How a Sundial works

Sundials record the passage of the Earth round the Sun. That’s what they do - accurately and precisely - they can’t help being accurate. So does anything that casts a shadow. They also help us tell the time, some more accurately and precisely than others; this depends on how accurately their markings are made. The huge variety of forms and styles of sundial all work according to the same principle. A shadow is cast by a rod or the edge of a plate, or a variation on these, onto a surface which is marked in some way to track the progress of the shadow over time.

how-sdial-1This rod or plate is called a Gnomon. This is set at an angle to match the latitude of the sundial’s location, and, in the northern hemisphere, is orientated to point due North. This means that the Gnomon is parallel to the Earth’s axis and points directly at the North or Pole Star.

Because the Pole Star is so far away the minuscule change in angle as the earth rotates and orbits the Sun makes no visible difference, and whatever the time of day or year, the Gnomon will always point at the Pole star, though this, of course, is only visible at night!

There are several styles of sundial in common use. At first sight the variety can look mind boggling, but most of the sundials on this website are variations on these main types:

The Horizontal Dial

This is probably the most familiar to most people; a flat table with either a rod or a triangular plate forming the gnomon. This is a very easy style to position as all it needs is to be pointed in the right direction. There is no need to adjust the configuration to suit the location as long as it has been made to suit the latitude.

A good example of this type of dial can be seen here:




This is almost as familiar as the Horizontal dial - usually wall mounted on a broadly south facing wall (though it can be configured to suit a wall facing in almost any direction). It is essentially the vertical projection of a horizontal dial. Because it needs to be configured to suit the exact angle the wall faces there is a little more to setting it up and it is less easily moved to a new location than a Horizontal dial.


A good example of this type of dial can be seen here:




This dial has a rod for a gnomon which is set at the correct angle for the latitude and pierces a disc at its centre. The gnomon replicates the Earth’s axis as with all other sundials, and the disc replicates the Equator. The disc is then marked with time markings on both sides.


In this way the shadow from the gnomon falls on the top surface of the disc in summer when the sun is high, and on the underside of the disc in the winter when the sun is low. At the two midway points (the Autumn and Spring Equinoxes) the sun will move round the rim of the disc parallel to the Equator and both sides of the disc will be in the shade.


A good example of this type of dial can be seen here:




This dial takes the form of a sphere set on an axis matching the latitude of its location, thus replicating the Earth in miniature, so as the Sun is orbited by the Earth, the light that falls on the sphere moves across its surface in exactly the same way as the Earth itself, so in summer the sun, arcing high, will shine directly on the northern hemisphere of the sphere, and in winter, arcing low, it will shine on the southern hemisphere.



The band round the middle of the sphere, corresponding to the Equator, is marked out with time markings and to form a gnomon a rotating hoop or bracket is fitted to the top and bottom of the axis rod. To tell the time this hoop gnomon is rotated till its shadow is at its narrowest, and a reading can then be taken from the time markings round the sphere.




A good example of this type of dial can be seen here:

website by clooti