Wooden Clock Spot: A Third Order Perpetual Calendar

The Perpetual Calendar Module

Perpetual calendars for clocks keep coming back to haunt me. This is the fifth different ne I have made but the first one was only a partial success and does not figure in this blog. It needs to be revisited.

This version which is just a calendar module is based on a design which is part of a magnificent and intricate skeleton clock created by Mark Frank. To see his whole project and locate the calendar module visit www.my-time-machines.net/astro_index.htm

First some notes on calendars through the ages.

The ancient Egyptians and others used a year with exactly 365 days. But the average tropical year has 365.24219 days and calendars soon got out of sync with things like crop planting and growing seasons.

Julius Caesar instituted the Julian Calendar in which a year has an extra day if its number is evenly divisible by 4. Thus the average year has 365 + 1/4 = 365.25 days. which was much closer but eventually accumulated an error amounting to 0.78 days per 100 years.

By 1582 the drift of the Julian Calendar amounted to about 10 days and was quite noticeable. So Pope Gregory XIII decreed that 10 days were to be skipped and instituted the Gregorian Calendar. According to this calendar leap years usually occur every 4th year but are skipped every 100 years unless the year is divisible by 400. Therefore 400, 800, 1200, 1600, and 2000 ARE leap years but 1900 and 2100 are not. A four-century period will be missing 3 of its 100 Julian leap years, leaving 97. So the average year has 365 + 97/400 = 365.2425 days. The error now is .03 days per hundred years.

This is now an almost universally accepted calendar but there are others, although these are not so easily understood as the Gregorian calendar.

But how to produce a mechanical analogue of the Gregorian calendar with its varied days of the month and those pesky leap years. A mechanical calendar which takes into account just the varying lengths of the months is relatively easy to design but will need resetting every four years. An enhancement will add one extra day in February every 4 years, giving a leap year. This may require resetting every 100 years because some century years are not leap years. Finally we can take into account the 100 year and 400 year cycle which really is overkill for a clock but interesting from an engineering point of view.

This is what Mark Frank's design can do and so can my interpretation.

It is based on a series of cams which insert or remove teeth into a count ring. In horological terms these movable teeth are known as 'surprise pieces'. The count ring has 28 teeth and 3 spaces for the variable days. One cam raises or lowers the 31st day tooth depending on the month. Another cam raises or lowers the 29th and 30th teeth leaving these teeth up for all months except February. All three teeth are down for a 28 day February.

There are three other cams and sub cams to take account of Februarys which have 29 days.

These are annotated on the images of the mechanism below.

The 4 year cam (here labelled 20 year cam) which unsurprisingly rotates once in 20 years has four lobes on it and one smooth space where a lobe should be, These are equally spaced. The lobes' raise the 29th February tooth to give a leap year for years 4,8, 12,16. On year 20 the tooth is not raised because this cam is smooth.

However the 100 year and 400 year cams provide the missing tooth as necessary

Every rotation of the 24 hour wheel moves the index wheel on by one day. The power for moving the index wheel comes from a remontoire wound by the motion of the 24 hour wheel. This is shown below.