sundial Theory:

Sundials are naturally limited in their usefulness and a cynic might complain that they are of little more than academic interest in a climate such as that 'enjoyed' in the British Isles. Nevertheless the study of sundials, or gnomonics as it is sometimes called, will also provide a good understanding of some fundamental astronomical principles.

As the Earth rotates on its axis, so the Sun appears to move uniformly across the sky, and if a rod is placed parallel to the Earth's axis, its shadow will naturally move uniformly around it. In other words, as the Sun moves through an arc of 15° in the sky in one hour, so will the shadow move at the same rate. This is the principle on which most (but not all) sundials are based, and in fact the same idea is used with telescopes which are then said to be 'equatorially mounted'.

Because the Earth's distance from the Sun varies throughout the year, and also because its equator is inclined to its orbit (by 23.5°), there is a difference between Apparent Solar Time (time told by the Sun) and Mean Solar Time which is the time kept by mechanical and electrical clocks. In fact it is possible for the Sun to be as much as a quarter of an hour fast or slow when compared with a clock which keeps mean solar time (i.e. Greenwich Mean Time). This difference is called the equation of time and is described in the Leaflet 'The Equation of Time'.

If we know this correction as a function of the date, it is possible to adjust certain types of sundial (those where equal intervals of time are indicated by equal angles) to allow for the change in the equation of time; or alternatively, for any type of dial, to apply a correction to the time read from the dial.

Another correction that has to be made, is to allow for the longitude of the place. We are all familiar with the fact that 'New York is five hours behind Greenwich' meaning, for example, that when it is midday at Greenwich it is only 7am in New York. This is because New York is 5 hours of longitude west of Greenwich. Even if we move only as far west as Bristol, we find that this town is 10 minutes of time west of Greenwich, so that the Sun crosses the meridian 10 minutes later than it does at Greenwich. Therefore, if you had a sundial in Bristol and wanted to find the Greenwich Mean Time, you would have to add 10 minutes to the time from the dial, unless this longitude correction had already been allowed for in the construction of the dial.

Types of Sundial:

Most sundials have the gnomon lying parallel to the Earth's axis. If the dial plate lies in the equatorial plane, then the time scale is equiangular (all the hour lines are exactly 15° apart). If the dial is placed in any other plane, then the time scale is no longer equiangular and the angles between the time marks have to be calculated using trigonometrical formulae. Dials can be classified according to the plane in which the dial lies, as shown in the following table:

Another type of dial depends on the fact that the Sun's Altitude (angular height above the horizon) changes with time. The dial has to be calibrated with date marks as well as time marks. These dials can be carried around and used anywhere within a few degrees of the Latitude for which they were constructed, merely by pointing the gnomon in the direction of the Sun. An Altitude dial inscribed on a cylinder, with a horizontal gnomon, was used by shepherds in the Pyrenees.

Portable dials were made in large numbers in the 16th-19th centuries, and a fine selection of them is on display at the Old Royal Observatory at Greenwich, now part of the National Maritime Museum.

Another type of dial is the equiangular dial, although it is not mentioned as such in the literature. It is defined as one that is constructed so that equal intervals of time are indicated by equal angles. Equiangular dials can be horizontal, vertical or reclining.

Sundials in Cambridge:

There are many interesting sundials in Cambridge. There is only space here to mention three. In Senate House Passage, a gateway to Gonville and Caius has a group of simple vertical dials mounted about an hexagonal tower, whereas the beautiful example in Queen's College is a vertical reclining dial with additional markings to indicate, not only the time, but also the Altitude and Azimuth of the Sun, its zodiacal position, the date, and the sunrise time. It is also a moondial, a notoriously inaccurate construction.

The dial chosen to commemorate the Tercentenary of the founding of the Observatory at Greenwich (in 1675) is called a reclining equiangular sundial. The dial is situated in the grounds to the east of the new RGO building. It was designed so that the viewer can read the time to within one minute of GMT. This necessitates regular adjustment of both the rotation of the dial and the position of the gnomon. Unfortunately there is not sufficient manpower available to do this on a regular basis. This dial is further described in another Leaflet.

See also; 'The Equation of Time' and 'The Tercentenary Sundial and the Greenwich Water Tanks'

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Tue Apr 16 15:18:59 GMT 1996

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Updated: December 6 '97, June 27 '14

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