Regiomontanus had been working on the principle of a traveling sundial that could be used for different geographic latitudes or polar elevations since 1457 at the latest. The basic idea probably dates back to the 14th century and was based on the Navicula Venetiis , the Venetian boat: so named after the shape of the body, which was based on the silhouette of this type of boat.
However, it is irrelevant to the functionality of the instrument. Regiomontan's considerations were initially based on the writing of an unnamed old manufacturer ("antiquus compositor") entitled Organum ptholemei aut quadratum horarium, in which the ship's shape was abandoned in favor of a square with fixed sights on the upper edge. In a first step, Regiomontan developed the vertical lines of the hours of equal length (equinoctial hours), but worked with only one pointer on a horizontal slider, which was not sufficient for the exact determination of the sun's position.
The complete description of the instrument as PDF for download
