Article of the month January

Regiomontanus Card Dial

A well-known sundial is the Regiomontanus Card Dial, which determines time from an altitude of the sun. The user should know date and latitude.
This dial is universal, i.e. it may be used in any latitude between 0 and 66.5 degrees.

Its triangular shape, with horizontal date scale and sloping latitude scale, is characteristic.
A brachiolus, or small arm, serves to position a weighted string on the correct date and latitude.

With the string held taut, the bead is moved to reach the correct point on the second date scale beside the hour lines.

With the card dial in a vertical plane, the sun altitude is measured using the sight on top.
The bead will now indicate time on the hour lines. The user must know whether it is before or after noon.

Card dials like this were used as portable sundials, and were aimed at the sun while handheld.
The photograph below shows a table model, which was easier to use.
The time was 7 AM or 5 PM.