The Venetian boat is a traveling sundial that can be used for different geographic latitudes and polar elevations. Developed in the 14th century, it owes its name to the silhouette of this type of boat. The instrument serves as a portable sundial: When it is correctly set and aligned with the sun, the plumb line hanging from the mast falls on the hour lines and the sliding bead on the plumb line marks the true local time. The body of the boat has date scales on three sides, a degree scale at the top and the hour lines (see illustration below). The height and angle of the mast can be adjusted, and the geographic latitudes of selected cities can be fixed on its scale.
Johannes Regiomontanus (1436-1476) worked intensively on the still rather imprecise predecessor instrument and in 1474 applied the principles of his general dial(s) to it, achieving a decisive improvement. The true local time can be determined from the change in altitude of the sun during the course of the day if the date - i.e. the position of the sun in the zodiac -, the geographic latitude and the position of the bead on the plumb line are set correctly.
The complete description of the instrument as PDF for download
