The noon mark

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 January 1986

In these days of radio and television time signals and the telephone speaking clock, it is salutary to consider how clocks were set before these innovations became commonplace. Greenwich observatory was disseminating time signals by time ball and telegraph, but how were clocks and watches set in the days before Greenwich? One answer is seen in figure 1 which is taken from La Gnomonique Pratique by Bedos de Celles, 1780. This picture has always fascinated me. It shows three gentlemen of the era waiting to check their watches at noon. Fixed to the south wall of the building on three supports is a circular disc with a hole in its centre. A sunbeam passing through the hole produces a spot of light on the wall. When the spot lies on the vertical line marked 'XII' it is noon local apparent time. It; is necessary to have a line on the wall not just a small mark, because the position of the spot will vary with the height of the sun, being close to the XII in summer and near the top of the line in winter. On this page last October I showed how to obtain clock time from apparent time.

The noon mark has the advantage of simplicity in that it can be put on any wall which catches the sun at midday - the wall does not have to face due south - and it requires no complicated calculations in its design as does a sundial. The price to be paid for this simplicity is that only the time at noon is shown, but this is no great disadvantage because clocks and watches can carry one through the other hours of day and night and through sunless days, and they did in the past.

Anyone can make a noon mark and it is instructive to do so. A plate with a small hole is mounted a few feet - the distance is not critical - from any wall with a southerly aspect, and a vertical line is drawn on the wall. The only precision requirements are that the line is truly vertical (check it with a plumb bob), and that the line and the hole lie on the meridian, that is, the aperture in the plate lies due south of the line. The details of design can be left to the skill and ingenuity of the maker, but it is important that the support or supports are rigid so that the plate does not move in the wind and is not easily displaced by accidental knocks.

Figure 2, which was taken four minutes before noon by the sun, shows a noon mark which has been set up on the south wall of my house. The problems of making and mounting the plate have been overcome by transposing the positions of the line and the perforated plate. The vertical line is a flag pole, and the 'hole' consists of two short black vertical parallel lines on the slanting weather board above the window. It is noon when the pole's shadow lies centrally between the lines. As the pole is supported by guy-lines with screw strainers it is easy to position the mast vertically, it being checked in two directions at right angles to each other by a plumb line.

Although the pole is about 9ft away from the wall and the shadow therefore has rather fuzzy edges I find that, with practice, it is easily possible to judge noon to an accuracy of plus or minus ten seconds. An idea of the speed at which the shadow moves can be gained from figure 3 which shows three close-ups of the shadow crossing the lines, taken at intervals of 20 seconds. Unfortunately my wooden pole is slightly bent and this introduces an error of about 15 seconds between winter and summer. To hold for all seasons the mast must be straight. I suppose I could calculate an Equation of my Mast!

In two books, I have read that, 'numerous examples of noon marks or noon dials are to be found on cathedral and church walls' (1) and 'Over the door or on the sill, or by a convenient window a mark was made to record the midday hour. Some of these may still be seen on the older houses in New England' (2) (my italics). I have seen and photographed hundreds of sundials and scratch dials but have yet to see a noon mark. It would be interesting to hear from readers who have come across and perhaps photographed such marks.