A sundial is an instrument that is used to determine the time from the sun by using an indicator called a 'gnomon' to cast a shadow or to project a spot of light on to a graduated surface. It differs from accurate clock time by a small amount known as the 'equation of time', caused partly by the varying speed at which the earth travels in its path around the sun, and partly because the earth's axis is tilted.
© Islington Artefact Library
Sundials were first used by the ancient Egyptians in about 800 BC. For centuries in Britain vertical sundials were the most common type and were mounted on the outside of buildings, just high enough so they were out of reach. They were used to regulate public clocks and to indicate time to the passer-by. Many of the oldest sundials have been marked to divide the period of daylight into 12 'hours'. Because there are more daylight hours in summer than in winter, an hour in summer was longer than an hour in winter. It was the invention of the mechanical clock that eventually led to our days being divided into hours of equal length.
Because sundials rely on the position of the sun to tell the time, the numbers are arranged in a clockwise direction on sundials in the Northern Hemisphere, whereas in the Southern Hemisphere they are arranged anticlockwise and the gnomon points in the opposite direction.