Altitude sundial, English, c 1570.
© Science Museum / Science & Society Picture Library
Most sundials work by charting the sun's movement using the shadows cast by a fixed style or gnomon, but an altitude dial depends on the Sun's altitude (or height above the horizon) to tell the time. The dial is calibrated with date marks as well as time marks, and the user must know these before pointing the gnomon directly at the sun and measuring the angle of the height of the sun from the shadow of the arc on the scale. This example was made in silver by Humphrey Cole (d 1591).