The Quadratum horarium generale represents a travel sundial that Regiomontanus had set at the end of a German and Latin calendar in his own printing press in Nuremberg in 1472. The structure of the calendar was revolutionary. Columbus carried a copy with him on his travels to America. With this sundial, also known as the Capuchin sundial, not only can the time of day be determined, but also the respective height of the sun, the time of sunrise and sunset as well as the duration of twilight, separated into three twilight levels. The clock plates were determined experimentally.
The lines diverging upwards correspond to the sun's change in declination over the course of the year. The months are shown below and the latitudes are shown on the right. The suspension of the plumb bob is set to the date and latitude. The plumb line is then held to the right and the bead is set to the right date. Finally, supported by the rear sight and front sight, the sun is aimed at over the upper edge and the time is read using the bead on the hour marks. If you bring the tablet back to the horizontal, you can read the time of sunrise and sunset from the bead. The clock was also called the Capuchin sundial because of its shape. The board is richly decorated. On the back there is a brass plaque with a perpetual calendar (Calendarium Motus Solaris Perpetuum).
Since the sun clock was easy to produce in a simpler form, even the less wealthy could afford such a clock long before a wheel clock was affordable.
The experimentally determined values were later confirmed by calculations.
If the time is known, the latitude can also be determined using the clock board.
