Constellation Co-Axial 31 Yellow Gold / Diamond / MOP Supernova

In Context

At 31 mm it is small by current standards, in the range most wristwatches occupied before cases began growing through the 1990s. The case back is open, so the movement is visible without taking the watch apart, and the sapphire crystal will resist everything short of deliberate abuse. 100 m is genuine swimming resistance — more than the shape of this watch suggests it would need.

The movement winds itself from the motion of your wrist. 50 hours of reserve is enough to set it down on Friday evening and find it still running on Monday. At 25,200 vph the seconds hand advances 7 times a second.

123.55.31.20.55.002 stands on its own in this catalogue — no other reference here shares its base number. It carries a white dial.

The 8521 is the 8520 with an 18-carat red gold rotor and balance bridge, the version of Omega's small co-axial movement fitted to gold-cased watches. It powers 86 watches in this library. The [8520](/calibers/omega-8520) is the co-axial caliber for ladies' and mid-size cases, with a free-sprung balance, a silicon balance spring, bidirectional winding, 28 jewels, a 50-hour reserve and a beat of 25,200 vph. The 8521 changes nothing mechanical: the rotor and the balance bridge are made in red gold so that the movement seen through a display back matches the case around it. Omega does the same with the [8501](/calibers/omega-8501) and the [8807](/calibers/omega-8807). In the library it sits under 37 [Ladymatic](/brands/omega/ladymatic) references, the 34 mm model in red or yellow gold and two-tone, 26 [Constellations](/brands/omega/constellation) at 27 mm and 31 mm, 19 [Aqua Terras](/brands/omega/aqua-terra) and 4 [Planet Oceans](/brands/omega/seamaster-planet-ocean) at 37.5 mm. The 34 mm case is the most common at 50 references, then 27 mm at 16 and 31 mm at 10. The [8700](/calibers/omega-8700) is the Master Chronometer movement that took over the smallest cases in 2018.

85 other watches in the library run the Omega 8521.

Omega

Seamaster Aqua Terra 150M Co-Axial 34 Yellow Gold / Black

231.53.34.20.01.001 · 34 mm

Omega

Seamaster Aqua Terra 150M Master Co-Axial 34 Sedna Gold / MOP / Bracelet

231.50.34.20.55.001 · 34 mm

Omega

LadyMatic Co-Axial 34 Red Gold / Diamond / MOP

425.68.34.20.55.001 · 34 mm

Omega

LadyMatic Co-Axial 34 Red Gold / MOP

425.63.34.20.55.001 · 34 mm