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Maker, Ferdinand Berthoud, Paris, France, c. 1785. Three train pinwheel escapement, 1/2 second gridiron compensated pendulum, 30 second Robin remontoire, full perpetual calendar, epicyclical equation of time, moon's age and phase, quarter strike on solar time Chime and strike can be silenced. 25.5"h x 12.25"w x 10.75"d, (65 x 31 x 27.5cm)

The equation of time complication is examined below in detail. This is located behind the main dial. The complex gearing behind this dial as well as the others requires this movement to employ a double frame construction. Only the more extreme complicated movements employ this construction.

The two gold engraved hands depict mean solar time or regular clock time as is commonly known. The black hand with the sun decoration just past the three o'clock position indicates how fast or slow sun time is from the regular time. That is how fast or slow the sun is at any time to mean time and this varies from approximately 15+ to 15- minutes depending on the season. The quarter striking on this clock has the unusual feature of being let off when the solar hand is on the quarter hour of the dial, not the minute hand of the mean solar time. Apparently the owner of this clock was more concerned with knowing 'sun time' than the regular time we so respect today. The equation kidney is silvered for contrast.

Detail of the engraved minute and hour hands.  

Detail of the engraved minute and hour hands. 

Shown here are details of the fine engraving associated with both the equation of time kidney cam and its drive wheel. The cam has elegant script describing each zodiacal month of the year. The circular wheel below shows the number of days in each month. This wheel which just over 3 inches in diameter (8 cm) has 365 teeth. An incredible precision on this small scale.

Here the engraving is following the sinuous path of the equation cam rim. 

This photo shows the epicyclical cage upon which the equation kidney cam is mounted. Normally an equation cam rotates once per year, giving the difference between mean solar and solar (sun) time. The simplest way to achieve this output is to have a follower riding upon the kidney cam's circumference and this is then attached to a hand reading off a sector dial delineated in 15+ to 15- minutes. One then simply takes this reading and adds or subtracts from the time currently being seen on the main clock dial. Combining both functions onto the main dial where a separate solar hand is either ahead or behind the the the clock's minute hand requires the further sophistication of epicyclical gearing, but in nearly all cases the kidney cam still rotates once per year. Here Berthoud employs an additional complication to get the correct reading with the kidney rotating once per day. Why do this? The reason is that Berthoud also employs a first order perpetual calendar in this clock. That is, the quadrennial leap years are accounted for with a day added to the month of February every four years, also known as the intercalary day. This system will make the calendar good for four years. By having the kidney rotating daily, but the readout for purposes of the main dial through the complex gearing yearly, the solar time will stay in step with the perpetual calendar for the four year period. For a discussion on the characteristics of a first vs. second and third order perpetual calendars click on my section of the astro skeleton construction project dealing with this.

This photo shows the complex set of gearing contained in the second frame and associated with the perpetual readout of the solar time. 

A detail of the epicyclical drive mechanism is shown in this photo. The input gear is 1, it is connected via an arbor to a worm gear shown as 2. That is connected to the long blued drive shaft arbor 3. This is then connected via a worm gear, 4 to a pinion 5. This drives the 365 tooth ring gear 6 which is attached directly to the kidney cam, 7.

A conventional equation kidney rotates once per year. In this clock it rotates daily and the readout is translated to the calendar via epicyclical gearing resulting in the correct annual reading. The calendar is a full perpetual, allowing for the differing lengths of the months as well as a February of 28 days every four years (leap year).

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Continental and American Skeleton Clocks, Derek Roberts, p. 24