Question:

“What is a Perpetual Calendar watch and how does it work?”

Maurice Duproz in Lyon, France

Answer by Professor J.C. Nicolet

The early “calendar” watches, dating from the 16th century, were equipped with a mechanism giving the day, date and month in addition to the hour which was still imprecise at that point in time.

In a calendar watch, the days and months follow sequentially but the same cannot be said of the dates which are either 28, 29, 30 or 31 depending on the month and whether it is leap year or not. In a “simple” calendar watch, it is necessary to correct the date five times during the year, i.e. the ist day of March, May, July, October and December.

Abraham-Louis Breguet is usually credited with having invented the mechanism which made these corrections automatically.

His invention led to today’s “perpetual calendar” watches as opposed to “simple calendar” timepieces. These models are based on the Julian calendar rather than the Gregorian calendar in use today. As a result, leap years are not deleted at the end of three out of four centuries, thus making it necessary to correct the watch three times in 400 years. Regarding leap years, February 29 has been deleted in the years 1700, 1800 and 1900. It won’t be deleted in 2000 but will be in 2100, thus today’s ads for perpetual calendar watches are right in their claims that these models will not have to be corrected for over a century. The actual duration of a year is 365.2422 days. The perpetual calendar counts the year as having 365.25 days while the simple calendar counts 12 x 31 = 372 days making it necessary to remove 6 or 7 days every year.

How it works:

To explain how the perpetual calendar works, we will discuss a mechanism devised by the author for an astronomical clock (see diagram). The principal part of the mechanism is the perpetual lever (B) which pivots on (b). It returns to its position by an action of the spring (rb) and it normally pushes against the perpetual cam (P). A small finger (D) completes one turn per day around point (d) and drags the lever between the hours of 23hOO and midnight by sliding on its inclined plane. The perpetual level (B) is equipped with two pawls (C1) and (C2) which are acted upon by their two respective springs. Each day around midnight the beak (Bj) moves the seven-pointed day star which is held in place by its jumper-spring. The diagram shows the position at midnight just before the jump to March 1. Normally the date is changed by the pawl (Cl) while the pawl (C2) slides onto the cam (L).

Date change

Five times per year, when the date changes from the 30th to the lst (or for leap years, from February 28 to 29), it is the pawl (C2) positioned behind the catch of the cam (L) which causes the hand to move from 30 to I (or from February 28 to March 1). For the month change, the lever (M) pivoting on (m) held by a pin on the cam (L) moves the month star from February to March.

The secret of the perpetual calendar is in understanding the way that the perpetual lever engages the pawl (C2) behind the catch of the cam (L) on the appropriate date. We have seen that the perpetual lever at rest pushes against the cam (P). This cam is the memory for the perpetual calendar. It has seven ridges corresponding to the months with 31 days, four indentations corresponding to the months with 30 days and a movable rectangle for February. The cam thus determines the three levels of rest for the perpetual lever.

The pawl (C2) which is engaged behind the catch of the cam (L) can occupy three different levels, This pawl can then become engaged behind the catch on the evening of the 30th and will not act until the 31st at the same time as the the pawl (Cl).

This then is the case of 31-day months corresponding to the seven ridges. The pawl (C2) becomes engaged behind the catch on the evening of the 29th when the lever pushes on the base of the indentation. The evening of the 30th, between 23h00 and midnight, it causes the date to change to the 31st. Finally for February the lever, pushing on one side of the rectangle and always lower than the bottom of the indentation, allows the pawl (C2) to move the date from February 28 or 29 directly to March 1.

An ingenious addition is that the small movable rectangle has three sides equidistant from its center of rotation and the fourth side which is positioned higher than the others. Thanks to this small simple mechanism hidden behind the date star, it rotates one-quarter of a turn each year so that once every four years, the highest side pushes on the lever.

For that year, the pawl (C2) will only act on the 29th of February, corresponding to the leap year. If we simplify the mechanism by replacing the small movable rectangle by a fixed indentation, the jump will always occur on February 28 and the calendar would then have to be corrected for leap years. This simplified device is called a “semi-perpetual calendar”.

The date is changed from 31 to I by the action of a pin placed on the cam (L) acting on the lever (M) which pivots on (m). As soon as the pin of the cam (L) escapes from the lever, the latter is drawn behind the next tooth by a spring. The end of (M) is jointed to allow it to pass behind the next tooth, thus causing it to move at the end of the following month.