Pocket Longitude Clock

This longitude watch is a precision instrument that is part of the same category as deck watches or wrist watches. Housed in a wooden, consular-style case, it has a convex white enamel dial on which are painted 4 concentric circles. The hours are marked in Roman numerals between the 2 inner circles. The outer circle is a dotted line with the minutes shown in Arabic numerals, in increments of fives. There is a subsidiary dial above the number VI that shows the seconds. It still has its winding key.

In theory, calculating longitude is a matter of measuring the time difference between a reference point and the current position of a ship. But this proved so laborious that, in 1766, the British government appointed the astronomer Nevil Maskelyne to produce some annual astronomical tables that would eliminate the need for the calculations. However, the practical solution to the problem of longitude came from technological advances in the measurement of time, with the manufacture of increasingly precise marine watches (chronometers). Most of them, including this one, were made in England by the British watchmaker John Arnold. Arnold's innovations led to the manufacture of marine chronometers that were more affordable, but still precise.

Cecilio Pujazón, director of the San Fernando Naval Institute and Observatory, left a handwritten note with this object that tells us about its provenance. It was acquired in London to be used by the commission responsible for compiling the American Atlas, and given to the commander of the brigantine ''Vigilante," Antonio García de Quesada, on November 15, 1792. It was returned to the Observatory on November 15, 1795 and subsequently used on several other commissions.