Universal equinoctial dial for all latitudes. It has a shaped wooden base and back support. The main brass disc of the instrument is bolted through its centre to the support and is free to revolve in a vertical plane. This disc has another rotatable disc set within it. One semicircle of the outer disc is hinged and can be moved to measure the azimuth of the sun. Behind this semicircle are scales for correcting the observed altitude of the sun and for the equation of time.

The outer disc has a hinged semicircle and the inner disc also has a hinged armillary semicircle. The rest of the inner disc is engraved with a globe. There are two fixed armillary semicircles perpendicular to the main disc, one attached to each disc. The maker’s signature, ‘Ferguson London’ is engraved on the declination semicircle.

This instrument was designed by Mungo Murray, a shipwright, and was first seen by James Ferguson in 1757. Ferguson later made his own version and wrote a short tract about its use. The tract explains many uses for solving problems of astronomy and spherical trigonometry, and for constructing pedestal horizontal dials and mural dials.

For more information regarding this dial please refer to the OUP & NMM catalogue, 'Sundials at Greenwich'.