On 19 October 2026, Jaeger-LeCoultre introduced a 36 mm Master Ultra Thin Moon in 18K pink gold, reference Q1242521. Its proportions, automatic Calibre 925 and combined moon-phase/date display offer a compelling example of contemporary classical watchmaking. Here's what makes this release worth understanding.

What is new about the 2026 Master Ultra Thin Moon?

The 36 mm pink-gold case is 9.3 mm thick. Its silver-toned sunray dial features gilt indices and Dauphine hands. At six o’clock, a starry blue moon-phase aperture sits within the date indication. A sapphire caseback reveals the decorated self-winding mechanism and 22K pink-gold rotor.

Jaeger-LeCoultre Q1242521: key specifications

• Case: 18K pink gold, 36 mm diameter, 9.3 mm thick
• Movement: in-house automatic Calibre 925
• Power reserve: 70 hours
• Functions: hours, minutes, seconds, date and moon phase
• Water resistance: 5 bar
• Strap: black alligator leather with interchangeable pink-gold pin buckle

The manufacturer's announcement does not establish a numbered production limit or retail price; neither is assumed here.

How does a mechanical moon-phase watch work?

A moon-phase display uses a mechanically driven disc to represent the Moon’s changing appearance as seen from Earth. According to NASA, the cycle from one new moon to the next lasts about 29.5 days. Watchmakers translate this astronomical rhythm into wheels and discs that advance with the movement. The accuracy over many years depends on the particular mechanism; no specific correction interval is claimed for this reference without manufacturer confirmation.

Why do the 36 mm size and 70-hour reserve matter?

A smaller dress watch can feel balanced and discreet, although comfort also depends on lug geometry, thickness and strap. The stated 70-hour power reserve means the fully wound automatic movement can keep running for almost three days without additional winding. The sapphire caseback also invites appreciation of the movement’s finishing.

Moon phase versus open heart: two different ways to enjoy mechanics

Moon-phase watches interpret an astronomical cycle on the dial. Open-heart watches expose a portion of the mechanism itself. They are different complications and experiences, but both reward curiosity about mechanical timekeeping.

At our Swiss atelier, EP Watches L’Origine Open Heart offers an independent approach to visible mechanics, hand-assembled around an automatic movement. It is not a moon-phase watch and is not technically equivalent to the Jaeger-LeCoultre. You can also explore our watch collection for different expressions of traditional watchmaking.

What this launch means for modern watchmaking

The Master Ultra Thin Moon makes a case for restraint. Its relatively compact dimensions, pink-gold construction and mechanical lunar display prioritise harmony over size. At the same time, it demonstrates why explaining what a movement actually does matters more than repeating marketing claims.

Final thoughts

The 2026 Master Ultra Thin Moon brings the familiar poetry of a lunar display to a refined 36 mm case with a 70-hour automatic movement. Whether your preference is for celestial complications or the visible architecture of an open-heart watch, understanding the mechanism makes the experience richer.

Official sources

Jaeger-LeCoultre — official announcement; NASA — Moon phases. Editorial imagery of Jaeger-LeCoultre supplied by the publisher; rights and credits subject to the publisher’s permission. EP Watches is independent and not affiliated with Jaeger-LeCoultre.