Mr Daniel, formerly of the National Maritime Museum at Greenwich, discusses a 19th-century sundail from the Old Deanery garden. Extract from The Friends of Rochester Cathedral Annual Report for 1987.

In the garden of 2 Kings Orchard there is a most unusual sundial, shaped like an anchor, which is set upon a pedestal, and which marks the boundary between the parishes of St Margaret and St Nicholas. The dial and pedestal are of grey stone, standing some 7ft 3ins overall in height. On the south side of the pedestal there is fixed a large brass plate engraved with a table of the equation of time. The base of the plinth is incised with the letters S.M.P. and the date 1825.

Anvone could be forgiven for not recognising this structure as a sundial, since it does not resemble a conventional dial of any pattern, but rather a nautical monument of some sort. Basically, sundials may be divided into four main categories or classes (1) equinoctial or equatorial dials, (from which, fundamentally, all sundials are derived,) in which the plane of the dial-plate lies in the plane of the equator; (2) polar dials, in which the plane of the dial-plate lies in the plane of the polar axis; (3) vertical dials, where the plane of the dial lies in a vertical plane; and (4) horizonta/sundials where the dial lies in the plane of the horizon. This last category is the one with which most people are familiar, at least it is the common or garden horizontal dial which they think of, when they read or hear the word 'sundial. There are other secondary classes of sundial, where the plane of the dial lies in a plane other than in one of those of a primary class. However, the Rochester dial actually falls into the first category and is an equinoctial/equatorial sundial, albeit of a peculiar design.

The dial resembles a short thick 'Admiralty pattern' anchor, the 'crown' of which is raised up towards the south, so that the plane of the 'anchor' (the plane of the dial), lies in the plane of the equator at an angle of 39 degrees above the horizontal level of the plinth, ie. the co-latitude of Rochester.

Hence it falls into the equatorial-sundial class. The 'flukes' of the 'anchor' are the gnomons or indicators, the edges of which lie parallel to the polar axis of the earth, and which cast their respective shadows onto the cylindrical surfaces of the 'stock' of the 'anchor'. As with the anchor of this type, there are two 'flukes', ie. there are two gnomons. This means that, in reality, the dial is a double dial, or two individual component dials, one on the east side of the 'anchor' and one on the west. The hour-lines were engraved on the east and west cylindrical surfaces of the sides the 'shank' of the 'anchor', and the 'stock'. They are no longer easily visible to the eye, having been weathered away over the years, although traces of the numerals and hour-lines can just be detected under the protective arms of the dial-head. As with a direct east-facing dial, the hours on the east side of the dial would have been marked from 4 am. through to 12 o'clock (mid-day), whilst the west side of the dial would have been marked similarly from 12 o'clock through to 8 pm.

The photograph shows the sundial viewed from the east (photograph taken at about 10.00 hrs GMT on 2nd luly 1986) and the fiducial edge of the shadow can be clearly seen. Although the time cannot be read-off due to the lack of visible hour lines, the dial was sketched by Warrington Hogg in 1893 for an article on sundials, published that same year in The Strand Magazine.

Photograph of the Old Deanery Sundial, taken at about 10 a.m. GMT.

Sketch of Sundial made by Warrington Hogg in 1893. The time is shown as 8 a.m. GMT.

The sketch subsequently appeared in the fourth edition of The Book of Sundials, published in 1900, originally compiled by Mrs Alfred Gatty, enlarged and re-edited by Eden and Lloyd. It was again used by Warrington Hogg in The Book of Old Sundials, (opp p. 14) first published by T. N. Foulis in 1914, which ran to several editions. It was also copied by Henslow in Ye Sundial Booke (1918). Fortunately the sketch clearly shows the hour-lines, the division of the hours and the Roman numerals. Not only does it illustrate how the dial works - it can be seen that the shadow of the gnomon indicates the time to be 8 am. (local apparent time) - but it also provides sufficient information to enable the sundial to be properly restored to working order at some future date.

In 1825, the year when the sundial was made or set up in the garden, mechanical clocks and watches were rapidly becoming more accurate, easier to produce and cheaper. Consequently, the power of the sundial, which had always been the primary instrument for the determination of time*, since dials were first constructed, was beginning to wane. However, it was not until a hundred years later that radio communications finally eclipsed

*Except where superseded by meridian or transit instruments in astronomical observatories.

the sundial altogether as a scientific instrument. Sundials, except those specifically designed to indicate mean time ('clock' time), normally show what is known as apparent (solar) time. In 1825 communications were still slow and inefficient. Consequently, towns and cities around the country were still keeping their own local 'clock' time, or local mean time. It was not until the advent of the railways, when trains had to run to some uniform timetable that 'railway-time' was introduced and zone times, based upon the particular meridians of capital cities became recognised. In Britain the time zone was based on the meridian on which the Royal Observatory at Greenwich was situated, but it was not until 1884, when Greenwich was agreed on internationally as the prime meridian of the world, that all longitudes were based on the meridian of Greenwich, as longitude zero, and on which all time-zones were established.

In 1825, for all practical purposes, it was only necessary to know local 'clock' time. Thus, by applying a 'correction' to the reading obtained from the sundial, 'clock' time could easily be ascertained. The equation of time table, engraved on the brass plate, gives this correction in minutes and seconds. The equation of time is the difference between apparent 'sundial' time and mean 'clock' time. It is due to the effect of the earth's axis being tilted to the plane of its eliptical orbit around the sun, combined with the effect produced by the earth's varying orbital velocity. This results in the sun appearing to be sometimes ahead of schedule and sometimes behind schedule, when compared with an accurate clock. For the most part, this error amounts to only a few minutes, but is most noticeable about 3rd November, when it reaches a maximum of 16 minutes and 20 seconds fast, when the sun is 'ahead of schedule, and about 11th February when it is 14 minutes 27 seconds slow, when the sun is 'behind schedule'. By applying this correction to the sundial, it would provide an accurate time-check for all clocks and watches in the vicinity.

It is not known who constructed this very remarkable sundial, but it is a fine example of the stone-mason's art, and a notable demonstration of the mathematical skills of the Art of Dialling. For this reason alone, it would be worthy of full restoration to working order: but, as part of our national heritage and the only known dial of its kind, it should receive proper treatment at the earliest possible opportunity.

Christopher St J. H. Daniel

Mr Daniel, formerly of the National Maritime Museum at Greenwich, is an authority on Sundials. He has written the Shire Album (176) and designed several sundials, including one on the tower of the church of St Margaret's Westminster.