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Equinoctial (or equatorial) 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 May 1987

The equinoctial is another name for the celestial equator and equinoctial sundials are so called because their dial plates lie in the plane of the celestial equator. The word 'equinoctial' can be used either as a noun or as an adjective. Because the celestial equator is a projection of the terrestrial equator on to the celestial sphere it follows that the dial plates of equinoctial sundials lie also in the plane of the earth's equator. They are therefore frequently referred to as equatorial sundials. Thus there are two names for the same thing - one of those little annoyances which confound the beginner!

So which term should be used? To resolve this and other sundial nomenclature problems, in 1970 the British Museum set up a committee representing museums in the United Kingdom. The names of the museums and the outcome of the meetings are given as an appendix in A Catalogue of European Scientific Instruments by F A B Ward, published for the Trustees of the British Museum. The committee recommended the term 'equinoctial' because this was used by the majorityof writers in the 17th and 18th centuries.

An example of a universal equinoctial sundial is shown in figure 1. It is universal because the mounting allows the angle of the whole instrument to be adjusted: at the poles the circular dial plate would be set horizontally, at the equator vertically and at the latitude of London in the position shown.

The style, which is always exactly at the centre, is perpendicular to the dial plate and therefore lies parallel to the earth's axis. Because the earth rotates at a constant speed (constant for diallists, not for precision astronomers!) of 360 degrees in 24 hours, all the hour points are equally spaced 15 degrees apart. It is therefore an equiangular sundial. This has the very great advantage that adjustments for longitude, the Equation of Time, and summer time can be made by rotating the dial plate one degree for every four minutes adjustment.

Rotation in the plane of the dial plate cannot be carried out on vertical and horizontal dials because their hour lines are not equally spaced. For these dials correction can be made by rotating them about their styles at the rate of one degree for every four minutes, but this will result in their dial plates assuming unconventional and undesirable angles!

The equinoctial dial of the type shown in figure 1 has two disadvantages. First, it does not function at the equinoxes because the sun is then edge on to the dial plate disc which therefore cannot have a shadow cast upon it by the style. Second, from a few days after the autumn equinox to a few days before the spring equinox, the sun will be below the plane of the disc. If the lower side has been calibrated, readings can be taken but not without a certain amount of contortion! For the other half of the year all is well, for the sun will be above the plane of the disc.

Near the equator conditions are better. between the march and September equinoxes the sun will shine o the north side, the hour lines running clockwise, while for the other six months of the year the sun will shine on the south side, the hour lines running anticlockwise. But the dial is still functionless at the equinoxes. Note that the dial of figure 1 cannot be used in the southern hemisphere unless the hour lines are renumbered to run anticlockwise on the upper surface of the dial plate.

The disadvantages mentioned can be overcome by having a ring instead of a disc on which to cast the shadow, with the plane of the ring parallel to the style, as seen in figure 2 which shows a simple equinoctial dial exhibited at the Wembley Engineering Exhibition last January. It is designed for the latitude of London and therefore is not universal.

The dial ring is a semicircle and therefore covers a range of only 12 hours. This range can be increased by making the dial rings more than half a circle; in an armillary sphere, which is an equinoctial sundial, it is often a complete circle. However, one then runs into the problem that at the equinoxes the front part of the ring casts a shadow on to the back part, eclipsing the shadow of the style.

It is possible to overcome this problem by tapering the ring, making the front narrower than the back, and engraving the hour lines across the whole width of the ring. Then at the equinoxes some light will fall on at least part of each hour line.