SUNDIALS - How do they work?
The first diagram shows how the Sun appears to move across the sky from East to West during a given day, causing continual changes in both the length and position of the shadow cast by any solid object. The seasonal change in the sun’s height above the horizon causes further changes in the shadow’s position.
From the earliest times mankind has used the movement of shadows produced by the apparent movement of the Sun for the reckoning of time and for the determination of important days such as religious festivals and when to plant crops for the following season. Any device which uses the SUN to divide the period between sunrise and sunset into units of time is known as a SUNDIAL.
The second diagram above shows how a sundial is located on the surface of the Earth. We see a shadow only when it falls onto a surface, and clearly the position of the receiving surface will affect the direction and length of the shadow. Most sundials tell the time by measuring the direction of the shadow cast by the sun using divisions marked on a dial plate set at some known angle. As the sun moves across the sky then the shadow of the gnomon moves across the scale around the dial plate. (A gnomon is the part of the sundial which produces the time-telling shadow.) Less commonly, time can also be told by measuring the changing length of a shadow.
The various sundial types are usually classified according to the orientation of the dial plate upon which the hour lines are marked. The dial plate can be horizontal or vertical or somewhere in between (known as reclining, or polar in the special case where the angle of recline is equal to the latitude angle). The dial plate can also take the form of a circular ring (armillary sphere sundial) or part of a ring (equatorial sundial) whose plane is perpendicular to the gnomon. All these sundial types tell the time from the direction of the shadow.
The pillar sundial tells the time from the length of the shadow. The positions of the hour lines are found using formulae derived from spherical trigonometry.
Variation with the seasons of the Sun's daily path across the sky, as viewed from Earth in the Southern hemisphere.
Sundial location on the surface of the Earth at 35 degrees South
Some of the great variety of sundial types available