Article of the month June

Sundials by projection

This model shows the connection between an equatorial, a horizontal, and a vertical sundial.

Consider such a model made out of transparent material. In general, we would see all the hour lines at once and running through each other.

It is another matter when we look at the model with one eye, from a point on the extended pole style (see arrow).
We see corresponding hour lines merge into one, as if only the equatorial pattern remained.
This is an illustration of the fact that the hour line pattern of a plane sundial is a projection of that of an equatorial sundial.

This shows the way towards realising a sundial on an irregular surface, for example on a beautiful stone or pebble.
After the pole style is fixed to the stone, a transparent equatorial sundial is mounted at right angles to it.
Then, using a point source of light on the extended pole style, the pattern is projected onto the stone, and the hour lines are copied by hand.
Another possibility is to project the hour line pattern using a slide projector.

This local apparent time sundial was made in this manner.