Shadow stick

The shadow stick is probably the earliest and simplest form of sundial. As the sun moves across the sky so does the shadow and a graduated scale can be used to indicate the time.

The problem with this as a time keeper is that the sun does not move across the sky at a constant rate throughout the year. It could be set up in the summer but it would not work in the winter.

Equatorial Sundial

The solution is to point the pole or style towards the pole star, i.e. it will be parallel to the axis of the earth. The shadow will then move at a constant rate with the earth’s rotation. Every 15o on the dial will represent one hour.

There are problems with this dial as well. At RLS with a latitude of 52o N, the sun would have to be at an altitude of at least 38o to be able to cast a shadow on the dial. This means that for half of the year the dial cannot be used. In Britain this runs from 21st September to 21st March.

X = Observers Latitude

Sundial

To make a sundial which will work throughout the year, we must keep the style at an angle equal to the latitude of the observer, but the dial must be horizontal. The dial can then be calibrated to take into account the changing rate of sun across the sky.

The sundial will give us local time, not GMT. To find the GMT time we must know our longitude.

For example, the RLS has a longitude of about 1o. This equates to a time of 4 minutes. We are west of Greenwich and are hence running behind. If our sundial shows 10:00 it will actually be 10:04 GMT.

Sundials can be both vertical or horizontal. In each case the style must point towards the pole star.

Prague

Perranzabuloe Millennium Sundial

The Samrat Yantra in Jaipur, India is the largest sundial ever built