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Arnold & Son A&S high-precision moonphase

High confidence

The accuracy claim is arithmetic and follows from the gearing. A standard moonphase uses a 59-tooth wheel and drifts roughly a day every two and a half years; a high-precision one uses a much larger count and drifts by a day only after centuries. The claim is therefore verifiable in principle by counting teeth through the display back — laborious, and one of very few specification claims in this catalog that admits a definitive physical check.

Specifications

Movement manufacturer
Arnold & Son
Winding
manual
Complications
High-precision moonphase

Architecture

A manually wound caliber driving an oversized moonphase disc through a high tooth-count gear train, approximating the synodic month far more closely than a conventional 59-tooth arrangement.

Open questions

OpenMissing primary sourceMedium

Tooth counts behind the Perpetual Moon accuracy claim

What gear ratio drives the Perpetual Moon's disc, and does it support the quoted accuracy?

Scope Unusual among the gaps in this expansion because it is answerable by arithmetic once the tooth count is known, and the tooth count is visible through the caseback. Recording it would turn a marketing figure into a verified specification — and would give an examiner a definite number to check against.

References using this caliber

Provenance of this record

Sources & Evidence

Primary sourceManufacturer documentation

Arnold & Son — collection and caliber specifications

Arnold & Son (Citizen group / La Joux-Perret)

Accessed
2026-08-19

Cited for

This referenceHigh confidence
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Last verified: Not yet reviewed