Question

Sundial is used to measure time in some monuments. Explain the basic principle used to construct a sundial.

Answer 1

A sundial is a device that tells the time of day by the apparent position of the Sun in the sky. It consists of a flat plate (the dial) and a gnomon, which casts a shadow onto the dial.

As the Sun appears to move across the sky, the shadow aligns with different hour-lines, which are marked on the dial to indicate the time of day.

Sundials work from the shadow movement of a fixed object based on varying positions of the sun throughout the day. Imagine a stick placed vertically at the North Pole. As the Earth rotates on its axis, the shadow of the stick traces a full circle in a period of 24 hours. The relative location of the sun keeps changing by 15 degrees every hour for any point on the Earth.

The basic sundial has a circular base with markings for hours, minutes, and seconds. The stick or flat-edged plate at its center is a gnomon. When it is pointed towards the north, its shadow position on the dial determines the current time at a location.

Answer 2

The sundial is an instrument which uses the Sun's altitude or azimuth or both to show the time. It is based on the projection of a shadow that is cast by a gnomon onto a surface on which hour lines have been drawn.

The gnomon can be a rod, wire or an elaborately decorated metal casting. The style is the time-telling edge of the gnomon. It must be parallel to the axis of the Earth's rotation so that the sundial can work accurately throughout the year. Its angle from the horizontal is equal to the sundial's geographical latitude.