Theory of Sundials

Sundial Principles

The principle of sundials is simple. As the Earth rotates on its axis, the sun appears to move across the sky. So the shadow of a gnomon (the shadow caster) will move across the hour band of the dial. The sun travels 15 degrees westward every hour; the gnomon's shadow on the dial plate moves at the same rate. You read the time by noting the hour line closest to the edge of the shadow (the centre of the shadow in the case of armillaries).

For a sundial to work correctly the angle of the gnomon and hour lines must be calibrated for the latitude in which the sundial is being used. All Border Sundial pieces are made to the correct latitude for where they will reside. They can be made for different areas and suitable for all countries of the world, (horizontal sundials do not work close to the equator).

You may have noticed that sundials do not always agree with your watch, the reason for this is extremely complicated, but as long you accept the maths the principle is quite simple.

The Earth orbits the Sun at a distance of some 93 million miles, making an orbit once every year. The axis of the earth always points in the same direction (towards Polaris, the pole star).

The earth moves around the sun in an ellipse (egg shape). As the earth approaches the sun it takes less time for the rotation to occur. Therefore the sun appears to rise slightly earlier; but as the earth recedes from the sun the rotation takes longer and the sun appears to rise slightly later.

This happens in a regular and predictable manner every year, so as everyone lives on the same earth it shouldn't matter.

Problems started When the first clocks were made problems began. Proving impossible to design a clock that could keep irregular time the days were averaged out into Greenwich Mean Time (Mean being Average).

So if you are told that your sundial's not telling the right time, point out that it's the clock which is wrong!

Sundials and the equation of time

Sundials use the light from the sun to cast a shadow onto the hour lines, so they tell 'Local Apparent Time' rather than 'Mean Time' as conventional clocks do.

For each degree of Longitude to the West of Greenwich the dial will appear to be four minutes slow, and for every degree to the East it will appear four minutes fast.

This is the time it takes for the sun to travel one degree, known as Longitudal Variation.

The sun also varies in its timekeeping throughout the year, sometimes 'losing' and sometimes 'gaining'. This is known as the Equation of Time.

Equation of Time

By adding or subtracting the Longitude Variation and the Equation of Time you will be able to read GMT at any position in the country.

Sundials do not understand British Summer Time so in the summer an hour needs subtracting from your reading.