Blancpain creates the first wristwatch with 'Equation marchante' >

08 august 2006

Blancpain once again distinguishes itself in the highest spheres of watchmaking by creating the Equation Marchante, the first wristwatch to feature a 'running' equation. This means that the dial is fitted with two coaxial minute hands, one of which indicates mean solar time (that displayed on clocks) and the other real solar time (that shown on sundials). This original system, which is particularly functional, enables direct read-off of real solar time, without any mental arithmetic.

The difference between real solar time and mean solar time

A day is the interval of time between two consecutive passages of the sun through the meridian of a location (the highest point in the sky). But the earth does not describe a perfect circle in its orbit around the sun. Like most planets in the solar system, it pursues an elliptical path. Moreover, the axis of the earth is inclined in relation to the plane of the orbit.

Since the movement of the earth around its orbit is not uniform, the passage of the sun through the meridian of a given place does not take place at perfectly equal intervals of time. The 'real' day, the interval between two 'real noons' varies in length according to the season, according to a successive pattern that is exactly replicated from year to year. Sundials take account of these differences.

Due to its irregular duration, real solar time could not be taken as a unit of time - especially with the development of mechanical measuring instruments (clocks, then pocket-watches).

A 'mean' day was thus taken as the standard unit, by imagining a fictitious sun that would cross the celestial equator in a uniform movement. 'Mean' time (also called 'civil' or 'legal' time), corresponding to that shown on our watches, was divided into days of equal duration throughout the year, resulting from the average of all real solar days in a year.

The difference between real solar time and mean solar time at a given moment is called the 'equation of time'. The equation of time is expressed in minutes (+/-), according to the following formulae:

Equation of time = Mean time - Real time

Mean time = Real time + Equation of time

Real time = Mean time - Equation of time

Real solar time and mean solar time coincide four times a year: April 16th, June 14th, September 2nd, December 26th (these dates change with the leap years). But the difference reaches as much as a quarter of an hour on certain dates. On February 12th, the equation of time is equivalent to around +14 minutes; the sun thus passes through the meridian when our watches show 12.14. On November 14th, the equation of time is around -16 minutes; real noon thus corresponds to around 11.44, civil time.

Features

Equation of time

The dial of the new Blancpain 'Equation Marchante' watch also features an equation of time display at 2 o'clock, with a slow retrograde hand moving over the arc of a circle graduated from -14 to + 16 minutes.

Moon phase

Blancpain has chosen a very original moon phase display: a retrograde hand sweeping over the arc of a circle located at 10 o'clock and pointing to tiny globes representing the new moon, the waxing crescent, the full moon and the waning crescent.

Real solar time

Another hand in yellow gold, adorned with a small sun, enables one to read off directly, and at any time, the real solar time as shown on sun dials, without having to perform any mental arithmetic. The user can thus visualise at a glance both mean time and real time, as well as the difference between them.

Equation cam

Variations in real solar time are mechanically reproduced by means of an equation cam ? shaped like a figure-8 'analemma' - which completes one revolution around its axis in a year. The design of this cam called for extremely precise calculations based on the irregular path of the earth around the sun.

To celebrate this fascinating encounter between astronomy and Haute Horlogerie, and for the first time ever, a circular dial aperture at 6 o?clock allows one to contemplate the equation cam as well as the feeler-spindle in action. The seconds hand also turns in the centre of this opening.

Calibre

Perpetual calendar - The world's thinnest perpetual calendar module.

Bridges - View of the 4 bridges supporting the gear and the barrel.

Weight - The fitting of the weight.

More information from: www.blancpain.com