Mean time dials

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 1986

A Sundial which reads mean time directly must incorporate adjustments for longitude and for the Equation of Time. Longitude was discussed on this page last June. As most mean-time dials are of the equinoctial type, which are equiangular, allowance is easily made by rotating the dial plate about its polar axis by an amount equal to the longitude. Equiangular means that all hour lines are equally spaced.

In a few mean-time dials the Equation of Time is allowed for by a daily adjustment to the position of either the hour scale or the gnomon, but in the majority of such dials use is made of the analemma, explained last month, whose characteristic curves can be incorporated either in the hour lines or in the design of the gnomon. I show an example of each, both of them equinoctial.

Figure 1 shows part of the dial plate of the Dolphin Jubilee Sundial at the National maritime Museum, Greenwich. Here all the lines, which are at ten minute intervals, are identical in shape and are one half of the analemma, the dotted part of the curve illustrated last month. At the summer solstice the dial plate is changed for one in which each curve is the solid line part. Six-monthly changing at the solstices is usual for all dials with analemmatic hour lines as it ensures that the correct half of the analemma is sued. The whole analemma can be engraved for each hour line but then the correct half ahs to be selected. Furthermore, because the total variation in the Equation of Time is about half an hour, if intervals of less than half an hour are shown, as in figure 1, adjacent analemmas overlap, producing a muddling jumble of lines.

The time is read by the position of the gap between the shadow of the tail fins of the dolphins. The gap acts as a nodus and ensures that the correct point on the curves is sued, according to the date. I took the photograph at 1.56 pm GMT on 26th April.

Figure 2, taken in the London Science Museum, shows a mean-time dial in which the analemma is cut away from the centre of the gnomon, seen in detail in figure 3. The edges of the cut-out form the style, their shadows falling on the semicircular equinoctial hour scale. Some of the months are shown round the periphery of the cut-out to indicate the correct edge. The narrow hour scale acts like a nodus and ensures that the right part of the analemmatic curve is used according to the date.

A disadvantage of this dial is that the gnomon has to be turned to face the sun each time a reading is taken - the gnomon is mounted on pivots lying parallel to the earth's axis. One way to overcome the nuisance of having to adjust the gnomon is to make it solid and everywhere circular in cross section. The outside edge instead of the inside edge then becomes the style.

This type of gnomon has two disadvantages. First, the correction for the Equation of Time cannot be reduced to zero; to do so the gnomon would have to have no thickness at that point and would fall apart! Second, two gnomons are required, one for each half of the analemma, and they are interchanged at the solstices.

Sundials which show apparent solar time function infallibly year in and year out without attention. Mean-time dials need some intervention by man and are therefore by no means foolproof!