Azimuth compass sundial, 1731-1776.
© Science Museum / Science & Society Picture Library
Wooden dyptich sundial made by the well known 18th century mathematician and astronomer, James Ferguson (1710-1776). The invention of the magnetic compass in the 13th century made possible the development of portable direction dials which required accurate orientation along the meridian. This later example's string gnomon (which would cast its shadow on the sundial's marked surface, indicating the time), is missing.