Observatory Chronometer

In the world of mechanical timepieces, accuracy is paramount. In the times before electronics, mechanical timepieces called marine chronometers were developed to a very high degree of accuracy and used for maritime navigation. In order to test the accuracy of such marine chronometers, watchmakers looked to the series of astronomical observatories existing in Western Europe to conduct accuracy assessments of timepieces. The observatory, having a slate of accuracy tests, would either reject the timepiece, or, if it passed the stringent testing, it would be certified as an observatory chronometer. As a result, the observatory trial was developed and became the standard for determining accuracy of timepiece movements.

Once mechanical timepiece movements developed sufficient precision to allow for accurate marine navigation, there eventually developed what became known as "chronometer competitions" at the astronomical observatories located in western Europe. The Neuchatel Observatory, Geneva Observatory, Besancon Observatory, and Kew Observatory are prominent examples of observatories that certified the accuracy of mechanical timepieces. The observatory testing regime typically lasted for 30 to 50 days and contained accuracy standards that were far more stringent and difficult than modern standards such as those set by COSC. When a movement passed the observatory, it became certified as an observatory chronometer and received a Bulletin de Marche from the observatory, stipulating the performance of the movement.

Of the millions of watches produced in Switzerland each year by all manufacturers in the mid-1960s, approximately 250,000 would receive official chronometer status (similar to what would be COSC standards today), and only a few hundred of the very best from the total production would be sent to an observatory for chronometer accuracy competitions. Watch movements that could compete for accuracy certification at the observatory had typically been specifically built for that purpose alone, they were slow beat movements, oscillating at from 18,000 to 21,600 bph, typically with oversized balance wheels, tweaked and prepared by the best watchmakers often for many years to render ultimate accuracy before they were submitted to the observatory. Typical examples of these specialized competition movements were the Peseux cal 260, the Zenith cal 135 and the Longines cal 360. Because of the development time to create and tweak such movements, watch manufacturers tended to enter very few movements at observatory competitions.

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