Arnold & Son HM Perpetual Moon in a steel case

Blue dials and moon phase indications have been popular watch themes for the last couple of years, and in this piece, Arnold & Son put them together and make the moon phase bigger and better. The whole dial is dedicated to the moon phase window, something that is usually relegated to just a subdial, and the moon alone is 11.2 mm in diameter. The “perpetual” in the name refers to the precision of the moon phase, with one day’s deviation every 122 years; at that stage it would take just a single push on the corrector situated on the caseband to bring it back to total precision. So it’s “perpetual” because even if you wear it day in day out for your entire life, it’s going to be pretty much in synch with our beloved satellite.

The accuracy of a moon phase function varies from the most common version, in which the disc with the two moons are driven by a wheel with 59 teeth which is advanced by one tooth every 24 hours, corresponding to a lunar month of 29.5 days. The actual lunar month is 29.53 days and so this version goes one day out of synch every 2 years and 7 months. A moon phase accurate to one day every 122 years is based on a wheel with 135 teeth.

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In this watch, the moon itself is moulded from a hand-sculpted original, with craters that create a shaded, perspective effect giving the moon a degree of visual three-dimensionality. Setting the moon phase is made easier by another display that can be seen through the transparent caseback, with a graduated dial and a pointer hand showing the day of the lunar month. The moons are set onto a translucent blue guilloché disc which has the same finish as the dial itself.

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The photos that Arnold & Son have provided don’t really show the watch’s profile, but its case is narrower below, and it expands outwards as you move up towards the watchglass, maximising the space dedicated to the dial and moon phase, and making the watch comfortable to wear. It is also a clever way of concealing the pusher that adjusts moon phase, and it also makes it easier to pull out the winding crown, which in any case is shaped so that it is easy to use. The piece is 42 mm in diameter with an alligator strap, and it is water-resistant to 30 metres (splash-resistant only, no showers or swimming).

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The movement is the A&S1512, a hand-wound calibre with 27 jewels, running at 21,600 vph (3 Hertz) with two mainspring barrels providing a power reserve of 90 hours (just under four days).

Reference and price

The reference is 1GLAS.U02A.C122S, and this stainless steel watch is a new version of an existing piece in a red gold case. Price is $17,200, about €15,600, and the watches will be available at points of sale around the world this month, December 2015.

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