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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 December 1985
Three topics will be discussed briefly this month. First, how any horizontal sundial can be moved to a different latitude and be made to function correctly. Figure 1 illustrates a sundial purchased in the USA, designed for a latitude of 43 degrees north, and now in London at latitude 52 degrees. It is mounted on a wedge-shaped slab which was cast in the mould shown in figure 2. The base of this mould is a piece of hardboard (or ply-wood) tacked to four pieces of wood which are joined to form a square, wide at the back, narrow at the front, and slanting each side. The angle of these slanting pieces is nine degrees, being the difference between the latitude of the place for which the dial was designed and the place where it is to be used. The short length of broom handle in the centre of the mould is to provide a hole in the concrete for the securing spigot on the underside of the sundial.
The wedge - it does not have to be concrete: any suitable material would do - is mounted on an accurately levelled surface with the narrow edge facing south. If the sundial is being transferred from a higher to a lower latitude then the narrow edge must be turned to face north. In fact, if the wedge of figure 2 were placed with the wide edge facing south then the dial shown would function correctly at a latitude of 34 degrees north (43-9=34). The function of the wedge is to tilt the dial so that the style lies parallel to the axis of the earth. When setting a dial to a new angle the gnomon must still be aligned with the meridian, that is, it must lie due north-south.
The linear movement of the dial from its old to its new location does not affect its time-keeping. The earth's diameter of 8000 miles is negligible compared with the 93,000,000 miles to the sun, and the dial will show the correct time anywhere on earth provided it is properly oriented.
The second topic also involves angulation, but in a vertical direction. Compared with a declining sundial, a direct south-facing sundial is much easier to design; the gnomon lies vertically on the noon line and the hour lines on each side of it are symmetrical. If such a dial is put flat on a wall which does not face directly south then it will not function correctly. The dial must be set at an angle as seen in figure 3 which shows the direct south-facing dial on the south wall of the church of St Aldhelm, Doulting, in Somerset. Because this wall faces 12 degrees west of south, the angle which the dial makes with the wall is also 12 degrees, but in the opposite direction.
Angled vertical dials can either be set in the wall or they can be fixed to the wall by specially made brackets. They are not uncommon and a few places where they may be seen are o the churches at Boreham in Essex (where there is a table of the Equation of Time inside the church), Thornhaugh in Cambridgeshire, Hadingham in Suffolk, Bledlow in Buckinghamshire, Beckley in Oxfordshire, Broad Hinton in Wiltshire, Stocklinch in Somerset, Milton Abbot in Devonshire and Michaelstow in Cornwall.
The third topic is the incorporation of longitude correction in the hour lines of a sundial, so that only the Equation of Time has to be taken into account when calculating clock time. Figure 4 shows a delightful modern sundial made by Richard Jones for the Methodist Church Centre at Bromsgrove, Warwickshire. Because Bromsgrove is two degrees west of Greenwich, local solar time of Bromsgrove is eight minutes later than that at Greenwich. Therefore in making the dial the positions of the hour lines have been calculated for eight minutes past each hour and then labelled as hours. That this dial has been corrected for longitude is evident from the fact that the noon line (one o'clock BST) is not vertical - compare it with the vertical sides. The noon line on any wall sundial showing local solar time is always vertical.
A similar correction may be made to a horizontal dial by repositioning the hour lines. It cannot be done rotating the dial plate; this procedure is only possible when all the hour lines are equally spaced, as they are on an equinoctial (equatorial) dial.