How sundial works.
The basic idea is simple. A sundial has a pointer – called a gnomon – and the shadow cast by the gnomon when the sun shines follows a predictable path through the day. The path can be marked with hours to tell the time.
Sounds too simple? Well, it is a little bit more complicated than that.
The trouble is that the predictable shadow path changes through the year. There are two reasons for this:
- The earth follows an elliptical orbit around the sun
- The earth’s tilted axis
As a result, the shadow cast by the gnomon moves at different speeds and to slightly different places at different times of the year. Dialists – sundial makers – call this “solar time”.
Luckily, the difference between solar time and clock time can be corrected with maths. Dialists use angled gnomons and the “equation of time” to correct their sundials. See below for links to explanations of this.
There is a further correction that needs to be made if a sundial is to match clock time.
Modern time zones (for example the Greenwich Mean Time zone) tell the time for large areas. The shadow cast by the gnomon of a sundial tells the time for the exact position of the sundial.
So the hour lines on a sundial need to be adjusted so the shadow tells the time for its time zone instead of its exact position. Unless, of course, a sundial is set exactly on the Meridian.