A sundial can be used to measure time. Time measurement is based on the position of the sun. The type of shadow that is cast by the sundial will determine the time, as the shadow will hit specific lines on the dial that relate to time. As the sun moves across the sky the shadow will align with different marks. In order for a sun dial to tell the correct time is aligned with the earth’s rotational axis.

There are many different forms of sun dials. Several different versions include the equatorial sundial, horizontal sundial, vertical sundial, polar dial, vertical declining dial, reclining dial, and spherical sundials. The moist well known sundials use shadows to tell time. Others may use a line of light to tell time or some will allow a spot of light to shine through a small hole. Shadow sundials will use a thin rod or other object, known as a gnomon. This gnomon is the piece that casts the shadow. Sundials may be fixed in place of portable. Fixed sundials must be installed properly, taking into account the current latitude and direction of true north. Most portable sundials are self-aligning and can be read properly at any location.

A sundial works on the principle of the suns movement or the apparent movement of the sun. In actuality it is the earth that is moving around the sun, while the sun is actually stationary. For a sundial to work scientists approximate that the sun “moves” around the earth once every 23 hours and 56 minutes. If the gnomon used in the sundial is aligned with the north and south poles then the shadow will move at a constant rate, with the time markings all at the same distance apart. This is the most common design of sundial. However if the gnomon is not aligned with the poles then the time markings will not be the same intervals apart as a correction factor is required. The shadow will not trace out a perfect circle but instead will make an ellipse or hyperbola.

The very earliest sundials have been dated to 3500 BC. These were obelisks and shadow clocks that were the foundation of Babylonian and Egyptian astronomy. One of the oldest descriptions of a sundial is found in the Old Testament of the Bible. The “dial of Ahaz” is mentioned, referring to a sundial. In 1570 an Italian, Giovanno Padovani, published an entire document on sundials including instructions and diagrams on vertical and horizontal dials. As it was quite clear that sundials were used extensively during Roman times.

Look to the following resources for more information on sundials.