Before the Global Positioning System (GPS), before radar, before any of the electronic systems we now take for granted, long-range aviators and mariners found their way by the stars. A navigator would raise a sextant, locate a star, then reach for a pencil and begin converting the time on his chronometer into an angular value he could actually use. That conversion, hours and minutes into degrees and arc, was where mistakes crept in, and in a cockpit covering seven miles a minute it could be the difference between making landfall or ending up in the ocean. Aviators like Amelia Earhart were likely lost due to navigational problems.
Every navigation watch of the 1930s, including Longines’ own celebrated Weems and Lindbergh models, still spoke the language of civil time. They told you what hour it was. The rest, the laborious arithmetic of turning that hour into an angle, was left to the navigator and his calculations. Longines’ answer was groundbreaking. What if the watch itself spoke the language of the stars?
The result was the Siderograph, quietly unveiled in 1938 and quite unlike anything the Swiss watch industry had ever produced. It is not a watch in any conventional sense, as it does not tell the time. There is no hour hand, no minute hand, no twelve o’clock. Instead its dial is printed in degrees, and its movement is geared not to the twenty-four-hour solar day but to the slightly shorter sidereal day, the time the Earth takes to complete one rotation relative to the stars rather than the Sun. Three colour-coded hands sweep concentrically across the face: black, red and blue, displaying a single angular value, the Greenwich Sidereal Angle.
When a star met the horizon in the navigator’s sextant, a press of a button froze the blue hand in place while the movement kept running. That frozen reading, combined with the star’s known position from an almanac, gave the navigator’s longitude in one subtraction. No conversion was needed, no racing to finish the arithmetic in an aircraft flying hundreds of miles an hour. The entire chain of calculation that had plagued celestial navigation for centuries collapsed into a single mechanical reading.
Siderographs are scarce, and the wrist version scarcer still, with only six examples known. They were never catalogue watches. The Longines reference cards show a family of some twenty references, most of them built for a specific military or scientific client. The idea itself is older than has generally been recognised. Longines’ own records trace it to a 1934 visit from the American airmail pilot Wesley L. Smith, which led to a first patent for an arc-graduated navigation dial in 1935. In 1937 Captain Francisco Radler de Aquino of the Brazilian Navy proposed a timepiece reading directly in angular degrees, a commission also documented in the Chronique des Métiers de l’Industrie Horlogère (1948). The instrument that resulted was refined with French long-distance aviators and made its public debut at the 1939 New York World’s Fair.
For the serious Longines collector, the Siderograph represents Longines at the absolute peak of its capabilities during the golden era of mechanical navigation.
The following article is an in-depth evaluation of this unique instrument. I’d like to give special thanks to Longines’ exceptional heritage department, and in particular to Daniel Hug, Head of Branding & Heritage, without whom this article would not be possible. Longines gave me unique access to its heritage archives: the original patents, the cliché photographs, the reference cards for every Siderograph model, and factory delivery records, including a documented consignment to Ostersetzer in Milan in May 1943 and wrist variants with serials 5,810,014 and 5,810,032.
Before the advent of GPS, long-range aviators and mariners lived (and in some cases died) by the angle, not by hours and minutes. Every celestial calculation was ultimately working toward a Greenwich Hour Angle, the one number that could place a navigator on the chart and tell them where on the planet they were.
Traditional navigation required laborious calculations, converting time into arc, referencing ephemerides, the printed astronomical almanacs that tabulate the positions of stars, planets, the Sun and the Moon in angular units and applying corrections. Tasks feasible for a ship’s navigator with the luxury of time, but daunting for a pilot in flight. Two developments converged to attempt a solution.
The first was new ephemerides, whereby international observatories (by 1935) had published tables giving star positions directly in angular units degrees, minutes, and tenths rather than in solar time. The angular language of the sky was now available on paper.
The second was that Longines, steeped in aerial chronometry, decided that if they could use their technical expertise to design an instrument that could take advantage of the new angular ephemerides, it would be invaluable to aviation.
Their answer was the Siderograph unveiled in 1938, arguably an engineering feat on a level of sophistication and construction neither rivalled nor attempted by any other manufacturer of the time. Gear the train to sidereal rate, print the dial in degrees and arc-minutes, and add a split-seconds hand to freeze the instant of the sight. Drive three nested hands so the instrument directly displays the Greenwich Sidereal Angle. Add a rattrapante so the navigator can capture the exact moment a star hits the sextant’s horizon. For rough conditions, add two rattrapantes to read the bisector, giving a mean reading that forgives the jolting motion from turbulence or rough seas. In some deck and cockpit instruments, they even went so far as to control the environment itself with a thermostated heater and gimbals, so the balance is isolated from the aircraft as much as possible.
The end result compresses the multi-step arithmetic to a single subtraction or addition of a co-right-ascension exactly where speed and accuracy matter most, in theory a revolutionary navigational instrument.
The Siderograph fascinates me because it sits at the junction of chronometry, astronomy, and engineering design; a purpose-built machine that encapsulates the final pre-electronic moment of celestial navigation. It bridged a brief, but historically significant gap, between the new angular ephemerides and the arrival of radio navigation that would render both celestial fixing and the Siderograph obsolete within a decade.
In the decade between Lindbergh’s 1927 crossing and the outbreak of World War 2, aviation’s pace of change was extraordinary. Flight speeds doubled, ranges extended well beyond shore-based radio beacons and military planners on both sides of the coming conflict began to seriously grapple with the demands of long-range navigation over open ocean. The bubble sextant had introduced celestial fixes to the cockpit and purpose devised air almanacs had followed. The procedures to complete the calculations now existed. The bottleneck was human, the speed and reliability at which a trained navigator could execute them.
Longines had worked closely with two of the era’s leading navigation practitioners. Philip Van Horn Weems, a naval officer, aviator, and navigation reformer collaborated with Longines on the Second-Setting Watch, devising a rotating seconds track allowing pilots to synchronise precisely to a radio time signal without having to stop the movement and compromise the watch’s accuracy. Charles Lindbergh, fresh from his 1927 Atlantic crossing, worked with Longines on the Hour Angle Watch, it built on the Weems to create a more sophisticated watch whose dial helped convert civil time into an angular hour angle for celestial calculations. Both were genuine, revolutionary advances. Neither went far enough, still leaving the conversion from time to angle in the navigator’s hands.
The Siderograph’s story, it turns out, begins earlier than anyone has recognised – not with the patent of 1938, but four years before it. According to Longines’ archives, a visit to Saint-Imier in 1934 by the American aviator Wesley L. Smith gave the company the idea for a navigation watch whose dial was marked in arc rather than in hours, and a letter from Smith preserved in the archives offered further advice. Longines built three prototypes, a labour-intensive and expensive exercise, and then applied for Swiss patent protection.
Smith is a largely forgotten figure today, but he was one of the most experienced instrument pilots of his generation. He flew for the Post Office Department’s Air Mail Service from November 1919 to June 1927, one of its longest-serving pilots, and in 1924 he set a transcontinental record carrying a 500-pound mail load from New York to California. When the service passed to private contractors he joined National Air Transport, later part of United Air Lines, becoming its Cleveland superintendent in 1928. He was later chief pilot of Transcontinental and Western Air (TWA), and in 1936 he crash-landed an airliner in a Chicago lot, saving all fifteen people on board. In October 1934 he flew as Jacqueline Cochran’s co-pilot in the MacRobertson London to Melbourne air race, which would have brought him to Europe in the autumn of that year. He died in February 1963, aged 69.
Because Longines was already working with Philip Van Horn Weems, the company took care to submit the new design to him. In a letter dated 9 January 1935, Weems confirmed that it did not infringe his own patents.
The result was Swiss patent CH 184 338, dated 21 September 1935. It describes a timepiece for determining the hour angle of the Sun relative to the Greenwich meridian: a chronograph watch with an arc-graduated dial, and a mechanism controlled by the rotating bezel so that the equation of time could be allowed for simply by turning the bezel through the corresponding angle. In other words, it took the principle of the Lindbergh Hour Angle Watch and moved the angular scale from the bezel onto the dial itself. It was the first of six patents Longines would file around celestial navigation over the following six years.
In 1937, following a proposal by Captain Francisco Radler de Aquino of the Brazilian Navy, Longines began work on a timepiece that displayed time directly in angular degrees. Radler de Aquino (1878–1953) was an internationally recognised pioneer of marine and air navigation, known for his simplified, uniform methods of solving the astronomical triangle using log tangents and log secants. He understood exactly where a navigator’s time was lost, and he wanted an instrument that removed the conversion altogether.
His name survives on the Longines reference cards. Reference 4305 is recorded as “Calotte aluminium, angle horaire ‘De Aquino’”: an hour-angle dial with centre seconds, cal. 19.71, in a round aluminium cockpit capsule carried on four spring mounts. Reference 4350, “Calotte Avion Radler – De Aquino”, followed with the larger cal. 37.9, a centre-seconds rattrapante and a balance-stop. Reference 4351 added a rotating bezel with index, but its card is struck through with the note “Annulé, voir réf. 4384”. The three-scale dial in black, red and blue is already present on these first instruments.
The Aquino prototype was then improved with modifications suggested by some of France’s leading longdistance aviators: André Japy, Commandant Maurice Rossi, and Paul Codos, chief pilot of Air France. Codos and Rossi were not casual advisers. In August 1933 they had flown the Blériot 110 “Joseph Le Brix” non-stop from New York to Rayak in Lebanon, 9,104 km (5,657 miles), a world distance record, with Longines chronometers on board.
Out of that process came reference 4384 and, with it, the name. The reference cards capture the very moment it changed, and for me this is the most evocative discovery in the whole archive. The cards for references 4384 and 4470 were originally typed “Chronomètre Avigation”, using the period contraction of “aerial navigation”. On each, the typed designation has been struck through and “Sidérographe” written over it by hand in green ink.
What follows is a detailed examination of the Siderograph, how it was conceived, how it worked, who used it, and why the handful of surviving examples occupy a unique position in the history of mechanical horology. It draws on the Longines patents, internal brochures, period trade publications and factory delivery records, including consignments traced through Longines’ heritage archives to clients from the French Aéronavale to the Italian Navy.
Longines protected the invention with a family of six Swiss patents filed between 1935 and 1941. The rotating bezel appears in several of them, but it is only one component among more complex devices, all designed to speed up navigation, whether by fixing a sequence of times rapidly or by translating time into hour angle and minutes of arc.
The 1935 patent for the Sun’s hour angle (CH 184 338) was supplemented by a more general one, CH 203 730, filed on 14 April 1938 and published on 1 July 1939. It extended the arc-graduated dial to the Greenwich hour angle of all celestial bodies. The main claim is for a watch movement driving at least three hands against three concentric scales, exactly what became the Siderograph, and the sub-claims extend protection to chronograph functions and to single and double split-seconds mechanisms operated by a push-piece.
CH 204 297, filed on 20 May 1938, does not concern the rotating dial itself but the winding and setting mechanism, with a control crown that operates an auxiliary rotating dial or rotating inner ring for chronometric observations. It remained in force for a decade and was cancelled only on 12 February 1948, a measure of how long this type of watch stayed in use.
CH 210 856, filed on 21 April 1939, covers the rhumb-line bezel, the lunette loxodromique: at least one tabular element, placed on the dial or on a bezel that may be movable, carrying trigonometric values to help solve navigation problems when fog or storms make star sights impossible.
CH 214 225, dated 15 April 1941, is the first to refer explicitly to a watch case with a rotating graduated bezel. Longines had been using a spring-loaded rotating bezel for some time; the patent adds a mechanism that stops the bezel precisely at each mark as it is turned.
CH 216 203, published on 17 November 1941, completes the series. It describes a chronometer with a large centre-seconds hand, at least one of which is split, so that the mean of two or three observations can be taken, together with a rotating dial and a balance-stop. The aim was to overcome a limitation of the traditional chronometer: the seconds hands, including the split hand, could be set to the nearest fraction of a second against a radio time signal, which brings the invention close in spirit to the Weems watches and their rotating inner disc.
In 1939 Longines also trademarked the names “Siderograph” and “Sidérographe Aviation/Navigation”, a move assertive enough to force Breguet to market a similar device under the name Sidéromètre instead.
| Patent | Date | What it protects |
|---|---|---|
| CH 184 338 | 21 September 1935 | Arc-graduated dial for the hour angle of the Sun; equation of time set on the bezel |
| CH 203 730 | Filed 14 April 1938, published 1 July 1939 | Three hands on three concentric scales for the Greenwich hour angle of any body; single and double split-seconds |
| CH 204 297 | Filed 20 May 1938, published 1 August 1939; cancelled 12 February 1948 | Crown-operated rotating dial or inner ring |
| CH 210 856 | Filed 21 April 1939, registered 15 August 1940, published 16 October 1940 | Lunette loxodromique: tabulated trigonometric bezel for dead reckoning |
| CH 214 225 | 15 April 1941 | Rotating graduated bezel with a precise stop at each mark |
| CH 216 203 | Published 17 November 1941 | Chronometer with split centre-seconds, rotating dial and balance-stop, set to the radio time signal |
After 1943 – LORAN (Long Range Navigation) – changed the calculus entirely. Developed in the United States from 1940 and operational across the North Atlantic by early 1943, it was a radio-based system that allowed ships and aircraft to fix their position by measuring the difference in arrival time between signals from pairs of land-based transmitters. Crucially it required no clear sky, no sextant, no almanac, and no arithmetic, just a receiver.
By 1945, 75,000 LORAN receivers were deployed and the system covered the Atlantic and most of the Pacific. Where the Siderograph had eliminated the arithmetic of celestial navigation, LORAN eliminated the need for celestial navigation altogether. The Siderograph did not fail, technological advances merely eclipsed it and made it effectively obsolete. Surviving examples migrated to laboratories, schools, and storage. The wrist variant never became a catalogue line.
Naming. Period French uses Sidérographe (accent often omitted in export texts). English circulars retain Siderograph. The rhumb accessory is consistently Lunette Loxodromique.
Sidereal time is the natural clock of the stars. Unlike solar time which tracks the Sun and forms the basis of the civil clocks we use every day, sidereal time tracks the Earth’s rotation relative to the distant stars. Because the Earth is also orbiting the Sun, a sidereal day is approximately four minutes shorter than a solar day: 23 hours, 56 minutes, and 4 seconds. The stars therefore rise and set slightly earlier each night and a clock geared to sidereal rate stays in step with them.
By late 1938 the first Siderographs were leaving Saint-Imier. Longines designated them Aero-Navigational instruments, signing dials Longines Aero-N. No. XXXXX – a serial convention that underlined their status as individual technical apparatus rather than catalogue watches. Each was built in collaboration with a specific military or scientific client. Specifications varied with the client’s need: heated and gimballed aluminium capsules for cockpit and bridge use, simpler open-cased instruments for observatory and laboratory work, sextant-mounted versions for in-line celestial fixing (ref. 4486), a pocket version (ref. 4584), and a small run of wrist examples (ref. 4592) produced in 1942–43. In its marketing, Longines was unambiguous about the ambition: period advertising positioned the Siderograph as enabling star navigation entirely independent of radio signals – what would later be recognised as a precursor to satellite-based positioning systems.
With ephemerides tabulated in angular units for the Sun, Moon, planets, and bright stars, a navigator using the Siderograph can reduce a sight directly by subtracting a star’s Right Ascension (RA) from the dial’s reading, dispensing with the need to make a time-to-angle conversion. The Siderograph is in essence a special-purpose mechanical computer, built for a narrow but critical task: expressing the Greenwich Sidereal Angle with enough resolution (0.2 arc-minutes) and with a capture mechanism that maps precisely to how celestial sights are actually taken. Longines described it as the “Weiterentwicklung und Perfektionierung der Stundenwinkeluhren” the evolution and perfection of the hour-angle watch, meaning the Lindbergh.
The tri-colour dial is designed for error-elimination. Blue for arc-minutes, red for degrees within the current ten-degree window, black for the coarse 0–360° sweep. The hand speeds mirror the quantities they represent. The heated capsule is a laboratory instrument with a handle, designed to compensate for the metrology of steel expanding, oils thinning and fogging glass.
Quick breakdown:
The Siderograph displays Greenwich Sidereal Angle (GSA), the GHA of the vernal point, not civil time. With a body’s right ascension (RA) expressed in degrees, the reduction is immediate:
GHA(body) = GSA − RA(body) (mod 360)
(Many aeronautical tables of the period tabulate the versed ascension instead, which is 360° minus the right ascension. In those cases you add, and subtract 360° if the total exceeds it.) Mechanically, the inner dial ring rotates independently inside the fixed chapter, and that moving ring rather than the hands alone, is what you align to the sidereal reference before reading blue → red → black. This suppression of the HMS→angle conversion is the instrument’s defining mechanical achievement.
At first glance a Longines Siderograph dial is mystifying. There are no hour or minute numerals instead a geometric scale of degrees and fractions in three colours, each hand sweeping at a different rate. Longines described the standard dial layout in 1939 as follows:
Black Circle (Middle Scale) – Marked 0° to 360° in 10° increments. The black hand makes one rotation in 23 hours 56 minutes one sidereal day moving continuously to indicate approximate sidereal time as an angle. This hand shows the Earth’s rotation relative to the stars: the hour angle of the vernal equinox. Set to 0° at Greenwich sidereal midnight, it reads current Greenwich sidereal time directly in degrees.
Red Circle (Inner Scale) – Subdivided into 10° units. The red hand rotates once every 40 minutes of sidereal time, corresponding to one 10° division of the black scale. It refines the black hand’s reading to the exact degree. As Longines noted, it “performs one revolution in the same interval the black pointer covers one division on the middle circle.” If the black hand lies between 130° and 140°, the red hand indicates precisely how many degrees (0–10) beyond 130°.
Blue Circle (Outer Scale) – Printed 0–60 (minutes of arc) with numerals every 5′. There are 300 hash-marks per degree; each tick = 0.2′ (12″). The blue hand makes one revolution per degree (~4 sidereal minutes). Read minutes 0–59.x at the index; tenths of a minute (~0.1′) are estimated by interpolation between the 0.2′ ticks.
When taking a reading, always read the blue hand first, it moves fastest. Then red, then black. Add the three values: a blue reading of 0.2′, red of 7°, black of 90° gives 97° 0.2′. The instrument can be read to 0.1 of arc, which is 6 seconds of arc, or about 0.4 seconds of time.
Blue rattrapante sweep freezes the instant of a sextant sight; double graduated rhumb line bezel aids dead reckoning. Oversized onion crown and auxiliary pusher for gloved use; LONGINES BREVETS signature, an ultra rare wrist variant believed made in only 6 examples.
To make this functionality practical in a turbulent cockpit or at sea, Longines incorporated rattrapante (split-seconds) features. Period texts (FH 1939/40; brochure MAY 1939) and surviving dials confirm the arrangement:
Reading order: blue → red → black. Example from the brochure’s Aldebaran observation: blue 47′, red 7°, black 270–280° band ⇒ 277° 47′.
To synchronise: rotate the outer ring so that at the Greenwich radio tick, the printed “60” sits directly under the blue hand, then push the blue crown to re-engage. The slide marked START/STOP (MARCHE/ARRÊT) hacks the balance for signal alignment. Some capsules add a power-reserve indicator at 12.
In practical aerial work, reading precision beyond 0.2′ (12″) is illusory, turbulence, bubble wander and instrument parallax dominate long before the scale becomes the limiting factor. The outer ring’s 300-tick density is therefore an engineering choice calibrated to real-world conditions rather than theoretical maximum resolution.
Almost all Siderographs share a common dial layout: a two-tone silvered face, outer brushed chapter and inner matte, with three concentric scales (black 0–360°, red 0–10° within each block, blue with 300 ticks to the degree). Several of the cal. 21.29 references add a power-reserve arc at 12. Many carry an inner rotating dial or ring, described in German as innere Drehlünette and on the Longines cards as cadran tournant. This is the arrangement protected by patent CH 204 297, in which a crown turns the ring for sidereal calibration. The outer lunette loxodromique of patent CH 210 856 is a separate device for course calculations. One ring is for setting, the other for computing.
On the wrist-worn Siderographs the crown engaged the movable inner dial ring, allowing rotation for alignment or calibration. Surrounding this were one or sometimes two graduated outer bezels engraved with degrees of arc and trigonometric scales described in period texts as double lunette tournante graduée. Most likely one bezel acted as an hour angle index ring while the other embodied the rhumb line calculator of patent CH 210856. Surviving examples show bezels engraved with sine and cosine values and interpolation factors, confirming real computational use. The combination of a rotatable inner dial and data-laden bezels makes the wrist Siderograph a uniquely sophisticated piece of 1930s navigation engineering, a mechanical calculator worn on the wrist.
Single split. At the exact moment of your sextant reading, the blue hand freezes the instant while the running blue continues. Read the frozen hand (blue → red → black), note the value, press again to reunite.
Double rattrapante (averaging). A single sight in rough conditions can be skewed by random motion; capturing two stops and reading their mean cancels much of that error. Two stop hands share the arbor and are controlled by one pusher. Take a rapid series period literature suggests six sights in ~3 minutes. Freeze the first (α₁) and last (α₂). The mean is ᾱ = (α₁ + α₂)/2 found on the dial by bisecting the angle between the two stopped hands. This forgives aircraft motion and bubble wander in air-sextants. In some variants a flexible cable release or electromagnet fires the split remotely, so the observer never leaves the eyepiece.
Remote actuation. The reference cards show how seriously Longines took the problem of keeping the observer’s eye at the sextant. The cockpit-capsule instruments (refs. 4384, 4470, 4487, 5129) use a flexible cable release, exactly as on a camera, clipped to the side of the capsule. The later boxed instruments go further: refs. 4618, 5397 and 6081 are described as having commande électrique à distance, an electrically fired split operated from a pear-shaped hand switch or a separate push-button block, so the navigator could work at some distance from the instrument. Patent CH 216 203 of 1941 brings these ideas together: split centre-seconds, rotating dial and balance-stop, with the hands set against the radio time signal.
What makes the Siderograph so coveted among collectors is not just its rarity, but also its uncompromising engineering. Longines took their finest chronometer movement and extensively modified it for this specialised task.
The Siderograph began with cal. 37.9. After the first Aquino instrument, ref. 4305, which used cal. 19.71 with centre seconds and no split, every early cockpit reference was built on Longines’ 16½-ligne split-seconds movement: ref. 4350 for Radler de Aquino, the cancelled ref. 4351, ref. 4384, which first carried the name Sidérographe, ref. 4470 with its index bezel, and the three-instrument ref. 4635 set. The cards describe them uniformly as “37.9, 2de au Centre, Rattrapante”, the earliest with “arrêt du balancier”. Refs. 4486 (sextantmounted) and 4487 (with lunette loxodromique) are listed as “37.9 ou 21.29”, showing the point at which the larger calibre became an option. The same movement, in a 48 mm steel pocket case (ref. 4584) and a 46 mm steel wrist case (ref. 4592), made the concept portable.
Base calibre. Longines cal. 37.9 split-seconds, manual wind; 16½ lignes (~37.9 mm), height 7.7 mm, 16 jewels, lever escapement at 18,000 A/h, bimetallic balance with Breguet over-coil, six-bridge nickel finish. Production of the base calibre begins in 1932; the museum wrist example (no. 5810014) is from this family.
Adaptation for Siderograph duty. Sidereal gearing: the going train is re-ratioed so the black hand completes 360° per sidereal day (≈ 23 h 56 m 4 s). Indication drives: secondary ratios deliver the red hand (0–10° per 40 sidereal minutes) and the blue sweep (1° per ≈ 4 sidereal minutes) over the 300-tick outer scale (0.2′ / 12″ per tick). Rattrapante works: the split-seconds clamp on the blue sweep’s train allows the observer to freeze a sight while the runner continues; some instruments show provision for first/last capture and reading the bisector for averaging. Hacking & zeroing: MARCHE / ARRÊT slide to stop the balance for radio-tick synchronisation; crown/outer-ring arrangement to set the blue sweep to the printed “60” index at the signal.
Contrast with the cal. 21.29 instruments. The boxed navigation Siderographs use cal. 21.29 (21 lignes), often with a power-reserve indicator, a double rattrapante, and in some references electric heating and lighting. The cal. 37.9 instruments omit the heater and power reserve, yet package the same triple-scale, split-capture logic in a movement small enough for a cockpit capsule, a pocket or a wrist.
For the instruments intended for the chart table and the ship’s bridge, Longines turned to cal. 21.29, a large 21-ligne (~47.2 mm) movement originally built for deck chronometers and observatory trials, the most accurate in Longines’ 1930s stable. The reference cards list it in ref. 4458 (the boxed “Chronomètre Navigation”), refs. 4510 and 5353 (gimballed mahogany boxes with lunette loxodromique, single and double rattrapante respectively), refs. 4512 and 4618 (double rattrapante in an aluminium box), ref. 5129 (double rattrapante in a sprung cockpit capsule for the Marine Nationale), refs. 5397 and 6081 (electrically controlled), and the later derivatives 8299 and 8385. For the Siderograph it was regulated to sidereal rate (approximately 3 minutes 56 seconds per day faster than mean solar time) and equipped with purpose-built functions: a split-seconds rattrapante capture mechanism – on some instruments with two split hands for averaging – and an up/down power-reserve. Auction catalogue descriptions of surviving examples record a monometallic balance with self-compensating Breguet hairspring, a beryllium pallet fork for antimagnetism and a micrometric regulator, plus a hacking (MARCHE / ARRÊT) device for precise synchronisation. Finishing is chronometer-grade: Côtes de Genève, a large balance, and crisp bevels throughout.
Sidereal gearing. The going train is re-ratioed so the cannon for the black hand completes 360° per sidereal day. Secondary trains step up to move the red hand 36× faster (10° per black-hand division in 40 sidereal minutes) and the blue hand 360× faster (1° in ~4 sidereal minutes). The outer scale’s 300-tick/degree resolution means the blue arbor and hand-hub must run true with minimal radial lash a reason the split works are unusually refined.
Split-seconds works. The blue arbor carries a classic rattrapante clamp system with polished steel jaws and a resilient spring to arrest without bounce and to rejoin cleanly at speed. On double versions a second clamp and hand are stacked; careful jewelling and end-shake control keep drag low. Surviving examples show relieved levers and mirror-polished contact faces.
Hacking & control. A lateral lever halts the balance when the MARCHE/ARRÊT slide is pulled. A separate train links the blue crown and outer ring for zeroing to the signal.
Service implications. The heated references (ref. 4512, its successor ref. 4618, and related instruments) were designed to run warm. Period texts specify −30°C ambient with a thermostat holding the capsule at ~20°C. Modern oils extend range, but amplitude still collapses if the heater or thermal path fails. Split clamps must be set parallel – any twist drags the running hand.
Where the wrist variants of 1942–43 traded thermal management for portability, the deck and cockpit instruments went the other way. The case is itself a precision instrument, a heated, gimballed aluminium capsule built to keep the movement at a stable temperature, isolated from vibration, and legible at a glance in poor light. The principal elements are summarised below.
Several surviving deck Siderographs feature a circular aluminium container with a built-in electrical heater and thermometer, powered by the aircraft’s or ship’s circuit. A Siderograph auctioned by Sotheby’s was noted to have a heating system in its outer case, complete with an on-off switch and wiring (though untested at time of sale). This assembly was typically mounted on shock-absorbing springs within a gimballed box – the entire setup resembling a small marine chronometer: a mahogany box with brass gimbal rings suspending a round metal case beneath a glass cover. Serial 6425248 (Christie’s 2004) and 5810653 (Dr. Crott Auction 2014) both show this configuration. On military examples, “Marche/Arrêt” (Start/Stop) is often engraved near the external stop lever or cable attachment, reinforcing the instrument’s purpose-built character.
The Longines reference cards distinguish several families of case. The calotte avion is a round aluminium capsule carried on four coil-spring mounts, meant to be bolted to an instrument panel; it houses the early cal. 37.9 references and the Marine Nationale’s ref. 5129. The coffret acajou à cardan is the gimballed mahogany box of refs. 4510 and 5353, a marine chronometer in all but its dial, and ref. 4458 sits in a similar wooden box. The coffret métal or coffret aluminium of refs. 4512, 4618 and 6081 is a square aluminium housing with external terminals for the electrical supply, which powers the remote split, the dial lighting (regulated by rheostat on ref. 6081) and, on the heated references, the thermostat. Ref. 5397 is a wooden box expressly “sans cardan”, without gimbals. Finally there are the sextant mount of ref. 4486 and the steel pocket and wrist cases of refs. 4584 and 4592.
Capsule. Die-cast aluminium housing, typically 94 mm frontal diameter for cockpit legibility; four or more mounting bosses; locking glazed cover.
Heating. Electric element in the base, wired to aircraft supply; thermostat prevents drop below 20°C and suppresses fogging at altitude. The brochure shows the element and wiring.
Suspension. Gimbal ring (Cardan) and rubber isolation decouple vibration; some capsules include jewelled pivots for the gimbal.
Lighting. Small bulbs, same circuit as the heater, illuminate the dial; some capsules include a thermometer aperture.
Controls. External START/STOP slide; colour-coded knurled crowns (blue relates to the outer ring); long-throw pusher for the split; port for remote cable.
Field ergonomics. The outer ring’s blue “60” alignment is visible at a glance; the power-reserve arc sits at 12; the split pusher is oversized for gloved use; and the START/STOP slide is engraved in clear capitals.
The Siderograph has usually been described in three configurations: deck or cockpit instrument, wrist version, and sextant-mounted. The Longines reference cards, made available by the heritage department for this article, show a far richer picture: some twenty-one references from 1937 onwards, each with its own cliché photograph. They are set out below in reference-number order, which is broadly the order of development.
| Ref. | Calibre | Complication | Bezel / dial | Case | Notes from the Longines card |
|---|---|---|---|---|---|
| 4305 | 19.71 | Hour-angle dial, centre seconds | – | Aluminium calotte | "Angle horaire 'De Aquino'". The first instrument for Radler de Aquino. |
| 4350 | 37.9 | Rattrapante, balance-stop | – | Calotte avion | "Calotte Avion Radler – De Aquino". |
| 4351 | 37.9 | Rattrapante, balance-stop | Rotating bezel with index | Calotte avion | Cancelled: "Annulé, voir réf. 4384". |
| 4384 | 37.9 | Rattrapante | – | Calotte avion | START/STOP indication, cable remote. Card retitled from "Chronomètre Avigation" to "Sidérographe". |
| 4458 | 21.29 | Rattrapante | Rotating dial, set by an extra crown through the box | Wooden box | "Chronomètre Navigation – Breveté". |
| 4470 | 37.9 | Rattrapante | Rotating bezel with index | Calotte avion | START/STOP indication, cable remote. "Chronomètre Avigation avec Index", retitled "Sidérographe". |
| 4486 | 37.9 or 21.29 | Rattrapante | – | Sextant mount | Electric lighting: "montage sur sextant". |
| 4487 | 37.9 or 21.29 | Rattrapante | Lunette loxodromique | Calotte avion | Cable remote. |
| 4510 | 21.29 | Rattrapante | Lunette loxodromique | Gimballed mahogany box | START/STOP indication, cable remote. |
| 4512 | 21.29 | Double rattrapante | Lunette loxodromique | Aluminium box ("coffret métal") | Power reserve, remote, heating for constant temperature. Replaced by ref. 4618. |
| 4584 | 37.9 | Rattrapante | Lunette loxodromique | Stainless steel pocket watch, 48 mm | Pocket Siderograph. |
| 4592 | 37.9 | Rattrapante | Lunette loxodromique | Stainless steel wristwatch, 46 mm | "Sidérographe bracelet". |
| 4618 | 21.29 | Double rattrapante | Lunette loxodromique | Aluminium box | Commande électrique à distance. Successor to ref. 4512. |
| 4635 | 37.9 | Set of three instruments | – | Calotte avion | "Sidérographe et montre civile pour utilisation Air Almanach": G.S.A., G.H.A. Soleil and G.M.T. |
| 4737 | 21.29 | Rattrapante, balance-stop | Rotating bezel without index; rotating dial | Aluminium calotte in wooden box | 12-hour dial. A time chronometer of the same family, without the degree dial. |
| 5129 | 21.29 | Double rattrapante | – | Calotte avion on springs; also supplied boxed | Power reserve, cable remote, MARCHE/ARRÊT. Made for the Marine Nationale Aéro. Card: "4384, mais 21.29 dble Ratt". |
| 5353 | 21.29 | Double rattrapante | Lunette loxodromique | Gimballed mahogany box | As ref. 4510 but with double rattrapante. |
| 5397 | 21.29 | Rattrapante | Rotating dial | Aluminium calotte in wooden box, no gimbals | Power reserve; electric remote for the rattrapante. |
| 6081 | 21.29 | Rattrapante | Lunette loxodromique; rotating dial | Aluminium box | Electric remote; dial lighting by rheostat. |
| 8299 | 21.29 | Double rattrapante | – | Aluminium box | Derived from ref. 4618. |
| 8385 | 21.29 | Rattrapante | – | Wooden box | Derived from ref. 5397. |
Two things stand out to me. First, the concept was settled early: the three-scale dial of ref. 4305 is recognisably the dial of every later instrument. What changed was everything around it, the calibre, the split, the bezel, the housing and the means of firing the rattrapante from a distance. Second, the line did not end in 1943. Ref. 4512 was superseded by ref. 4618, and refs. 8299 and 8385 are later derivatives again. The cliché for ref. 8299 shows the instrument posed on printed ephemerides, one of them in Italian and open at the tables for the Moon.
Deck / Cockpit (~94 mm)
The “canonical” Siderograph: aluminium capsule; power-reserve at 12 on many examples; one running blue + one or two split blues; remote provision. Period dials often carry AERO-N marks and a dial serial under the signature.
In the reference cards these are refs. 4384, 4470 and 4487 (cal. 37.9) and ref. 5129 (cal. 21.29, double rattrapante) in the sprung calotte avion, and refs. 4458, 4510, 4512, 4618, 5353, 5397 and 6081 in their various boxes.
Wrist Siderographs (cal. 37.9, c. 1942–43)
Six wrist Siderographs are known to the market. Longines archive entries record at least two wrist serials, 5 810 014 and 5 810 032, in 1942–43; the adjacent 5 810 015 appears to be a related deck/pocket variant. One wrist example is preserved by the Longines Museum, Saint-Imier. At least one 37.9 Siderograph has surfaced uncased, and a spare original dial is also known in collectors’ hands. As such, the wrist Siderograph ranks among the most important vintage Longines on the market.
Configuration
Steel case 46 mm (over-sleeve fit); cal. 37.9 split-seconds re-geared to sidereal rate; blue rattrapante for instant capture (occasional double-split for averaging); inner rotating ring coupled to the crown; double-graduated rhumb-line bezel (CH 210856); onion/mushroom crown with auxiliary pusher; LONGINES BREVETS signature. Compared with deck units, wrist pieces omit the heater and power-reserve but retain the full tri-scale sidereal display.
Pocket Siderograph (ref. 4584)
The reference cards also record a pocket version, ref. 4584: the same cal. 37.9 rattrapante and lunette loxodromique as the wrist model, in a 48 mm stainless steel pocket case with bow and a second crown at two o’clock. To my knowledge it has not previously been described in the literature. Its existence may explain movement 5 810 015, which sits between the two known wrist serials in the Longines registers and has long been thought to be something other than a wristwatch.
Sextant-Mounted
Longines supplied a Siderograph mounted directly on a sextant. It is ref. 4486, described on its card as “Chronomètre Avigation (sidérographe) avec éclairage électrique – montage sur sextant”, with either cal. 37.9 or cal. 21.29, and the cliché shows the small cylindrical lamp housing clamped to the bezel to light the dial. The brochure frames the coupling with the identity H = f(α) — the observed height H is a function of the sidereal angle α so both must be recorded simultaneously; the rattrapante makes that practicable.
The following is taken directly from the Longines internal catalogue of 1939, explaining the Siderograph’s purpose in the company’s own words:
In the past, the naval officer was required to make long and difficult calculations in order to determine both their current position and direction of travel. However, the slow pace of sea navigation made such calculations possible and plausible. But a pilot flying an aircraft at 400 miles per hour must be able to determine their position in just a few minutes. The success of the flight will depend on it. It was therefore an imperative necessity to adapt navigation methods to the requirements of aeronautics. The navy could also benefit from such an improvement.
First we had to make the ephemeris more practical. This was the task of the official office in charge of composing them the Bureau des Longitudes in France, H.M. Nautical Almanac Office in England. New tables appeared which give directly in degrees, minutes and tenths of a minute of arc the Greenwich hour angle of the vernal point, the right ascensions of the Sun, the Moon, the planets and the main stars.
But a parallel simplification was necessary in chronometry. The Longines watch company produced such a tool – no surprise to naval officers who already knew the quality of its on-board instruments, nor to aviators, since the most famous among them used the special watches it created for them. Today it provides them with a completely new device: the Sidérographe. Until now, we had to transform the times marked by the stopwatch into angular values. Set on the sidereal day, divided not into hours, minutes and seconds, but into degrees, minutes and fifths of a minute of arc, the Longines Siderograph directly indicates the Greenwich hour angle of the vernal point.
Thanks to this double improvement in ephemerides and on-board instruments, the Greenwich hour angle of any star can now be obtained by simple subtraction or addition. From the sidereal angle marked in arc on the dial of the new Longines device, we subtract the right ascension of the star expressed also in angular units or we add its versed ascension.
At heart the Longines Siderograph is not a clock at all but a mechanical computer for celestial navigation. Instead of showing hours and minutes, its dial is laid out in degrees and minutes of arc and is geared to turn at the sidereal rate – the rotation of the Earth relative to the stars rather than the Sun. Set correctly, it gives the Greenwich sidereal angle directly, so the navigator can work in the same angular units as the modern ephemerides.
In practice the Siderograph sits on the navigator’s table in its heated, gimballed aluminium capsule. The navigator sets the instrument using the star tables, then when a star “kisses” the horizon in the sextant he arrests the split-seconds hand. The reading on the dial, combined with the star’s right ascension from the tables, gives the Greenwich hour angle of the star with a single subtraction. What previously demanded a chain of time conversions and trigonometry collapses into one mechanical reading.
Because the Siderograph works in pure angles it bridges sea and air navigation – precise enough for observatory work, robust enough for the North Atlantic, and quick enough for an aircraft covering several miles a minute. That blend of theoretical astronomy and very practical engineering is what makes the instrument so unusual in Longines’ history and why surviving examples are so sought after today.
To understand the Siderograph’s significance, it’s useful to compare it with other aviation chronometers of the era and recognize how it streamlined the celestial navigation process.
Weems Second-Setting Watch
Introduced in 1929, this instrument served as a civil-time synchronization aid, featuring a rotating seconds ring that allowed pilots to align their timepiece with radio time signals without stopping the movement. However, it lacked computational capabilities and still required pilots to manually convert hours-minutes-seconds readings into angular measurements for celestial calculations. Its primary use-case centered on radio discipline and maintaining accurate logbook records.
Lindbergh Hour Angle Watch
Introduced in 1931, the Lindbergh Hour Angle Watch was Longines’ second collaboration with the aviator and used a more sophisticated conversion dial that could transform civil time into hour angles for celestial navigation. While this reduced some computational burden, pilots still faced the time-consuming HMS-to-angle conversion process during calculations. The Lindbergh represented an important stepping stone, cheaper, lighter and adequate for many flights, but still requiring mental arithmetic conversions that were error-prone under stress.
The 1935 Arc Dial (Patent CH 184 338)
The missing link between the two. Prompted by Wesley L. Smith’s visit in 1934, Longines moved the angular scale from the bezel onto the dial itself, in a chronograph whose bezel now served only to set the equation of time. The instrument read the hour angle of the Sun directly, but it was still a solar instrument and still limited to one body. Three prototypes were made. Generalising the idea to the stars, by regulating the movement to sidereal time, was the step that produced the Siderograph.
The Siderograph
In 1938 the Siderograph’s revolutionary contribution was eliminating the time-to-angle conversion step that plagued earlier instruments. Pilots still needed to take sextant sights and consult tables for right ascension/declination (or co-right ascension/versed ascension), but the Siderograph provided direct angular readings, eliminating a critical computational bottleneck and source of errors.
American and British air services experimented with bubble sextants with integral clocks, averaging devices, and periscopic sextants such as the Kollsman periscopic sextant used by the USAAF and the British Mark IX none of which integrated a sidereal angular dial with rattrapante in a portable watch-format the way Longines did. In this narrow niche, Longines stands at the fore-front of innovation during this period.
| Movement no. | Ref. | Type | Delivery / agent | Features | Appearances and notes |
|---|---|---|---|---|---|
| 5,810,653 | 5129 | Double-rattrapante cockpit instrument, cal. 21.29 | Marine Nationale Aéro pattern | Dial signed Longines Aero-N. No. 5810653; power reserve; MARCHE/ARRÊT | Dr. Crott Auction 89 (2014). Illustrated by Longines Heritage as ref. 5129. |
| 5,789,285 (case 5,676,429) |
To be confirmed | Gimballed deck instrument in mahogany box, cal. 21.29 | Delivered 21 May 1943 to Ostersetzer, Milan | Split-seconds, power reserve, stop lever | Antiquorum 2007, lot 691, CHF 32,000 hammer; Monaco Legend 2018, unsold (est. €20–30k), with original certificate papers. One instrument, several appearances. |
| 6,425,248 | Probably 5129 | Deck instrument, cal. 21.29, Marine Nationale | French Aéronavale | MARCHE/ARRÊT; shock mount; original box | Christie's 2004, CHF 28,680. |
| 6,425,252 | Probably 5129 | As above | French Aéronavale | As above | Christie's 2006. Same batch as 6,425,248. |
| 6,865,944 | 4618 | Double rattrapante in aluminium box, cal. 21.29 | – | Aluminium case with heating system and light; power reserve; 94 mm | Sotheby's Geneva 2018, lot 233, catalogued c. 1945. |
| 6,865,946 | 4618 | As above | – | Lunette loxodromique | Illustrated by Longines Heritage. Two numbers from the Sotheby's example, so evidently the same batch. |
| 5,810,014 | 4592 | Wrist Siderograph, cal. 37.9 | 1942–43 | Lunette loxodromique; LONGINES BREVETS | Longines Museum; illustrated in Montanari, Longines Legendary Watches, p. 284. |
| 5,810,032 | 4592 | Wrist Siderograph, cal. 37.9 | 1942–43 | As above | Recorded in the Longines archives. |
| 5,810,015 | Possibly 4584 | Unconfirmed; possibly the pocket version | – | – | Sits between the two wrist serials in the registers. |
| Not recorded | 5129 | Deck instrument, Marine Nationale Aéro, in glazed wooden box | French Aéronavale | Double split, power reserve, 124 mm | Private collection; photographed by Santi (mimandcroket). |
| Not recorded | – | Dial (Flückiger, stamped 1 6461 ZJ) and cal. 21.29 movement | – | Toothed dial rim | Collection of Louis F. Vuille. |
Working census of known Siderograph examples. Wrist variants highlighted. Additional examples likely exist in private collections or institutional holdings not yet documented.
How to Use the Longines Siderograph – Three Simple Steps
Step 1: Synchronise Before Your Flight. Set the Siderograph to Greenwich sidereal time using a radio signal. Rotate the outer ring so the blue “60” mark aligns with the sweep hand at the time signal, then use the START/STOP slide to hack the balance for precise synchronisation. Verify the heater is maintaining ~20°C and check the power reserve indicator at 12 o’clock.
Step 2: Capture Your Celestial Observation. At the exact moment your sextant sight is aligned, when the star hits the horizon, press the remote trigger or pusher to freeze the split-seconds hand. The dial now displays the Greenwich sidereal angle at that precise instant. Read the three scales in order: blue hand (minutes of arc), red hand (degrees within the 10° segment), then black hand (which 10° segment on the 360° scale).
Step 3: Calculate Your Position in One Step. Subtract the body’s right ascension from the dial reading to get the Greenwich Hour Angle: GHA = GSA − RA. Where the tables give the versed ascension (360° − RA) instead, add it, and subtract 360° if the total exceeds it. The method is the same for the Sun, the Moon, the planets and the stars. Use this GHA with your sextant altitude and standard sight reduction tables to plot your line of position.
For navigators taking multiple observations in rough conditions, the double-rattrapante version allows you to freeze both the first and last sights of a series, then read the bisector angle between the two stopped hands as your averaged value.
In practice, here is how an aviator or mariner in 1939 would have used it:
Pre-Flight Setup.
Longines even supplied the whole arrangement as a matched set. Ref. 4635, entered on its card as “Sidérographe et montre civile pour utilisation Air Almanach”, is a group of three identical cal. 37.9 cockpit capsules: one reading G.S.A., the Greenwich sidereal angle; one reading G.H.A. Soleil, the Greenwich hour angle of the Sun, the direct descendant of the 1935 patent; and one a conventional twelve-hour G.M.T. clock with which to enter the Air Almanac.
Taking a Sight.
Say it is night and you are taking a sight on Aldebaran (the brightest star in the Taurus constellation). At the moment the star touches the horizon in the sextant, press the remote – the blue hand stops. The dial reads, for example, 132° 15.4 . From the almanac, Aldebaran’s right ascension in 1939 is approximately 68° 06.5 . Subtract: 132° 15.4 − 68° 06.5 = 64° 08.9 . That is the Greenwich Hour Angle of Aldebaran at the moment of observation. Combined with the measured altitude and standard sight reduction tables, this gives a line of position. Repeat with a second star or the moon and you have a cross-fix.
The 1939 Fédération Horlogère worked example is equally clear. An aviator sights the Sun on 13 April 1939. The Siderograph reads 293° 35 . The Aeronautical Ephemerides give 339° 12 for the Sun, which is its versed ascension, 360° minus a right ascension of 20° 48 . Adding the two gives 632° 47 , and subtracting 360° leaves a Greenwich Hour Angle of 272° 47 . A second example, using Aldebaran (alpha Tauri) the following day, works the same way: a reading of 277° 47 plus a versed ascension of 291° 53.5 gives 569° 40.5 , less 360°, or 209° 40.5 . One method for all celestial bodies, where previously the Sun and the stars required separate techniques.
Dead Reckoning with Rhumb Bezel.
When weather or daylight precluded star sights entirely, the navigator could fall back to rhumb line navigation using the Siderograph’s bezel scales. Patent CH 210856 describes a bezel with pre-calculated trigonometric value sin and cos increments at 5° intervals allowing displacement from course and speed to be computed without manual sine and cosine arithmetic. A pilot flying an hour at 200 knots on heading 045° could read his northing and easting directly from the bezel. Longines was integrating what we would now call flight computer functions into a wearable instrument, decades before digital avionics.
Production of the Siderograph was limited. The serial evidence points to a client-driven programme rather than anything approaching general commercial production, but it was not a matter of one-off pieces either. A remarkable cliché in the Longines archives shows some thirty examples of a single reference, ref. 5129, laid out together for the camera, evidently a single batch of the Marine Nationale pattern. Known movement numbers run from the 5.78 million range to the 6.86 million range, that is from about 1938 to about 1945, with at least one confirmed delivery in May 1943, and the reference cards show derivatives still being created after that.
1938–39 – Introduction, prototypes, patent filings. By this point the cal. 37.9 cockpit references (4350, 4384, 4470) already existed. Movement 5,810,653, long thought a possible prototype, is identified by Longines as ref. 5129, the double-rattrapante cal. 21.29 instrument made for the Marine Nationale Aéro. Longines trademarked the name in 1939 and began advertising in technical journals.
1939–40 – Orders from confirmed military clients. The French Aéronavale was the client for ref. 5129. Two surviving deck instruments, movements 6,425,248 and 6,425,252, both cal. 21.29 and both marked for the Marine Nationale, are documented through Christie’s sale records in 2004 and 2006. A consignment went to Ostersetzer of Milan, Longines’ Italian agent, with a delivery confirmed by Longines heritage archive records dated 21 May 1943. The end user is not named in the extract. British RAF interest is documented anecdotally, with at least one example reportedly used on long-range Atlantic patrols.
1941–43 – Small-batch production. Serials 5,810,014 and 5,810,032 were delivered in this window. The Monaco auction piece (movement 5,789,285, case 5,676,429) was delivered May 1943 – the differing movement and case numbers hinting at in-house assembly from existing parts. Wider deployment was likely curtailed by the advance of electronic radio beacons and LORAN.
Post-1945 – Electronic and inertial navigation superseded the concept. By the late 1940s the need for a mechanical sidereal instrument had faded. Longines did not pursue civilian commercialisation; the Siderograph remained a bespoke tool. Daria Marozzi’s Longines reference book (1990) covers it on pp.100–107 – by then already a historical footnote known only to dedicated collectors.
The principal examples, summarised from the census above:
The Siderograph was designed to solve a specific, urgent, life-or-death problem faced by navigators flying at speed over a featureless ocean or terrain in the years before electronic aids made celestial navigation outdated. Longines solved it by reimagining the scope of what a mechanical instrument could be. And then other technologies superseded it.
It would be wrong, though, to say that the Siderograph stopped dead in 1943. The reference cards tell a longer story, one that begins with Wesley Smith’s visit in 1934 and the patent of 1935, not 1938, and that continues with ref. 4618, catalogued around 1945, and with the later derivative refs. 8299 and 8385. Aviation moved on to radio, but the reference cards suggest that some clients, for whom a star sight remained the fix of last resort, did not move on quite so quickly.
LORAN, the wartime radio navigation system operational across the North Atlantic by early 1943, did not merely improve on celestial navigation, it made it optional. No clear sky required. No sextant, no almanac, no arithmetic, only a receiver. The Siderograph’s design window, the years between the new angular ephemerides of the mid-1930s and the arrival of radio fixes, lasted perhaps half a decade. The concept did not fail, the problem it solved was itself resolved by other technologies, faster than anyone could have anticipated when Longines filed the Siderograph patent in April 1938.
This is probably the most technically important instrument Longines ever produced. More ambitious than the 13ZN flyback chronographs, more sophisticated than the Lindbergh Hour Angle watch that preceded it, a dazzling chronometer-grade movement regulated to sidereal time, with a rattrapante capture mechanism. Alfred Pfister, Longines’ own technical director, described it in 1942 as without doubt the supreme expression of the company’s aviation timekeeping work. It is difficult to argue with that assessment. Built to a level of complexity and precision that would have placed it in an entirely different category of expenditure from any catalogue watch of the period, it was from the outset an instrument for a very particular client with a very specific need.
Longines evidently understood and was proud of its historical significance. In 1955, the company formally donated a Siderograph to the Musée d’horlogerie in La Chaux-de-Fonds, where it was placed alongside marine chronometers and astronomical watches. Alfred Pfister, who wrote the technical description that introduced the instrument to the wider horological press, was present at the handover. It was a fitting acknowledgement that the Siderograph belonged not in a display case or a dealer’s catalogue, but in the horological equivalent of the hall of fame.
The Siderograph is a contribution to navigation that remains chronically underappreciated. Whilst owning one is a near impossibility, it beguiles as exemplary evidence of Longines at their peak, during the golden age of mechanical watchmaking, when, in my opinion, the brand was the single most important manufacturer in aviation horology.
Special thanks to Daniel Hug
This article would not exist in its present form without Daniel Hug, Head of Branding & Heritage at Longines. Daniel opened the Longines archives to me and generously shared his own research into the evolution of the Siderograph: the reference cards, the cliché photographs, the patent history, and the correspondence with Wesley L. Smith and P. V. H. Weems. The origin story and the reference catalogue presented here rest on his work. I am deeply grateful for his time, his expertise and his patience with my many questions.
My thanks also to Auro Montanari (Longines Legendary Watches); to Philippe Hebeisen for archive review and steady support; to Louis F. Vuille for photographs of his Siderograph dial and movement, and for documenting the reverse stamping; to Santi (mimandcroket) for permitting use of his Marine Nationale Aéronavale deck-instrument image; and to our dear friend and contributor Andy Tolley for his expertise, images and advise.
Appendix A — Glossary (Selected Terms)
Greenwich Sidereal Angle (GSA): The Greenwich Hour Angle of the vernal point; the angular measurement displayed directly on the Siderograph’s dial in degrees.
Right Ascension (RA): The angular distance eastward along the celestial equator from the vernal equinox to the hour circle of a celestial body.
Versed Ascension (co-RA): 360° minus the right ascension, tabulated in some period ephemerides so that the navigator adds instead of subtracting: GHA = GSA + versed ascension (less 360° if necessary). Modern almanacs call the same quantity the Sidereal Hour Angle.
Lunette Loxodromique: The rhumb-line bezel featuring tabulated trigonometric values and interpolation scales; operates independently of the watch movement.
Cardan: Gimbal suspension system used to keep the instrument level during navigation.
Trotteuse / Rattrapante: Running sweep hand / split-seconds hand mechanism.
Avigation: Period contraction of “aerial navigation”. The Longines reference cards first describe the instrument as a “Chronomètre Avigation” before renaming it “Sidérographe”.
Calotte avion: The round aluminium cockpit capsule, carried on coil-spring mounts, that houses the early Siderographs.
Cadran tournant: Rotating dial or inner ring, turned by a crown (patent CH 204 297), used to set the instrument to the time signal.
Commande à distance / commande électrique à distance: Remote actuation of the rattrapante, by flexible cable or electrically.
Appendix B — Mathematical Reference (Sexagesimal Conversions)
Angular Units: 1° = 60′; 0.1′ = 6″
Sidereal Hand Rates:
Sight Reduction Formula:
Longitude from Dead Reckoning:
Appendix C — Worked Intercept Example
Objective: Derive a line of position from a star sight using the Siderograph for direct GHA determination.
Setup: Assumed DR position φ = 48° 00 N, λ = 20° 00 W (mid-Atlantic). Celestial body: Aldebaran. For 1939: right ascension 68° 06.5 (versed ascension 291° 53.5 ), declination δ ≈ +16° 23′.
Key Advantage: Steps 4–6 follow standard celestial navigation practice. The Siderograph’s innovation is that Steps 1–2 provide direct angular readings without time-to-angle conversion.
Appendix D — Primary Sources and References
Patent Documentation:
Contemporary Trade Publications:
Company Documentation:
Secondary Sources and Archival Material:
