Pocket sundial, Chinese, c 1900-1925.
© Science Museum / Science & Society Picture Library
This wooden pocket sundial includes a compas and folding bras gnomon, which here is a hinged pin set at right angles to the plane of the dial, the inclination of which can be varied. At the summer solstice the dial plane is at its maximum inclination and is approximately in the equatorial plane. If it were kept in this plane no reading could be obtained when the sun was south of the equator, but as the sun moves southwards the inclination of the dial is decreased until it is a minimum at the winter solstice.