Horology & Navigation: Understanding the Longines Hour Angle Watch

An Enigmatic Watch

I remember the first time I saw the Longines Hour Angle air-navigators’ watch. It was on the front cover of the venerable John Goldberger’s Longines: Legendary Watches. Gold-cased, white dial, a massive ‘onion’ crown are the backdrop to what is on one hand a relatively simple time-only watch, but on the other hand is a complicated sea of numbers dotted across a dial and bezel. At first viewing, the enigmatic jumble of digits was lost on me, as if I were looking at the controls of a vintage aeroplane cockpit itself.

How do you even use that thing? is a question which did not come to mind until a few years later when I came across a review of one of Longines’ modern remakes of the watch on YouTube, where even the owner admitted to not completely understanding how it works. 

With my grandfather having been a second world war RAF Bomber Command radio signalman and navigator, I felt it my duty to find out how to use the inscrutable device. I knew the watch’s inventor, Charles Lindbergh, the American aviator who in 1927 flew solo 3,600 miles for over 33 hours non-stop from New York to Paris, created the hour angle watch to calculate longitude somehow, but I wasn’t sure how. As it turns out, to truly understand this watch one must understand the rudimentary and nowadays outdated methods of air navigation.

For this, a good start is a 1930s textbook titled Air Navigation by P. V. H. Weems. A US Navy Lieutenant Commander, Weems was also the inventor of the second-setting complication which features on the Lindbergh watch; a way of setting the seconds hand independently of the hour and minute hands by way of a rotatable inner seconds dial, handy for quick yet precise seconds adjustments. 

Lines of Longitude and Latitude - Weems’ Air Navigation

Weems’ textbook outlines the first step in understanding the Hour Angle watch works: understanding longitude and latitude. Any position on the globe is noted as a combination of your latitude (your lateral position on the axis north-south of the globe) and longitude (your position on the east-west axis). Longitude and latitude consist of three pieces of information: an angle, minuets and seconds. The angle is the angle of the meridian line of longitude to the local longitude. In the case of latitude, the middle is the equator, and for longitude, it’s the Greenwich meridian line (GMT). 

Celestial Sphere and Hour Angles - Weems’ Air Navigation

Another celestial term to know in order to comprehend the Hour Angle watch is solar noon; the time when the sun is at its highest point in the sky (or, Zenith) in that location. Civil time (often called GMT or mean time, the timezone of London) is defined as being 12 noon when the sun is at its highest point over the meridian line. All other timezones are defined as GMT± some hours depending on distance: Paris is +1hrs, New York City is -7hrs. Time is therefore expressed as either a number or a an angle from the sun’s highest point. 

Our final piece to the puzzle is the equation of time, which is simply the difference between local solar time and mean time (GMT). The term ‘equation’ here being used in the medieval sense referring to ‘reconciling a difference’.

So how does the Hour Angle watch work out longitude? The first step, which doesn’t even concern the watch, is to determine latitude. This is done with a sextant, instructions for the use of which are beyond the scope of this article. Once you know your latitude, the next step is to have the watch set, as accurately as possible, to GMT, which can be done via radio signal. This tells us the hour angle of solar noon at Greenwich. The watch is designed so that the navigator can easily read off various dials to give precise hour angles for GMT and local time, the difference for which is their precise longitude. The bezel even rotates to correct for the equation of time for that specific day, information for which is found in a celestial almanac. 

Seconds-Setting Watches - Weem’s Air Navigation

Now let’s get in the cockpit and work through an example as though we were Lindbergh himself…

You are cruising at 13,000 feet, hardly able to think over the deafening grrr of the engine and the freezing cold temperatures. A vast Atlantic void which offers no landmarks from horizon to horizon moves beneath you. You glance at your watch, fitted over the sleeve of your shearling-lined flight jacket: every hour difference in timezone from GMT is equal to a difference of 15º longitude, and the watch indicates this in arabic numerals just below the roman numerals. Every 4 minuets is equal to 1º of longitude, indicated on the watch’s bidirectional bezel and in small text on the seconds-setting subdial. 

You already know your latitude from your sextant, and know that you are heading east around 50º. 

A voice from the long wave radio gives you a chronometer-accurate reference of GMT as per your request, so you turn the seconds-setting dial so that the running seconds is perfectly synchronized to the time at Greenwich, London. 

You bring the sextant to your eye once more and consult your nautical almanac to calculate the local solar time. 

The difference between the angle, minuets and seconds of the watch’s arabic numerals and the angle, minutes and seconds from the solar time calculation gives you your longitude, and the seconds hand points to the precise degrees on the bezel. 

You now have your bearings once more, set your correct course, and press on to Paris. 

The Man Behind the Design

Portrait of Charles Lindbergh - Britannica.com

Equally difficult to understand as the watch itself is its creator, Charles Lindbergh. Although he attended university, he swiftly dropped out and became a stunt pilot with a circus. From there he became the original Top Gun Maverick, earning his daredevil reputation with his characteristic procedure of standing on the wing of his aircraft before parachuting off when things went awry, an act he carried out on multiple occasions as an aerobatics pilot.

Lindbergh had a minimalist philosophy when it came to flying. When planning the first solo flight across the Atlantic, he stripped the plane back to the absolute basics. He even considered the plane’s fuel gage to be excess weight and decided to measure fuel consumption by using his watch. The result was the Spirit of St. Louis, a modified Ryan A2 (called the NYP - New York to Paris). In realising its utility, Lindbergh decided to use a trusty wristwatch to also measure differences in hour angles, though he didn’t create the hour angle watch until after his epic flight. 

Lindbergh’s original pencil sketch for the Hour Angle Watch

Lindbergh became one of the world’s first global superstars, even completing a world tour later that same year to meet his fans. Some years later, on March 1st, 1932,  Lindbergh’s name was back in the spotlight when his baby son was kidnapped, assumably for random, and was unfortunately found dead some days later.

During the second world war, Lindbergh was invited to inspect the Nazi’s air-force, touring factories, inspecting and even test-piloting Nazi planes. He was deeply impressed with the uniformity and discipline of the Nazis, and even became a proponent of eugenics, believing Europe’s white race were superior. Back in the US, he criticised FDR and British and American Jews, echoing Hitler’s propaganda.

Lindbergh and Co. standing by the Spirit of St. Louis - Britannica.com

Lindbergh, simple enough a man on the outside, housed a scandalous and complicated web of lies and deceit. This culminated in an affair with his secretary in Berlin who he started a second secret family with using the alias Careu Kentwith. Soon after, he had another secret family in Europe with his secretary’s sister, having 7 children by them. He eventually had 4 families totaling 13 children, managing to keep the affaires undercover using a series of PO boxes. 

Lindbergh nonetheless contributed to the war effort, later joining the US navy, teaching flying techniques and even winning a dog fight. In his old age, Lindbergh went full-circle, becoming an environmentalist and founding a rainforest preserve in his name. 

Simple and clean at first glance, but hiding a whole host of complications in broad daylight, Lindbergh’s complex and controversial legacy lives on in the simple yet confusing wristwatch he created over 90 years ago. 

Collecting the Lindbergh

The original hour angle watch from 1931 was a large pilot’s watch in a 47.5mm silver case. Around 2000 examples are rumored to have been produced and were distributed exclusively in the US by Longines-Wittnauer.

1938 saw a second run on the watch in a more civilian-spec 38mm case and a higher contrasting champagne second-setting subdial. Soon after that, Longines created the smaller ladies’ version at 33mm. The 33mm version was re-released by the brand in the 80s and 90s. 

Being the low-key icon it is, Longines have a tradition of regularly celebrating anniversaries of Lindbergh’s historic flight with recreations of the original watch. The first came in 1987 to celebrate the flight’s 60th anniversary, in a 38mm stainless steel case. The following year, they release a 47mm, closer to the original size, but again in steel, limited to 1000 pieces. They also made just 10 examples of the 1988 model in solid yellow gold. They reissued the watch again in 1993, featuring the same 47mm case; 1000 in steel and 100 in gold. Some more limited edition pieces, some in two-tone, came in 2002.

Modern Longines seem to reinvigorate the Lindbergh every half-decade or so. In 2008, we saw 80 yellow gold examples in 47mm, and in 2012 a steel model very close to the original design. But perhaps the most exciting yet is 2017’s titanium hour angle watch with a black ceramic bezel. 

For vintage pieces, I encourage readers to consult John Goldberger's aforementioned book Longines: Legendary Watches. Longines themselves are also famously accommodating to owners of vintage pieces and regularly offer extracts from their archives so long as you have the watch’s serial number.