Theory of Sundials

The principle of sundials is simple

As the earth rotates the sun seems to move across the sky. So the shadow of a gnomon (the shadow caster) will move across the hour band of a sundial. The sun travels 15 degrees west per hour so the shadow of the gnomon on the sundial plate moves at the same rate. By noting the hour line closest to the edge of the shadow you read the time (or centre of the shadow on an armillary sphere).

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

You may notice sundials do not always agree with your watch. The reason for this is complicated but as long you accept the maths the principle is simple.

The earth orbits the sun at some 93 million miles distance, orbiting once a year. The earth's axis 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 on a regular and predictable pattern every year so as everyone lives on the same earth it shouldn't matter.

Problems started when clocks were first made. It proved impossible to design a clock to keep irregular time. So the days were averaged out into Greenwich Mean Time (Mean being Average).

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

Sundials and the equation of time

Using the sun's light to cast a shadow onto the hour lines, sundials 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 the sun to travel one degree, known as Longitudal Variation.

Equation of Time

The sun varies in its timekeeping through the year sometimes 'losing' and sometimes 'gaining' - known as the Equation of Time.

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

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

Jürgen Gieseno's Sun Declination and Equation of Time.