The equation of time is a rare and fascinating horological complication. It displays 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 24 hours. Mean solar time may show a discrepancy with true solar time ranging from minus 16 minutes to plus 14 minutes. On just four days a year, the two times are exactly the same. Given that the sun’s various positions in the sky are reproduced in an identical manner on the same dates, watchmakers can “program” them by means of a special cam. It is shaped like a figure 8 and mechanically reproduces the path of the sun’s successive positions, called an analemma curve. Requiring 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 (-16 to +14 minutes). The user then has 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 supersedes this principle. It simultaneously indicates civil time and true time by means of two separate minutes hands. The running solar hand, adorned with a facetted golden sun, provides a direct reading of solar time minutes. This apparently simplicity conceals a difficult construction process, as 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 that operate 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. Breguet has developed an extremely slim equation cam borne by a sapphire disc also serving to correct the equation of time by month.
Added to the equation of time, the complexity of a perpetual calendar complements this model. 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.
This timepiece also offers a third complication that is noteworthy in its own right: a 60-second tourbillon with a titanium carriage housing a Breguet balance with a silicon balance spring. This innovative characteristic notably enables the balance wheel to achieve a 4Hz frequency, while maintaining an 80-hour power reserve.
The dial features two types of engine-turning, including a “wave” pattern specifically developed for this new creation. The inscription “Marine royale” is engraved on the tourbillon bar, whose execution naturally draws the attention. Visible through a sapphire caseback, the bridges have been chased to depict in details 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 oscillating weight on the rim of the calibre, the self-winding movement deploys the full splendour of its decoration.
The new Marine Équation Marchante 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. This “Grande Complication” comes with a 43.9mm-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.
