Work of our members

A compact disc may gleam in all the colours of the rainbow, depending on nearby light sources. The vague bands or narrow stripes all go through the centre of the disc. Hardly anyone notices now. Years ago, though less obvious, this diffraction phenomenon was visible on gramophone records.

If we place a CD on a sunlit horizontal surface, the reflection is a sharply delimited, brightly-coloured line. Its direction depends on the location of our eye and the azimuth of the sun. With one eye straight above the middle of the disk, the line indicates the azimuth of the sun. We may draw a scale of degrees, with 0 to the south, on the edge of the disk (in gnomonics, azimuth is measured from the south towards the west).

The idea of using a CD for a sundial is obvious. Mario Catamo and Cesare Lucarini, among others, developed it for a horizontal, a vertical, as well as an equatorial sundial. They published it in the journal of the North American Sundial Society (The Compendium, Sept. 1999, pp 19-23).

The equatorial CD-sundial

The disc with hour numerals for an equatorial sundial.

The equatorial sundial is quite simple: all hour lines are 15 degrees apart. All we need do is draw a scale of hours on the edge and add the proper numerals (see photo). A finer division is of course possible.

Catamo and Lucarini did not describe a suitable construction for the sundial. I used an empty ‘jewel case’ CD-box and snapped the hour disc into it. It is now easily rotated for longitude, EOT and DST adjustments.

On the outside of the lid we apply the letters N (North) and S (South) to facilitate orientation (for example using a compass).

Now the lid should be fixed at an angle of 38 degrees (for our latitude) to the bottom of the box, so that the hour lines are in the plane of the equator.

To read the time, one must look with one eye, perpendicular to the hour disc. This is not difficult, because the disc reflects the face.

The disc fitted in the jewel-case lid.
The sharp line marks the time; it is ten to one local apparent time.

An inconvenience of equatorial sundials with a flat hour face is that in the winter period the time must be read on the underside of the dial face. That is not necessary here. If you use a blank CD, you can see the light through the top.

Horizontal and vertical versions

Horizontal and vertical CD-sundials are less simple. On the hour line face, a network of date lines and hour lines should be drawn. The hour lines are curved rather whimsically, and one should read the time on the intersection of an hour line and a date line. As an example, we show the network from the Catamo and Lucarini paper.

The network of date and hour lines on a horizontal CD-sundial.
Latitude 42 North.

Hans de Rijk