Time

Sundial

4.8 - Understand the structure and use of sundials

The purpose of a sundial is to tell the time of day based on a shadow the Sun causes on its surface.

The sundial is typically a horizontal or vertically aligned disc that has points around its surface marking time.

In the middle is a large protrusion called a gnomon. It is this that causes the shadow to fall on the disc. The angle of the gnomon should be set to the latitude of its position. The angle of this gnomon is important as it can make the difference between a correct and incorrect reading.

Sundials usually have a decorative inscription on them with a time or weather-related theme.

Sundials have been used for centuries to tell the time but telling time by the position of the Sun is a difficult process and there are two disadvantages to them:

They are useless when no Sun is visible

The time they tell is usually slightly inaccurate to the casual observer due to the equation of time and require special understanding. We do this by a formula called the equation of time.