Swiss aluminum and steel siderograph with split seconds Circa 1939

Aluminium and steel navigation watch
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A navigation watch, the siderograph Aero-N., with double split seconds made for the Aeronautique Navale, the air arm of the Marine Nationale of the French Navy, by Longines in Saint-Imier, Switzerland, circa 1939. The case of this watch was crafted from aluminium and steel. Its numbered, two tone silvered dial is marked Aero-N. and features a power reserve indicator, and three central seconds hands, of which two are for the split seconds. The numbered movement is fitted with a high end precision lever escapement.
The dial and movement are signed.
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Diameter: 94 mm
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Weight: 545 g
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Signed: Longines Fab Suisse
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Siderograph
The siderograph is a combination of a watch and a navigation instrument, and keeps a reference time corresponding to the time at 0° Greenwich longitude. This precision instrument was placed in a wooden box, which centrally contained an aluminium plate with five springs that held the measuring instrument in a suspended position. The holes for attaching the springs are located on the bottom of the case.
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The Longines siderograph was originally developed in 1938 to help with aircraft navigation. The instrument measures Greenwich sidereal and local time in hour angles, minutes and angular minutes. Its accuracy and precision made the siderograph the measure of all things in marine chronometry. It allowed the navigator to operate at night in the same way as during the day, far surpassing the systems of Weems and Lindbergh, as both depended on the time signals.
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The siderographs were equipped with the 2129 chronometer caliber, which was the most precise movement Longines possessed at the time. In addition, a split seconds and an anti magnetic beryllium lever were integrated and the movement was fitted into a hardened aluminium case. This material was perfectly suitable for aeronautical and maritime purposes due to its weight and non iron character. Versions were available with one and (extremely rare) as here with two split seconds.