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Disc 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 September 1987

On reading sundial literature one not infrequently comes across types of dials which are covered in one book only. One such is the disc dial. In may ever-growing sundial library, catalogues apart, I can find it described only in Positional Astronomy and Astro-navigation Made Easy by H R Mills, a book whose title belies its usefulness to the diallist, for it contains a wealth of theoretical and practical sundial information.

In my recent delving into the British Museum treasures which are not on exhibition I came across the disc dial illustrated here (Catalogue number 176). It is an altitude dial and functions in the same way as an altitude ring or pillar dial.

It is brass, French or German, 18th century, with an irregular outline, measures 62mm high and 48mm across, and ahs a suspension ring passing through a hole at the top. It is designed for use in a fixed latitude of about 48 degrees.

The obvrse (figure 1) has a double scale marked with the initial letters of the month, each month being divided into ten-day interval. Moving over the scale is the pointed ends of an alidade pivoted near its centre. An alidade (sometimes Alidad) is a revolving arm for sighting or taking angular measurements on an astrolabe, quadrant, or similar instrument.

The other end of the alidade carriers a hinged pin gnomon, folded in figure 1, which can be swung round to project on the reverse side of the disc (figure 2) where it moves against a scale of seven marks corresponding to the seven declination lines sometimes found on horizontal or vertical sundials.

In use the dial is suspended by the ring and rotated until it is edge on to the sun so that the shadow of the gnomon falls across the irregularly curved hour scale marked with dots for the hours and short lines for the half-hours.

As in any altitude dial, apart from noon when the sun culminates, each point on the hour scale corresponds to two times of day equidistant from noon, one as the height of the sun is increasing and the other as it is decreasing. If there is doubt which of the two readings is correct then a second reading ahs to be taken after a short time interval to determine the direction of the sun's change in height.

Here I should perhaps digress to explain that culmination (Latin, culmen - summit) is a term used by astronomers to indicate that any heavenly body has reached its highest altitude or, what is the same thing, that it is on the meridian.

Compared with the ring dial, the disc dial with its gnomon folded for storage is more compact. Not all disc dials have alidades; that described by H R Mills has a fixed pin gnomon. To allow for the sun's declination on this dial the curved hour lines, like those on an altitude pillar dial, are drawn across a series of vertical date lines. The time is read at the point where the shadow of the gnomon crosses the line, interpolated if necessary, corresponding to the date.