Abraham-Louis Breguet (1747-1823) stands among the most revered names in horology, celebrated for innovations that shaped modern watchmaking. While his tourbillon, self-winding mechanism, and anti-shock system are widely acknowledged, his work in astronomical timekeeping, particularly equation of time (EOT) watches, remain one of Breguet’s most intricate yet less-explored achievements.

With 2025 marking the 250th anniversary of his birth, this guide offers the most comprehensive study of every known EOT timepiece made during his lifetime, expanding on our prior analysis of the complication (in parts I and II).

Looking at the stars

Breguet operated in an era where precision timekeeping was dictated by astronomy, and his workshop, positioned at the heart of Paris’s scientific and commercial networks, was uniquely placed to serve scientists, navigators, and royalty who required accurate solar and mean time readings.

Rather than settling on a single method for displaying and correcting the EOT, he explored multiple mechanical solutions, refining some while revisiting earlier ideas when necessary. His approach does not follow a strictly linear evolution but instead reflects a dynamic cycle of innovation, mechanical experimentation, and adaptation.

Breguet’s cam-driven system with feeler to indicate EOT, as found in pocket watch no. 3862

As Breguet passed away in 1823, some of the timepieces included here were completed under the direction of his son, Antoine-Louis Breguet (1776-1858), who continued the firm’s work while maintaining his father’s standards. However, the lack of access to Breguet’s complete archives remains a handicap in reconstructing the full scope of his work. If anyone is aware of an EOT watch not included in this research, please let us know, we welcome any additional information that can help refine this study.

A Spectrum of Solutions: EOT Displays & Mechanical Evolution

Breguet’s approach to EOT was not a single trajectory of refinement but rather a continuous process of experimentation, revision, and the occasional return to earlier concepts. Over more than five decades, he produced 21 functional EOT pocket watches, moving between different designs rather than following a strictly linear progression. His willingness to reintroduce earlier ideas in later models suggests that he did not see one solution as inherently superior to another but instead tailored each execution to suit its movement architecture and intended owner.

The choice of approach was dictated by the function and audience of each watch. Scientists required precise solar readings, navigators relied on astronomical calculations, and aristocratic patrons sought technical ingenuity paired with aesthetic refinement. As a result, Breguet’s EOT mechanisms ranged from subsidiary displays (e.g., No. 92 and 160) to “running” EOT, also known as équation marchante, (e.g., No. 1348 and 2614) and, ultimately, to dual-dial layouts (e.g., No. 1226 and 3863).

The movement of watch no. 217. Image – Christie’s

The introduction of these indications, made possible by differential gearing, allowed both time scales to be displayed simultaneously on a single or dual dial, eliminating the need for manual calculations. Yet even after refining this system, Breguet continued producing cam and lever-driven EOT watches, reinforcing that his work was not about replacing one approach with another but selecting the most suitable solution for each watch’s technical demands.

Though Breguet spearheaded these advancements, he was not working in isolation. His son, Antoine-Louis, played a crucial role in refining precision during the 1820s, particularly in the adaptation of EOT mechanisms to what was then considered ultra-thin designs.

Meanwhile, his pupil Jean-Charles Oudin (1768-1840), better known as Charles Oudin, later expanded upon these principles in his own timepieces produced in the Palais-Royal workshop established after he left Breguet, demonstrating the master’s techniques influenced a broader network of watchmakers. Their combined efforts ensured that Breguet’s EOT watches were not only technically sophisticated but also highly adaptable, proving that his legacy was as much about versatility as it was about mechanical precision.

A Unique Experiment & Charles Oudin

The earliest known attempt by Breguet to display the EOT in a portable watch predates his mature system by several years. Long before the introduction of more conventional indications, he experimented with more direct solutions. The only example of its kind known to have been produced by Breguet, No. 444 stands as a unique experiment, offering rare insight into his earliest efforts to translate astronomical time into mechanical form.

Breguet No. 444 (c. 1779)

Breguet No. 444, completed in 1779 but sold only two decades later on April 7, 1799, to a Mr Renble, represents an early attempt at integrating the EOT into a pocket watch. Unlike later models with cam-driven subsidiary displays, No. 444 employed a rather unusual and exotic pivoted auxiliary hand mounted directly on the meantime hand. Though eventually replaced by more advanced systems, it remains a significant step in Breguet’s use of EOT indications.

Breguet No, 444 (ca. 1779). Image – The Art of Breguet by George Daniels

Oudin, one of Breguet’s most skilled apprentices, was actively engaged in horological advancements at the time of No. 444’s production. Later appointed Watchmaker to Napoleon I, Oudin’s early training under Breguet shaped his expertise in astronomical horology.

The dual signature on No. 444, of both Breguet and Oudin, suggests Oudin played a direct role in the development of its EOT system. Whether it was a collaboration or a delegated project, the watch aligns closely with Oudin’s later precision work. A second known example, signed solely by Oudin, is housed in the Musée Paul Dupuy in Toulouse, reinforcing his contribution. Another related watch, No. 4970, carries the inscription “Équation inventée par Charles Oudin Palais-Royal n° 52”, indicating that he continued refining this display method beyond his tenure in Breguet’s workshop.

The equation cam at the centre of No. 444 (ca. 1779). Image – The Art of Breguet by George Daniels

A fundamental limitation of No. 444 was that its EOT indication remained relevant only once per day. The watch’s calendar disc and equation cam completed a full 360º revolution annually, yet the EOT hand, pivoted on the hour hand, only reflected the equation deviation at noon. For the rest of the day, its position became meaningless, failing to provide a continuous reference for the equation of time.

Unlike later designs where a kidney-shaped cam allowed for dynamic, real-time EOT adjustments, No. 444’s system required the wearer to infer the correction manually, relying on a static offset that did not update throughout the day. While mechanically intriguing, this approach proved less practical than the continuously variable indications that followed, which ensured greater precision and legibility.

No 444 (ca. 1779) @ The Art of Breguet by George Daniels (this view illustrates the feeler system that connects the equation cam to the superimposed equation hand. As the cam rotates, the feeler follows its contour, transmitting the variations in the Equation of Time to the equation hand, which displays the correction on a time scale divided into 5-minute intervals)

Mechanically, No. 444 follows Breguet’s Souscription model, balancing precision with streamlined production. It features a manually wound central barrel, a three-armed plain balance, and a ruby cylinder escapement, a system Breguet favoured before transitioning to lever and tourbillon escapements. A spiral steel balance spring with a compensation curb ensured temperature stability. The engine-turned silver dial incorporates a rotating centre disc for the date, an unusual feature for early Souscription watches.

The movement of No. 444 (ca. 1779). Image – The Art of Breguet by George Daniels

Housed in a 62 mm gold guilloché case, No. 444’s cuvette bears the inscription “Inventée par Charles Oudin” alongside Breguet’s signature, further confirming Oudin’s involvement. Once part of the David Salomons collection, No. 444 stands among the earliest recorded portable timepieces featuring an EOT display, marking an essential step in the transition from astronomical tables to mechanical solar time indications.

Although later refinements improved readability and efficiency, No. 444 remains a key example of Breguet’s early attempts to reconcile solar variations in timekeeping. It also underscores Oudin’s influence, as his later work carried forward Breguet’s teachings.

From Experiment to Masterpiece – 1783 to 1790s

While No. 444 marked Breguet’s first recorded attempt at integrating an EOT indication, it remained an isolated experiment. It was only in the years that followed, beginning around 1783, that Breguet began developing a coherent mechanical approach to the complication, one that would reach full maturity in a series of extraordinary timepieces.

Breguet No. 92 (c. 1783-1785)

Four years after the inaugural No. 444, Breguet began work on two exceptional timepieces, No. 92 and No. 160, marking his first fully developed executions of the EOT complication.

No. 92, commissioned in 1783 by the Duc de Praslin, a patron of horology and member of the distinguished Choiseul-Praslin family, features a double-sided construction.

As a double-sided display watch, the recto side presents the primary timekeeping functions alongside a perpetual calendar with three retrograde indicators and an EOT indication on a subsidiary display.

Breguet No. 92, front and back. Image – Musée des Arts et Métiers, Paris

The verso side features a chased gold dial, incorporating a moon phase, a power reserve indicator, and regulators for both sonnerie speed and timekeeping adjustment. The movement is powered by a lever escapement, a compensated balance wheel, and a balance spring with a terminal curve, ensuring both precision and stability.

Breguet No. 160 (c. 1783)

Unlike No. 92’s double-sided design, No. 160 features a rock crystal dial, exposing its intricate mechanics. An enamel dial was also provided. Announced as the most advanced timepiece of its era, with no restrictions on cost or time, it incorporated every known complication. The transparent dial integrates EOT, perpetual calendar, thermometer, and state-of-winding indicator. The EOT mechanisms in both watches introduced the subsidiary display indication driven by the cam and lever system that calculated the difference between mean and solar time.

Breguet No. 160. Image – L.A. Mayer Museum for Islamic Art, Jerusalem

While No. 92 employs separate retrograde indications for the day, date, and months, making it functionally rich despite its double-sided configuration, No. 160 refines these concepts further in a single-dial format. Beyond the EOT, No. 92 includes a ten-minute repeater on a bell and a minute repeater on a gong, while No. 160 incorporates a more complex chain-driven system. Another shared attribute is the independent seconds, a rare feature at the time, highlighting Breguet’s focus on precision and scientific horology.

Breguet No. 160. Image – L.A. Mayer Museum for Islamic Art, Jerusalem

The French Revolution of 1789 disrupted the completion of both watches. No. 92 was eventually finished and sold in 1805 for 4,800 francs to the Duc de Praslin, a devoted patron of Breguet. No. 160, however, remained unfinished until 1827, long after Breguet’s death, when his successors completed the watch.

Sir David Salomons, a British horological scholar and collector, played a key role in preserving Breguet’s legacy, publishing influential works on the brand. He later acquired both No. 92 and No. 160, ensuring their survival and historical documentation.

Breguet No. 160 in profile. Image – L.A. Mayer Museum for Islamic Art, Jerusalem

No. 92 ultimately entered the collection of the Musée des Arts et Métiers in Paris, while No. 160 followed a different path. On May 3, 1917, Salomons first noticed No. 160 displayed in the window of Louis Albert Desoutter’s shop near Regent Street. Fascinated by its complexity, he researched its provenance before acquiring it. Upon his passing, it was inherited by his wife. It later became part of the collection of his great-niece, Vera Salomons, who housed it at the L.A. Mayer Institute for Islamic Art in Jerusalem.

The reverse of No. 160. Image – L.A. Mayer Museum for Islamic Art, Jerusalem

Curiously, both watches fell victim to theft. No. 92 was stolen from the Musée des Arts et Métiers in Paris but was later recovered when the thief attempted to have it repaired. No. 160 was taken in the notorious 1983 burglary in Jerusalem, along with the storied Breguet “Marie Antoinette”, vanishing for over two decades before being recovered in 2007.

Breguet No. 217 (c. 1790s)

Building upon this groundwork, Breguet continued refining his approach to the EOT complication, integrating it into sophisticated mechanisms. Nearly a decade after work began on No. 92 and No. 160, he adapted these principles to a self-winding design, resulting in No. 217, known as the “Havas perpetuelle” after one of its former owners, the founder of the eponymous news agency.

Breguet No 217 (ca. 1790s). Image – Christie’s

One of only two known perpetuelle watches with an EOT function, alongside the legendary No. 160 Marie Antoinette, this timepiece merges self-winding technology, an EOT display, and a perpetual calendar into a single, highly refined mechanism, securing its place as a landmark in horology.

Sold in 1800 to General Jean Victor Marie Moreau for 3,600 francs, it was later returned to Breguet after Moreau’s death in 1813. General Moreau (1763–1813) played a key role in Napoleon’s rise to power but later fell out of favour, leading to his exile to the United States in 1804. He returned to Europe in 1813, joining the Russian army against Napoleon, only to be mortally wounded at the Battle of Dresden.

Refurbished and resold on December 31, 1817, to Charles-Louis Havas (1783–1858) for 4,800 francs, the watch resurfaced over a century later, achieving US$3.3 million at Christie’s Geneva in 2016. Havas, the second owner, was a banker and publisher who founded Agence Havas in 1835, the world’s first news agency and the precursor to Reuters and Agence France-Presse, better known today as AFP.

Breguet No 217 (ca. 1790s). Image – Christie’s

No. 217 is a self-winding Perpetuelle featuring a quarter repeater à toc, which taps the case to indicate the nearest quarter-hour. Its EOT display graduated from +15 to -15 minutes, operates via a cam-driven system linked to the month calendar wheel for continuous solar time correction. The perpetual calendar integrates a subsidiary seconds dial with the month indication alongside a date aperture signalled by a gold arrow pointer. A 60-hour power reserve indicator ensures reliable operation.

Breguet No 217 movement with EOT cam visible (ca. 1790s). Image – Christie’s

The engine-turned silver dial features Roman numerals, outer dot minute divisions, and gold Breguet hands, with subsidiary displays for the power reserve and EOT indications. The movement incorporates a jewelled lever escapement, a four-arm compensated balance wheel with platinum adjusting weights, and a blued steel helical balance spring.

A platinum oscillating weight drives the winding system, safeguarded by Breguet’s parachute shock protection. Encased in 18K gold with a 55 mm diameter, the watch bears the signature “Breguet et Fils” on the dial and “Breguet No. 217” engraved on the movement plate.

Divergent Designs: Mechanically Distinct EOT Watches – 1794–1817

Breguet No. 1 (c. 1794)

At the turn of the 19th century, Breguet’s pursuit of mechanical innovation extended beyond escapements and power management to increasingly sophisticated mechanisms, particularly in the refinement of the EOT complication evident in No. 1 of Series 2 and No. 1226.

The Breguet No. 1 of Series 2 marked an evolution in his EOT designs by integrating this function alongside a full calendar. Although initiated before 1794, the watch was completed only in 1819.

Housed in a 63 mm case, it features secondes d’un coup, a jumping seconds mechanism that enhances timekeeping precision. Its Earnshaw spring detent escapement, combined with a three-armed compensation balance and parachute suspension, optimises both accuracy and shock resistance. A helical steel balance spring with terminal curves further improves isochronism, while a reversed fusee and chain regulate power delivery, ensuring stable performance over extended use.

Breguet No. 1, 2nd series (ca. 1794). Image – The Art of Breguet, George Daniels.

The white enamel dial prioritises clarity, featuring a subsidiary seconds display at the bottom, the date on the left, and the EOT on the right. A month aperture at twelve o’clock and a date hand pivoted at ten o’clock complete the calendar indications.

Notably, this was the first instance where the EOT hand, positioned at two o’clock, was driven by a substantially sized negative cam mounted on the annually revolving calendar wheel. The lever and its feeler traced the inside of the cam’s profile, transmitting precise equation corrections. This layout streamlined the display of three complications, date, month, and equation, but increased the watch’s thickness, limiting its suitability for later adaptations.

Dial side view of movement of Breguet No. 1, 2nd series (ca. 1794) and its oversized negative EOT cam. Image – The Art of Breguet, George Daniels

Levers at the edge of the movement allows for manual adjustment of the month wheel and equation, as well as stopping the central seconds hand. While the date hand advances automatically, it halts on the thirtieth day, requiring a manual reset to the first via a designated lever.

Movement view of Breguet No. 1, 2nd series (ca. 1794). Image – The Art of Breguet, George Daniels

Despite its unconventional design, No. 1’s equation system exemplifies Breguet’s approach to balancing innovation with legibility. Subsequent adaptations refined its functionality, allowing for greater flexibility in case dimensions and dial configurations. This iterative development is evident in No. 1226, which, though completed later, was sold two years before No.  1.

Breguet No. 1226 (c. 1817)

Completed in 1817, No. 1226 is a timepiece that displays both mean and solar time on separate dials, an exceptionally rare feature among Breguet watches if we exclude the Fatton inking chronographs.

Retaining the negative cam-and-lever system introduced in No. 1, this watch returns to a conventional-sized format, albeit with a fixed cam, while applying the equation of time indication to a centrally positioned hand on the reverse dial. However, in the accompanying image, the EOT hand is incorrectly displaced relative to the calendar hand’s position. It should accompany the calendar hand, deviating positively or negatively by a maximum of 15 minutes throughout the year.

Breguet No 1226 (ca. 1817). Image – Museu Medeiros e Almeida, Lisbon

The watch is housed in a gold case approximately 60 mm in diameter, featuring finely executed engine-turning. The front gold engine-turned dial displays hours, minutes, and seconds, along with a 36-hour power reserve indication. The reverse platinum dial incorporates a highly detailed calendar system, including indicators for the months, moon phase, day of the week, and date, as well as an EOT scale with a gold sun-adorned serpentine hand.

Breguet No 1226 (ca. 1817). Image – Museu Medeiros e Almeida, Lisbon

At its core, No. 1226 is regulated by a double-wheel Robin escapement, a hybrid system that combines elements of lever and detent escapements. The movement features a three-arm compensation balance, a spiral steel balance spring with a terminal curve, and a parachute suspension system. Additionally, the watch is equipped with a quarter-repeating function with a dumb mechanism, producing softer chimes for discreet use.

Breguet No 1226 (ca. 1817) movement, dial side, with the fixed negative EOT cam at its centre. Image – Museu Medeiros e Almeida, Lisbon

Breguet No. 1226 was purchased by Jean-Andoche Junot around the time he assumed command of the Napoleonic French forces in Portugal, where his early victories earned him the title of Duc d’Abrantes. However, after falling from favour, Junot died in 1813 from injuries sustained in a suicide attempt. His widow, Laure, known for her political manoeuvring and nicknamed “Petite Peste” by Napoleon, later sought the return of the Bourbons, possibly funding her efforts through the sale of the watch, an act unlikely to have pleased the ex-Jacobin Breguet.

The next known owner was William Noel Hill, Baron Berwick of Attingham, a British diplomat and close associate of General Rowland Hill, who later passed it to Arthur Wellesley, 1st Duke of Wellington. This transfer could have taken place after the withdrawal of the army of occupation in 1818, when Hill served as Wellington’s second-in-command, or in 1842 when Wellington succeeded him as General Commanding-in-Chief.

Eventually, No. 1226 entered the collection of the Portuguese collector Medeiros e Almeida in 1964 when he acquired it at auction for a then-record sum of £27,500 (approximately US$700,000 today). It is now housed at the Museu Medeiros e Almeida, where it remains a key piece in the institution’s horological collection.

Refinement and Standardisation: The Evolution of Subsidiary EOT Displays – 1827–1836

Following Breguet’s death in 1823, his workshop, under the direction of Antoine-Louis Breguet, continued refining the equation of time indication, preserving the principles and precision that had defined his legacy.

This period saw all EOT watches with subsidiary indications adopting the display at six o’clock, solidifying a design approach that would become increasingly standardised. The timepieces in this category, spanning from 1827 to 1836, exemplify not only advancements in mechanical execution but also the diversification of Breguet’s clientele, extending from aristocrats and industrial magnates to European monarchs.

Breguet No. 4214 (c. 1827)

No. 4214, sold in 1827 to Captain Richard Seymour-Conway, 4th Marquess of Hertford, for 7,800 francs, represents a key evolution in Breguet’s approach to the equation of time. The gold engine-turned case, measuring 41 mm, is decorated with an intricate map of France, covered in translucent enamel and encircled by a laurel wreath, underscoring its aristocratic ownership.

Breguet No. 4214 (ca. 1827) Half-Quarter Repeater, 7.5 minutes

This watch integrates a half-quarter repeating mechanism and an EOT display into a compact yet highly functional design. The movement incorporates a jewelled lever escapement, a compensation balance with parachute suspension, and a spiral steel balance spring with a terminal curve and regulator—features that reinforce both precision and resilience.

The silver dial features an eccentric chapter ring with Roman numerals, a jumping hour hand and symmetrically arranged subdials, one for the months on the right and one for seconds on the left. Additional apertures display the day of the week, the date, and the moon’s age and phase at the centre. A power reserve indicator sits above the main display, while the equation of time scale is positioned below.

Breguet No 4214 (ca. 1827). Image – Breguet (1747-1823) by Sir David Salomons

The equation function operates via an annual rotating cam that regulates a pivoted lever, translating the variation in solar time into the movement of the equation hand along the display scale. This allowed the wearer to manually adjust time according to a sundial, compensating for the natural fluctuations between mean and solar time throughout the year.

Breguet No. 4308 (c. 1827)

Sold in 1827 to King George IV for 9,650 francs, No. 4308 stands apart as a rare example of a half-quarter repeating montre à tact with an EOT display. This design allowed the wearer to determine the time either visually or by touch, a feature that had been an important element of Breguet’s montres de souscription since the late 18th century.

Breguet No. 4308 (ca. 1827) Half-Quarter Repeating Montre à Tact

The outer, engine-turned gold case features touch pieces and a revolving tact hand, while the inner case houses a sliding mechanism that activates the repeater.

Breguet No 4308 (ca. 1827). Image – Watches & Clocks in the Sir David Salomons Collection, George Daniels & Ohannes Markarian

The silver, engine-turned dial includes an eccentric chapter ring with Roman numerals, subsidiary dials for seconds, days of the week, and months of the year, and apertures for the date and regulator. Below the main dial, a finely calibrated equation of time scale enables precise solar time adjustments. The movement employs a lever escapement, a compensation balance with parachute suspension, and a spiral balance spring with a terminal curve and regulator.

Breguet No 4308 (ca. 1827). Image – Watches & Clocks in the Sir David Salomons Collection, George Daniels & Ohannes Markarian

No 4308, movement, dial side, with EOT cam engraved with the months of the year visible on the right @ Watches & Clocks in the Sir David Salomons Collection, George Daniels & Ohannes Markarian.

King George IV’s patronage further underscores Breguet’s prominence among European royalty, while the watch’s later inclusion in Sir David Lionel Salomons’ collection highlights its enduring historical significance.

Breguet No. 3832 (c. 1828)

No. 3832, delivered in 1828 to Comte d’Archinto for 8,000 francs, represents a refinement in both precision and presentation.

The watch is housed in a 53 mm double-case design with an à tact hand and touch pieces on the outer case, allowing time to be read by feel. The inner case features a slide that activates the half-quarter repeater mechanism, which chimes hours, quarters, and an additional strike at the half-quarter interval.

Breguet No. 3832 (ca. 1828) Half-Quarter Repeater with Calendar and À Tact Feature. Image – Christies

Breguet No 3832 (ca. 1828). Image – Christies

The engine-turned silver dial is arranged for optimal readability, featuring an eccentric mean time chapter ring, a serpentine equation hand positioned on the EOT sector, and subsidiary dials for months and days of the week.

The EOT mechanism relies on a cam-and-lever system, with a pivoted lever tracking the cam’s profile and transmitting the correction via a rack and pinion system to the equation hand. The addition of a full calendar alongside the EOT display reflects the increasing focus on astronomical precision during this period.

The movement includes a jewelled lever escapement and a compensated balance, ensuring stability across temperature variations.

Breguet No. 4254 (c. 1828)

Watch No. 4254, sold in 1828, represents another advancement in EOT integration within a half-quarter repeating format. This watch maintains the same fundamental principles as No. 4214 but introduces refinements in mechanical efficiency and dial legibility. The equation indication, regulated by an annual cam, provides a real-time correction that is easily readable, allowing the wearer to apply the necessary adjustment for solar time.

Breguet No. 4254 (ca. 1828) Half-Quarter Repeater. Image – Breguet (1747-1823) by Sir David Salomons

The gold case features an eccentric chapter ring with subsidiary dials for seconds, the month, and a calibrated sector for the EOT. Additional apertures display the day of the week, the age and phase of the moon, and the date of the month. The repeater mechanism chimes the nearest 7.5-minute interval with clarity, reinforcing its dual function as both an astronomical instrument and a practical timepiece.

Breguet No. 4290 (c. 1829)

No. 4290, sold in 1829, marks one of Breguet’s early forays into ultra-thin designs incorporating an EOT complication.

While few details are available regarding its movement architecture, the watch is known to integrate an EOT display in a highly refined layout, optimising space efficiency while maintaining accuracy. This model serves as a precursor to later ultra-thin equation watches, balancing mechanical sophistication with elegant proportions.

Breguet No. 4290 (1829) Ultra-Thin Equation of Time Watch. Image – Musée national suisse

Breguet No. 4290 (1829) Ultra-Thin Equation of Time Watch. Image – Musée national suisse

Breguet No. 4691 (c. 1831)

Sold in 1831 to Lord Henry Seymour Conway for 7,000 francs, No. 4691 represents Breguet’s most advanced ultra-thin equation of time execution. With a case thickness of just 7.7 mm, it embodies a remarkable synthesis of horological complexity and elegant proportions.

Breguet No 4691 (ca. 1831) prior to its restoration. Image – Sotheby’s

The movement is built on garde-temps principles, featuring a gilt brass lever escapement, a compensation balance with parachute suspension, and a spiral balance spring with a regulator for fine timekeeping precision.

Breguet No 4691 (ca. 1831). Image – Breguet

The guilloché silver dial is arranged for intuitive readability, featuring an off-centre mean time display with Roman numerals, subsidiary seconds dial at 9 o’clock, a month indicator at 3 o’clock, and apertures for the day and date. The EOT display, positioned between 5 and 7 o’clock, is marked with a blued steel serpentine hand featuring a gold sun motif. A moon phase display appears above 12 o’clock, while a power reserve indicator is positioned at the bottom. The watch was later acquired by the Breguet Museum in 2013 for US$1.1 million.

Breguet No 4691 (ca. 1831). Image – Breguet

Breguet No. 4517 (c. 1836)

No. 4517, sold in 1836 to Anatoly Demidoff, 1st Prince of San Donato, for 4,000 francs, is an exceptional ultra-thin pocket watch that refines Breguet’s equation of time execution within an elegant and highly compact case. Later part of the Belmont House Collection, it integrates an EOT display, temperature Gauge, moon phase, and annual calendar while maintaining mechanical precision in a remarkably slim form.

No. 4517 (ca. 1836) Ultra-Thin EOT with Moon Phase & Calendar. Image – Belmont House Collection

The engine-turned silver dial features a highly legible layout, with the EOT indication positioned below the main display, ensuring that the correction remains intuitive to read. The movement is built on chronometer principles, incorporating a lever escapement, compensation balance, and parachute suspension. This watch serves as one of the final refinements of the subsidiary EOT display in Breguet’s pocket watches.

The Running Equation: Differential Gearing in Breguet’s EOT Watches – 1805-1814

Breguet’s advancements in horology marked a pivotal shift from mechanically distinct EOT displays to the running EOT mechanism, a breakthrough in precision timekeeping. Early EOT pocket watches relied on discrete mechanical systems, such as cam-and-lever mechanisms, to indicate the difference between mean and true solar time. These designs required a manual calculation, with the wearer referencing a subsidiary display to determine meantime relative to solar time.

Breguet’s ingenuity, however, led to the application and development of the then-already-known running EOT system, where the equation correction was dynamically integrated into the motion work, allowing the displayed time to continuously account for solar variation without requiring user calculation.

By incorporating differential gearing and disengaging mechanisms, these watches applied solar time corrections in real-time, refining astronomical timekeeping while minimising operational friction and improving mechanical efficiency. Today, the running EOT system still represents a pinnacle of technical sophistication, blending horology’s artistic heritage with functional advancements that cater to both scientific precision and practical usability.

Among the most significant examples of Breguet’s innovation in this field are No. 1348, No. 1416, No. 2614, and No. 3862. Each represents a distinct phase in the evolution of differential running EOT systems, exploring variations in disengaging mechanisms, complex calendar integration, and adaptations tailored to the needs of the watchmaker’s international clientele.

Breguet No. 1348 (c. 1805)

Breguet No 1348 (ca 1805) running EOT with disengaging mechanism. Image – Antiquorum

Breguet No. 1348, produced in 1805 and sold to Messrs. Meyer and Tues, introduced a disengaging mechanism designed to minimise wear while ensuring precise solar time adjustments, engaging only once per hour. This innovation reduced unnecessary strain on the movement while maintaining the accuracy of the equation correction.

Breguet No 1348 (ca 1805) running EOT with disengaging mechanism. Image – Antiquorum

The watch also features a dual calendar system, accommodating both the Gregorian and Julian calendars, alongside a sectoral EOT display indicated by a serpentine Breguet hand. Housed in a 62 mm gold case signed by Guillaume Mermillod, No. 1348 remains a significant early example of Breguet’s advancements in disengaging EOT mechanisms.

Breguet No. 1416 (c. 1812)

Breguet No 1416 (ca 1812) running EOT with Half-Quarter Repeater. Image – Sothebys

Breguet No. 1416, produced in 1812 and sold to Prince Antoine de Bourbon, took a similar approach to EOT correction by integrating a half-quarter repeater with a dual-hand display. Instead of relying on a subsidiary dial, the watch employs two minute hands, one for meantime and the other for solar time, allowing for instantaneous visual comparison.

The equation correction is dynamically adjusted via an annual wheel system, and the repeater mechanism, chiming at 7.5-minute intervals, further enhances the watch’s usability, providing both visual and auditory means of tracking time. Now part of the Belmont House Collection, No. 1416 exemplifies an advanced execution of real-time equation correction in a dual-hand configuration.

Breguet No. 2614 (c. 1814)

Breguet No 2614 (ca 1814) Montre Turque with running EOT. Image – The Art of Breguet, George Daniels

Breguet No. 2614, produced in 1814, is one of the rare Montre Turque EOT pocket watches specifically crafted for the Ottoman market. No. 2614 integrates a differential gear system that continuously adjusts for solar time variations in real time, ensuring seamless accuracy.

Breguet No 2614 (ca 1814) Montre Turque with running EOT. Image – The Art of Breguet, George Daniels

Beyond its advanced equation function, the watch incorporates an annual calendar and a lunar display, with a subsidiary dial referencing both Islamic and European calendars, highlighting its dual-purpose design.

Breguet No 2614 (ca 1814) Montre Turque with running EOT. Image – The Art of Breguet, George Daniels

The dial, signed in Turkish script and featuring Ottoman numerals, reflects Breguet’s adaptability to international clientele, demonstrating his ability to tailor complex horology to the specific cultural and technical requirements of different regions.

Dual-Time Clarity: Watches with Separate Solar and Mean Time Dials – 1817-1829

Breguet’s development of the equation of time complication evolved towards greater clarity and mechanical efficiency. Unlike earlier models that employed a concentric running equation hand, this group of watches introduced dual-display designs, separating mean and solar time and applying the differential principle to calculate and display the equation correction. This refinement resulted in a series of significant timepieces that include Nos. 2807 (1817), 3832 (1828), 3862 (1821/24), 3863 (1824), 4111 (1827) and 4112 (1829). Though unified by their astronomical function, each represents a distinct approach to balancing complexity, legibility, and mechanical ingenuity.

Breguet No. 2807 (c. 1817)

Breguet No. 2807 (ca. 1817) EOT on differential with optimized energy efficiency. Image – Antiquorum

Sold in 1817 to Russian General Alexis Petrovich Yermoloff, Breguet No. 2807 introduced a new differential-based EOT mechanism aimed at reducing mechanical strain and optimising energy efficiency. Instead of maintaining continuous engagement with the train, its equation function activates only once every two hours, minimising friction while ensuring precise solar time adjustments. This mechanism operates via a specialised double-wheel and double rack-and-pinion differential system, governed by an equation cam mounted at the centre of the annual wheel, allowing seamless calculation of the discrepancy between mean and true solar time.

Housed in a 53 mm gilt brass case, the movement features a going barrel and a straight-line calibrated lever escapement, ensuring stable timekeeping performance. The dial, crafted by Louis Tavernier (1754–1840), presents a well-balanced layout with separate sub-dials for true solar time and mean time, complemented by complications such as moon phases and a date aperture. Tavernier’s father, Jean-Pierre (1714–1795), had already published a work entitled Table de différence du temps vrai au temps moyen in 1754, reinforcing the family’s deep engagement with astronomical timekeeping.

Breguet No. 3862 (c. 1821-1824)

Breguet No. 3862 (ca. 1821/24) EOT on differential, completed only in the 1960s. Image – Maison Breguet

Breguet No. 3862, started between 1821 and 1824 but left unfinished until its completion in the 1960s, further refined the differential-based approach by integrating a perpetual calendar system. Designed to complete one annual rotation, its equation cam interacts with the differential gearing to continuously adjust the solar time display, ensuring precise and automatic equation corrections throughout the year.

Breguet No. 3862 (ca. 1821/24) EOT on differential, completed only in the 1960s. Image – Maison Breguet

The movement is gilt-finished, featuring a straight-line lever escapement with 21 jewels, a bimetallic compensation balance with straight parachutes on both pivots, a spiral steel spring with a terminal curve, and a steel regulator with a gold index.

The silvered engine-turned dial presents two symmetrical sub-dials: the left displays solar time with Arabic numerals and an applied gold sun symbol above 12 o’clock. At the same time, the right indicates mean time with Roman numerals and an applied gold star symbol. An outer ring is calibrated for the annual calendar, indicated by a central blued steel serpentine hand. Additionally, the watch features a small subsidiary seconds dial, a weekday window, and a fan-shaped aperture displaying moon phases and the lunar calendar.

The case, crafted from 18K gold with an engine-turned finish, measures 61 mm in diameter and includes a numbered and signed gold cuvette. The movement remained incomplete until the mid-20th century, when it was finished with a newly designed dial, moon disc, hands, and case. It is now part of the Montres Breguet collection.

Breguet No. 3863 (c. 1824)

Breguet No. 3863 (ca 1824) EOT on differential à tact. Image –  The Art of Breguet, George Daniels

By 1824, Breguet had further refined the differential EOT system with No. 3863. The silver engine-turned dial of this watch features a sweep centre date hand and two subsidiary dials, with solar time on the left and meantime on the right. Above and separating both dials is a subsidiary seconds indication. Below, two apertures display the moon’s age and phase alongside the day of the week indicated by its first letter. The engine-turned case incorporates an à tact hand on the back and touch pieces along the edge.

Breguet No. 3863 (ca 1824) EOT on differential à tact. Image –  The Art of Breguet, George Daniels

Mechanically, No. 3863 employs a lever escapement, a two-armed compensation balance, a spiral steel spring with a terminal curve and regulator, and Breguet’s signature parachute suspension system for shock absorption. Its EOT mechanism adjusts both the hour and minute hands of the solar time display with minimal resistance, using a double spring-loaded wheel connected via two pivoted racks and a pinion for precise adjustments.

Breguet No. 3863 (ca 1824) EOT on differential à tact. Image –  The Art of Breguet, George Daniels

Breguet No. 4111 (c. 1827)

Breguet No. 4111 (1827) EOT with Half-Quarter Repeater, 7.5 min. Image – Christies

No. 4111, produced in 1827, represents an ultra-thin execution of the differential EOT complication with a dual-dial system. Originally sold to Mr Peyronnet before being resold in 1834 to Count Charles de l’Espine, the watch features a half-quarter repeater that chimes every 7.5 minutes, enhancing its practical utility as an astronomical timepiece. As previous models in this category, it arranges separate off-centre subdials, solar time on the left and meantime on the right, allowing for an intuitive reading of the equation correction.

Built on chronometer principles, its movement incorporates a lever escapement, a compensation balance, and a parachute suspension system for enhanced shock resistance, ensuring reliability despite its slim proportions.

Breguet No. 4112 (c. 1829)

Breguet No. 4112 (1829) EOT with Half-Quarter Repeater, 7.5 min. Image – Watches & Clocks in the Sir David Salomons Collection, George Daniels & Ohannes Markarian

Building upon this lineage, No. 4112 was introduced in 1829, integrating the advancements of its predecessors while also incorporating a half-quarter repeater. Sold to Mr Goding for 8,128 francs, this watch not only displayed the difference between solar and mean time but also made it audible through a striking mechanism that chimed every 7.5 minutes.

Breguet No. 4112 (1829) EOT with Half-Quarter Repeater, 7.5 min. Image – Watches & Clocks in the Sir David Salomons Collection, George Daniels & Ohannes Markarian

Housed in a 56 mm gold engine-turned case, it features a crystal cuvette, offering a clear view of the movement. A centrally placed equation cam governs solar time corrections through a precise lever mechanism. At the same time, additional complications include a leap year cycle, an age and phase of the moon display, and a fast/slow regulation indicator at 6 o’clock.

Breguet No. 4112 (1829) EOT with Half-Quarter Repeater, 7.5 min. Image – Watches & Clocks in the Sir David Salomons Collection, George Daniels & Ohannes Markarian

Conclusion

Breguet’s EOT watches mark a significant chapter in the development of mechanical horology, showcasing his ability to tackle complex astronomical indications with clarity and ingenuity. He treated such complications as serious technical challenges, integrating them into functional instruments that served both scientific and practical purposes.

These creations reflect the maturity of late 18th- and early 19th-century watchmaking, and they helped establish mechanical solutions to celestial problems as a legitimate pursuit within the field. The influence of these pieces remains visible in modern horology, where the equation of time continues to be explored as a mark of technical refinement and historical awareness.

Breguet’s contribution thus stands as a benchmark in watchmaking—where intellectual rigour, mechanical skill, and purpose-driven design converged to advance the discipline.

Sources and Bibliography

Primary sources

● Breguet Official Website:
○ For information on Breguet’s history, innovations, and some watch models. (www.breguet.com)
● Musée du Louvre, Paris:
○ For access to Breguet timepieces and related historical documents.
● Musée des Arts et Métiers, Paris:
○ For information and images of Breguet No. 92.
● L.A. Mayer Museum for Islamic Art, Jerusalem:
○ For information and images of Breguet No. 160.
● Museu Medeiros e Almeida, Lisbon:
○ For information and images of Breguet No. 1226.
● Christie’s Auction House:
○ For auction records and images of Breguet No. 217 and No. 4111.
● Antiquorum Auctioneers:
○ For information and images of Breguet No. 2807.
● Montres Breguet S.A. Collection:
○ For information and images of Breguet No. 3862.
● Sir David Salomons Collection:
○ Historical documentation and information about the watches within this collection.

Secondary sources

● Daniels, George. The Art of Breguet. London: Sotheby Parke Bernet Publications, 1975.
○ Essential for information on many of the Breguet watches discussed.
● Engel, Thomas. A.L. Breguet, Watchmaker of Kings. London: Sotheby’s Publications, 1994.
○ Provides detailed historical context and analysis of Breguet’s work.
● Clutton, Cecil, and George Daniels. Watches. London: B.T. Batsford Ltd., 1965.
○ A classic horological reference with information on Breguet.
● Daniels, George, and Ohannes Markarian. Watches & Clocks in the Sir David Salomons Collection. London: Sotheby’s Publications, 1980.
○ Provides information about the watches that were once owned by Sir David Salomons.