We go about our daily business to legal and universal time, which divides the year into 365.25 days of 24 hours, and the hour into 60 minutes. However, because the Earth’s orbit around the Sun is elliptical rather than circular, “true” solar time differs from legal time by an amount that varies over the course of the year. Hence solar noon, when the sun is at its highest point in the sky, varies from legal noon by extremes of -16 to +14 minutes, depending on the time of year. The two values exactly coincide just four times, on April 15th, June 13th, September 1st and December 25th. Does knowing this change our day-to-day existence? Is it a life or death matter that a “solar” appointment must take place a few minutes before or after the agreed legal time? Judging by the latest crop of new models, this equation of time and other complications of astronomical ilk have clearly tickled watchmakers’ fancy.
An A-list of names
Here are just some of the surprisingly numerous examples: the Panerai Luminor 1950 Equation of Time Tourbillon Titanio L’Astronomo, the most complicated wristwatch ever made by Panerai, comprises a 30-second tourbillon that rotates perpendicular to the movement plane, an equation of time, indication of sunrise and sunset times in the city chosen by the wearer and, on the back, a depiction of the night sky in that city, next to the four-day power reserve made possible by L’Astronomo’s three barrels. Jaeger-LeCoultre proposes the Grande Tradition Grande Complication which combines a minute-repeater, a sidereal zodiac calendar with month indication to show the position of the stars at any time of the year by means of an annual calendar, and a flying tourbillon which replaces the hour hand to indicate not mean time but the unit of time which astronomers use to keep track of the direction of the stars: sidereal time.
Vacheron Constantin revisits its Patrimony collection with the Calibre 2253, a combination of tourbillon, perpetual calendar and equation of time with indications of sunrise and sunset times. The calibre exhibits a breathtaking 14-day power reserve. Meanwhile, Audemars Piguet proposes the Royal Oak Equation of Time, complete with indication of sunrise and sunset times, a perpetual calendar and astronomical moonphases.
The magic of cams
As Vacheron Constantin explains, “The oldest clock showing the equation of time was made by the mathematician Nikolaus Mercator in the seventeenth century. It was a means of precisely adjusting time according to the passage of the sun at its highest point. Since then, the rare instruments calculating the equation of time have been the work of accomplished horologists. In order to function, this complication requires an equation cam in the form of a figure of eight or analemma, which plots the positions of the sun from a fixed position on earth every day over the course of the calendar year. This equation cam makes one revolution around its axis in a year. The extreme precision of its form determines the accuracy of the equation of time.”
As for sunrise and sunset times, says Audemars Piguet, “These indications depend on three parameters: the date, the longitude and the latitude of a given place. The hands are controlled by two cams performing one full rotation per year. Their shape is determined by the latitude of each location, while their position along the driving wheel depends on longitude. These cams must be machined with extreme precision, as a mere hair’s breadth corresponds to a discrepancy of eight minutes.”
The perpetual calendar is able to keep track of the meanderings of the Gregorian calendar with no outside intervention other than a single date adjustment every 400 years. Indeed, under the Gregorian calendar, the average length of the year is 365.2425 days. To ensure a whole number of days, an additional day is added every four years, i.e. February 29th in leap years. Centurial years are leap years only if they are exactly divisible by 400. A perpetual calendar watch will next need adjusting on March 1st 2100, a common year. As with the previous two complications, the perpetual calendar also relies on cams. The month cam, programmed for the different lengths of each month, makes one revolution a year. The leap-year cam fits inside a circular cut-out in the month cam. It makes a quarter-revolution a year and one complete revolution every four years. All in all, a “mechanical memory” of 1,461 days!