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. There is sometimes an interpretation chart on it as well.

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

Sundials usually have an inscription on them with a time or weather related theme. Some are classic and thoughtful, some are cheesy.

Sundials have been used for centuries to tell the time but there are two disadvantages to them:

  1. They are useless when no Sun is visible
  2. The time they tell is usually inaccurate to the casual observer due to the equation of time and require special understanding. This problem is addressed in the equation of time page.

Telling time by the position of the Sun is a difficult process.

We do this by a formula called the equation of time

The Sun does not move relative to Earth, the changing of the Sun's position is due to the movement of the Earth.

We will see what the equation of time is by looking at how it affects a sundial.


- What are the disadvantages of using a sundial to tell the time?
- What is the reason for a precise angle of a gnomon on a sundial?