A sundial is a clock that uses the position of the Sun to indicate the time. Typically a rod casts a shadow upon a plane or surface. On this surface are found markings that indicate the time by the position of the shadow.
There are several types of sundials. Each type serves the same purpose: to tell time by the position of the Sun. But the methods used to extract the data from each sundial are different.
The definition of 'sundial' referred to in most dictionaries is inadequate. In a well-known dictionary, one reads the following :
'A chronometer which shows local time through the position of the shade produced by a rod illuminated by the Sun. The rod must be positioned parallel to the Earth's axis. Therefore, the inclination angle in reference to the level of the horizon is identical to the geographical latitude of the position.'
In addition, a description of an equatorial, a horizontal, and a vertical sundial is provided in the dictionary. However, there are a number of 'special types of sundials', which the above definition does not cover or only covers inadequately.
In our website, we will introduce different types of sundials. However, in order to have a comprehensive understanding on sundials, we need to be familiar with some of the terms used in astronomy. Also, some background knowledge on astronomy is needed before we can appreciate the indigenous ideas behind sundials which allow us to tell time since ancient times. These terms and concepts are covered in the "You Need to Know" and "Background Knowledge" sections.