Quadrant or diptych sundial. This portable folding pocket instrument used sunlight to tell the time and was very popular among sailors until the 18th century. It works by casting a shadow onto a prepared surface, while a small compass serves to orient the horizontal plane of the quadrant.
Two coordinates are needed to determinethe exact ship’s position: latitude (distance north-south from the equator) and longitude (distance east-west from the Greenwich meridian).
Calculating latitude was no problem thanks to instruments such as the sextant and octant and mathematical tables.
The problem was to find a reliable way to determine longitude, which was achieved by using clocks. High-precision marine chronometers were made from the second half of the 18th century onwards.