In the early decades of aviation, pilots were not merely flying — they were navigating the unknown.
Long before radar, satellite navigation, or onboard computers, determining one’s position in the sky required a precise combination of mathematics, celestial observation, and accurate timekeeping. When an aircraft travels at roughly 200 miles per hour, a navigational error of only 30 seconds can translate into miles of miscalculation. In the vastness of the Atlantic or the Pacific, that margin could mean the difference between locating a tiny island fuel stop — or missing it entirely.
It was in this environment that one of the most important navigational tools in horological history emerged: the Weems seconds-setting watch. Part timepiece, part scientific instrument, and wholly revolutionary, the Weems would fundamentally reshape the relationship between aviation and timekeeping.
A Navigator’s Problem
The watch’s origin begins with a man rather than a manufacturer: Lieutenant Commander Philip Van Horn Weems, a U.S. Navy officer, aviator, and pioneering navigator.
Lieutenant Commander Philip Van Horn Weems - (Image by Air and Space Museum)
During the 1920s, aviation was advancing at a breathtaking pace. Pilots were setting endurance records, racing across continents, and attempting increasingly ambitious long-distance flights. Yet navigation techniques had not yet caught up with aircraft speeds.
Traditional celestial navigation — borrowed from maritime practice — relied on sextants and accurate timekeeping. But watches could only be synchronized to the nearest minute, leaving a potentially disastrous margin of error.
Meanwhile, the spread of radio technology created a new opportunity. Standardized radio time signals allowed navigators to hear exact time broadcasts anywhere within range. The challenge was clear: how could a pilot quickly synchronize a watch with these signals without stopping the movement entirely? Weems’ solution was elegantly simple. Instead of resetting the hands, the seconds scale itself could be adjusted.
The Adjustable Seconds System
The concept behind the Weems watch was a rotating seconds-setting chapter ring located inside the dial. By turning this inner ring, the wearer could align the seconds scale with a radio time signal without disturbing the watch's movement. In practice, this allowed corrections of roughly plus or minus 30 seconds — a dramatic improvement over existing methods.
For navigators performing celestial observations, this meant faster synchronization and more reliable calculations. But before the system became a wristwatch, it existed in a much humbler form.
A Waltham Vanguard ‘Weems’ second-setting pocket watch. - (Image Flight Birds)
Weems’ first experiments involved modifying existing pocket watches. Working with San Diego watchmaker Mr. Dadisman of Jessop’s Jewelers, Weems adapted several Waltham Vanguard and Hamilton torpedo-boat watches, adding mechanisms that allowed the seconds indication to be adjusted.
These experimental instruments proved the concept worked. The next step was to produce a purpose-built watch. For this, Weems turned to Longines, which had already established itself as a leading supplier of timing instruments for aviation.
Longines Enters the Picture
The development of the Longines Weems watch was the result of a remarkable collaboration between several figures deeply involved in early aviation: Philip Van Horn Weems, the inventor and navigator; John Heinmuller, Longines’ American agent at Wittnauer and a passionate aviation supporter; and Alfred Pfister, Longines’ technical director in Saint-Imier.
A Longines Weems prototype. - (Image by Flight Birds)
Together, they transformed Weems’ concept into a purpose-built wristwatch designed for pilots. These first Longines Weems prototypes appeared in 1928, and they were unlike anything that had come before.
The earliest examples were essentially experimental pieces. Both prototypes used repurposed Longines cal. 18.69N movements, originally manufactured more than a decade earlier for dual-time pocket watches intended for the Turkish market.
The watches were large — approximately 47 millimeters in diameter — and housed in silver cases with oversized crowns for operation while wearing gloves. Perhaps their most distinctive feature was the tiny experimental chapter ring measuring only 18.5 millimeters in diameter: this inner rotating scale mirrored the design of earlier Longines dual-time pocket watches and would later be enlarged in production models for better legibility.
An early Longines Weems seconds-setting watch that belonged to Weems himself. - (Image by Time and Navigation)
Two pieces were produced: serial 3585867 delivered in November 1928 and owned by Weems himself, and serial 3585868 delivered to the Longines agent Arthur Baume in London in 1930. Weems’ personal watch was later pictured in his 1931 book Air Navigation and today resides in the Smithsonian Institution.
Production Begins
Encouraged by the prototypes’ success, Longines introduced the first production run of Weems watches in 1930, supplying them through Wittnauer in the United States. The design evolved quickly, with Longines enlarging the inner rotating chapter ring from 18.5 millimeters to 21 millimeters, improving legibility for cockpit use.
An original Longines Weems watch with enamel dial, original guarantee, and presentation box. - (Image by Phillips)
Later versions incorporated additional refinements, including a dedicated crown at four o’clock to adjust the rotating seconds scale, larger chapter rings for improved visibility, and improved movements such as the Longines cal. 37.9 introduced in the late 1930s. While the earlier large Weems models were produced by Longines, the seconds-setting system would later appear in watches from other manufacturers — including Omega, Jaeger-LeCoultre, Zenith, and Movado — as the world catered to the demands of wartime aviation.
From Weems to Lindbergh
The Weems watch did not remain the final step in aviation timekeeping. In fact, it directly led to another famous navigational instrument: the Longines Hour-Angle watch developed in collaboration with pioneering aviator Charles Lindbergh, who learned celestial navigation from none other than Weems.
An advertisement for the Longines Hour-Angle watch dating to the late 1930s - (Image by Macau Timepiece Museum)
Where the Weems allowed pilots to synchronize time precisely, the Hour-Angle watch simplified the calculation of longitude during celestial navigation. Together, the two designs formed the foundation of airborne navigation tools in the 1930s.
Wartime Service
In the leadup to the Second World War, the Weems seconds-setting concept was adapted for military use by several Swiss manufacturers. The British Air Ministry specified a navigation watch based on the Weems system, known as the Mk VIIA (6B/159), which was issued to RAF pilots and navigators beginning around 1940. Produced by Longines, Omega, Jaeger-LeCoultre, Zenith, and Movado, these watches retained the essential rotating seconds-setting mechanism but were housed in more compact stainless-steel cases of roughly 33–34 mm designed for practical military wear.
Jaeger LeCoultre A11 Mark VII "Weems" U.S. Army Air Corps Pilots Watch - IN THE SHOP
Engraved with Air Ministry markings and serial numbers, they were issued as official navigation instruments rather than personal timepieces, intended to be synchronized with radio time signals before flight. Approximately 7,000 examples are believed to have been supplied to the RAF at the outbreak of the Second World War, and many saw service during the Battle of Britain and the wider air war that followed. In the cockpit, paired with sextants, charts, and radio time signals, the Weems watch helped pilots maintain the precise timekeeping required for dead-reckoning and celestial navigation — transforming an elegant interwar invention into a critical wartime tool.
A Tool for Aviation’s Golden Age
Today, the Weems watch stands as one of the earliest true pilot’s tool watches — a timepiece designed not for style or prestige, but for survival. Its rotating seconds scale may seem simple by modern standards, yet in the context of 1920s aviation it represented a significant leap forward in navigational accuracy.
Like so many great inventions, the Weems watch was born from collaboration: a visionary navigator, a determined watchmaker, and an industry eager to support the exploration of the skies. In the end, it proved a simple truth that every navigator understood: when you’re flying across an ocean with nothing but instruments and the stars to guide you, every second counts.
