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ZENITH Observatory Chronometer, Cal. 5011 O

CHF 4,750


Extremely rare and well-preserved fine ZENITH Observatory Competition chronometer SN. 6371019, Cal. 5011 O («O» stands for Observatory, 50mm, 22 lines, high frequency 21’600 alt/hour, Guillaume balance), comes with its original testing wooden housing. Made in 1967 exclusively for observatory competitions, that particular piece got “Primes” (11th place of around 180 participants in the category, with score 2.44) at the observatory competition in Neuchâtel in 1967 thanks to Jean-Pierre Vuille, who adjusted this movement for the trials at Zenith. To reduce the friction, 48-hour power reserve indication is not functional, corresponding functionality was removed by adjuster, during preparation for the trials. Moreover it’s very unusual, that the Guillaume balance is hand-signed by Jean-Pierre Vuille with “JPV” initials.

In 1960, the large 50mm Calibre 5011 set a record-breaking precision standard in its category and was used to power marine chronometers, pocket watches and table clocks. It was also used in the Pilot Montre d’Aéronef Type 20 in limited quantities (from about 350 unused movements). In 1967, Calibre 5011O was the most accurate movement ever tested to date by the Neuchâtel Observatory. Absolute record and serial prize for the 4 best timepieces from the same manufacturer.

Case: two-body wooden protective/testing case, circular case, polished bezel, glazed transparent display back, large crown. Dial: silvered matte dial, Arabic numerals, blued steel baton hands, engine-turned subsidiary seconds. Movement: golden Cal. ZENITH 5011-O 50mm, 22 lines, high frequency 21’600 alt/hour, Guillaume balance with gold poising screws, hand-signed “JPV” balance. Movement and dial signed, Diam. 59 mm; case 75 x 75 x 31 mm.

Competition chronometer-quality wristwatch movements are not “ordinary” C.O.S.C. certified movements. These movements were designed and finished to excel in the rigorous world of Observatory Competitions, as conducted in Geneve and Neuchatel in the years 1944 through 1967.

Up until 1967, the exact length of a second was determined by the Earth’s motion around the sun, and the Neuchatel observatory’s precise optics allowed the fine measurements necessary for precise timesetting to be made. Because of this, Neuchatel began to host observatory chronometer trials – a series of the most stringent, thorough accuracy tests in the history of mechanical watchmaking. The trials, held over a period of forty-five days, encompassed ten separate movement timekeeping tests in five different positions and two temperatures. These observatory trials quickly outgrew their roots as a simple certification test, and before long the trials at Neuchatel were a competitive battleground for the finest watchmakers in Europe. Names like Longines, Zenith, Peseux, Omega and many more traded records for decades, fighting to produce the most accurate movement possible. These movements were never intended for regular use, and bore little to no resemblance to what these companies offered their consumers. Along with the technical prowess of the watchmakers building the movements, in the “racing team” there were specialists, the “regleurs de precision”, who worked tweaking a number of movements until they could find a few of them to bring to the Observatory Competitions. By means of fine adjustments, replacement of key parts, use of special lubricants, they could create a few movements for the Competitions. And often they were the same movements for many years.

Watch is in overall very good condition 8/10.


Brand Zenith
Model Observatory Competition Chronometer
Year Circa 1960s
Movement Hand-winding
Calibre Zenith Cal. 5011-O
Case material Steel, Brass, Wood
Case size Diameter is 59 mm, wooden testing case 75 x 75 x 31 mm
Features Observatory Chronometer, Got “Prime” in 1967
Box/Papers No box, no papers


In fully working order. Very good condition 8/10, minor surface and crystal scratches, scuffs and discolorations through the age. The 48-hour power reserve indication is not functional, corresponding functionality was removed to reduce the friction by Jean-Pierre Vuille (Zenith adjuster), who prepared the movement for trial. The balance is hand signed by adjuster “JPV”. See photos.

Watches are sold as-is, and there is no guarantee of to be waterproof or movement accuracy, due to the nature of vintage watches. Please view all the pictures carefully as they form an integral part of the description. Any defects not mentioned but visible in the pictures will be considered as described.


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