Full-sized marine chronometers, equipped with detent escapements and running for 56 hours, were mass-produced by the Japanese company Daini-Seiko-sha during World War II. This watch manufacturing company was part of a group, which also included Hattori, the watch and jewelry retailer, and Seiko-sha, mainly a manufacturer of clocks. Today we know this firm by the names Seiko Instruments Co. Ltd., and Seiko Epson Co.¹

The design of the Seiko marine chronometer is an exact copy of the famous Ulysse Nardin chronometers made in Switzerland, right down to the location of the trademark stamp beside the ratchet wheel on the movement backplate. The timepieces and their components were, however, entirely made in Japan.

Prior to the advent of Seiko marine chronometers, Seiko-sha produced five-day running deck clocks (Figure 1) and eight-day fusee ships clocks (Figure 2), using good quality lever escapements.² Although suitable for merchant ships, they were inadequate for use in military navigation. For that purpose, the Imperial Navy used imported marine chronometers by Nardin, Kullberg, etc. The Seiko-sha deck clocks had a rated accuracy of less than plus/minus 12 seconds per day, whereas the naval accuracy standard was plus/minus 1 second per day, during a 20-day test. This was why the navy used full-size imported chronometers for navigation.

The author, with the help of his colleagues, has investigated the short history of Seiko’s full-sized marine chronometer, which was produced for only about five years from 1941 to 1945. In this paper I describe its history with quotes from previously secret military documents, interviews with participants of those times, and photographs of a Seiko marine chronometer as well as of an early Nardin chronometer retailed in Japan. These latter timepieces are in the author’s collection.

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