More than 3,500 years ago, people realized that the sun could be used to tell the time of day. As a result, they learned how to construct instruments called Sundials.
How it works
A shadow cast by the Sun indicates the time of day on a Sundial. On a sunny morning, set the Sundial in direct sunlight, with the line on the board running from north to south. A magnetic Compass is needed to get the direction. (If you don't have one, position your board on a cloudless night and line it up with the Polaris). The length of the shadow that falls on the Sundial and the indications on the Sundial will tell the time of the day.
Although the Sundial is very simple to make and to use, it also has many disadvantages. A Sundial is no longer accurate after a month. This is because the obliquity of Earth causes the 'path' of the Sun to change over the months. The same Sundial cannot be used in two different places. The Sun has different 'paths' for two different places .
Diagram of the Sundial