The Regiomontanus sundial - AstroMedia

Sturdy AstroMedia cardboard kit for a universal sundial to measure True Solar Time between Oslo and Cairo.

Difficult problem, resourceful solution
This sundial was invented as early as the 15th century by Johannes Regiomontanus, the famous astronomer and mathematician from Königsberg in Lower Franconia. It is a typical example of his ability to respond to a difficult problem with an ingenious solution: in terms of its basic type, his solar chronometer belongs to the group of high-altitude sundials. However, these must always be constructed for a certain geographical latitude and become increasingly inaccurate further south or north. This gave Regiomontanus no peace of mind: he invented a special line arrangement on which the True Local Time could be determined for all latitudes between 30° and 60° with the help of a movable pointer and a plumb line. In addition, the Regiomontanus sundial also allows the determination of the rising and setting times of the sun for any date and place. With the plumb quadrant on the back, the sun's altitude (distance angle above the horizon) can be easily determined.

How it works
Find the horizontal line corresponding to the latitude of your location. Find the date in the upper date scale and trace its line downwards. Place the pointer tip at the intersection of the date and latitude lines and hold it there. Locate the day's date in the right date bar and stretch the plumb bob so that it crosses the 12 o'clock line at the level of the day's date. Move the bead to this intersection of the 12 o'clock line and the date line. Hold the board vertically and turn and swivel it so that the pinhole is directed at the sun and its light images the star-shaped aperture exactly in the centre of the target. Read off the True Local Time from the position of the sphere.