Mastering The Equation Of Time
Mastering The Equation Of Time

The equation of time is one of the rarest and most fascinating horological complications.

The equation of time is one of the rarest and most fascinating horological complications. It serves to display the difference between mean solar time (corresponding to civil or standard hours and minutes) and true solar time, meaning the actual solar hours and minutes.


Since antiquity, the sun has been used as the basis of time. Nonetheless, the visible motion of the sun — the true solar time indicated on sundials — is irregular. With the improvement of timekeeping precision, watches and clocks became the basis of time, and true solar time was replaced by mean solar time, within which each day has the same duration of exactly 24 hours. Mean solar time may show a discrepancy with true solar time, ranging from minus 16 minutes to plus 14 minutes. The two times are exactly the same on just four days in a year.


Watchmakers have nevertheless, over the years, created timepieces that display the time difference between the true and mean solar time. They reproduce the various positions of the sun in the sky in an identical manner on the same dates, by means of a specially ‘programmed’ cam. The latter is shaped like a figure 8 and mechanically reproduces the path of the sun’s successive positions, called an ‘analemma’ curve. Requiring extremely accurate execution, the cam is coupled with a feeler-spindle that drives an equation lever serving to indicate the difference between civil time and solar time (minus 16 to plus 14 minutes). This read-off is generally provided on a sector, or subdial. It is then up to the user to mentally add or subtract the difference displayed in relation to mean time, in order to calculate true solar time.


The new Marine Équation Marchante from Breguet supersedes this principle. It simultaneously indicates civil time and true time by means of two separate minute hands. The running solar hand, adorned with a facetted golden sun, provides a direct reading of solar time minutes that is both quicker and more user-friendly. Its apparent simplicity conceals an arduous construction process that few watchmakers are capable of achieving.


The solar minutes hand has to meet two imperative demands: it must sweep in a conventional way around the dial, like the civil minutes hand, while also daily moving away from the latter by a distance that varies in accordance with the analemma curve, in order to display the equation. Breguet was able to accomplish this by equipping its running solar hand with a differential gear, powered by two rotation sources operating entirely independently: the rotation of civil minutes, and that controlled by the lever in contact with the equation of time cam, which makes one full turn per year. This specially developed and extremely slim equation cam is borne by a transparent sapphire disc, which also serves to correct the equation of time by month.


The complexity of the running equation of time is complemented in this watch by a perpetual calendar. Two apertures — one between 10 and 11 o’clock and the other between 1 and 2 o’clock — respectively display the days of the week as well as the months and the leap-year cycle. The date appears inside the chapter ring by means of a retrograde hand, tipped with an anchor motif and sweeping across an arc running from 9 to 3 o’clock.


Driven by the self-winding 581DR calibre, the watch also flaunts a third important complication: a 60-second tourbillon with a titanium carriage, housing a Breguet balance with a silicon balance spring. This innovative characteristic enables the balance wheel to achieve a 4Hz frequency, while maintaining a comfortable 80-hour power reserve, the indication for which can be seen through an aperture between 7 and 9 o’clock.



The technological feat this timepiece represents is accentuated by the spectacular decorative work of the artisans of the House of Breguet. It features new aesthetic signature codes, giving it a modern and dynamic appearance: central lugs combining polished and satin-brushed surfaces; more open fluting, with visible flanks; a crown topped with a polished ‘B’ against a sandblasted background as well as a crown adorned with a chamfered and satin-brushed wave motif. The front dial features two types of engine-turning, including a specially developed ‘wave’ pattern. The inscription ‘Marine royale’ is engraved on the tourbillon bar.


Visible through a sapphire caseback, the bridges have been delicately chased to depict in meticulous detail the Royal Louis, a first rank vessel in the French Royal Navy. The barrel is adorned with a windrose motif, in reference to astronomical navigation. Thanks to the carefully chosen positioning of the oscillating weight on the rim of the calibre, the self-winding movement deploys the full splendour of its decoration.



The watch comes with a 43.9 mm diameter case in rose gold or platinum. The rose gold version frames a silvered dial and an anthracite movement, while the platinum interpretation has a blue dial and a rhodium-plated movement. Elegance, legibility and aesthetic balance are the key features of these two equally sumptuous models.

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