Accession No

6047

Brief Description

Wheatstone's Polar Clock, polar sundial, by Watkins and Hill, English, 2/4 19th Century

Origin

England; London

Maker

Watkins and Hill

Class

dials

Earliest Date

1819

Latest Date

1856

Inscription Date

Material

metal (brass, lead); glass; liquid; stone (calcite, selenite)

Dimensions

height 179mm; diameter 88mm (base)

Special Collection

Provenance

Purchased from Tesseract, Box 151, Hastings-on-Hudson, New York, 10706, USA on or before 26/05/2005. Tesseract catalogue 79, Spring 2005. Purchased with the assistance of a grant-in-aid from the PRISM fund.

Inscription

'Watkins & Hill
Charing Cross.
London' (on base)

Description Notes

Wheatstone's Polar Clock; brass polar sundial by Watkins & Hill; English; 2/4 19th C

Lead-weighted base with three levelling screws, two perpendicular spirit levels, 8 point compass labelled 'N,NE,E,SE,S,SW,W,NW' divided to 5°. From the base extrudes a long arm whose vertical angle is adjustable on an arc scale labelled 90-0, numbered by 10, divided to 1°. At the top of the arm is the optical sight, comprised of a brass cylinder containing the eyepiece and a removable piece containing a selenite crystal. The eyepiece can rotate about a semicircular scale, labelled VI-XII-VI, numbered every hour, divided to 5 minutes, and can be unscrewed to detach from the rest of the optical sight.

Condition: good, some surface loss to hour scale; complete

References

Events

Description
The "polar clock" is an invention of Charles Wheatstone. It is based on the principle that sunlight is polarised by the atmosphere, and the maximum effect of this is observable at the celestial pole. As the earth rotates and the sun appears to move across the sky, so does the direction of the polarisation of the sunlight. It is this effect that is used to determine the time of day: when the eyepiece is rotated to the point when the sky appears darkest (corresponding to maximum polarisation) the time of day can be read off the scale.

Wheatstone argued that his "polar clock" had three advantages over standard sundials. The observer only needs a view of the celestial pole, which stays still in the sky, unlike the sun. The "polar clock" works for a while before sunrise and sunset, when the sky is lit but the Sun not visible. Finally it even works with a small amount of cloud cover.
12/07/2005
Created by: James Hyslop on 12/07/2005

FM:46516