2- Description of the astronomical clock

1. Historical
2. Description: General   Inscriptions   The astrolabe    The computus    The going train
3. Automata and chiming mechanism

General

The various parts of the movements are set inside a square tower made of lime stone, dating from the origin of the clock. The masonry is 0m25 thick ; its base is a square of 1m80 on each side ; its height is 2m15. Above, corbelling out, is another square structure, 2m15 on each side and 1m60 high, ending as a cornice in Greek style. A wooden terminal structure is attached to a wrought iron frame, sealed into the masonry. Set in are the days of the week, the scenes with automata, the small bell chimes, the campanile with the Swiss guard and the big bell. The rooster dominates the top of the structure at a height of more than 9 meters above ground level.

 Inscriptions

A critical investigation is required. Some of the captions on post-cards follow the inscription reproduced on fig. 28, referring to Lippius as the inventor, but the latter did not build the dock but only helped restoring the rooster. The inscription reproduced indicates that the frontons were finished at the end of the XIXth century. Furthermore the text on the left side of the clock misses the restoration of 1598, after the « injuries produced by the heretics», and directly accounts for the reconstruction in 1651 by Guillaume Nourrisson.

The astrolabe

Located on the main façade, the astrolabe gives the following indications: the time, the month, the sign of zodiac, the positions of the sun and the moon. Here the figure indicates 14 hours and 50 minutes, the 10th of April, sign of Aries (the ram), the 21st day of the moon.. The main plate or « tympanum» is immobile; it is engraved with the projections of the earth and the celestial sphere . Above the tympanum is a mobile part or « rete » on which an eccentric circle, the « ecliptic », shows the path of the sun.

The hour hand, or « alidade », guides the sun on the ecliptic circle ; it performs one revolution in 24 hours. The rete revolves in the same direction as the alidade but indicates sidereal time ; it performs one revolution in 23 hours 56 minutes. As a result, one full revolution of the alidade corresponds to one full revolution of the rete plus 4 minutes or 1/365 addition al revolution which is one division on the rete or one day in the year.

In the centre of the astrolabe the phases of the moon are shown by sphere gilt on its half. A star indicates the age of the moon. The precision of the gearing which conducts the moon is remarkable, the error with respect to the real moon being 3/10,000.

The Computus Ecclesiasticus and the perpetual calendar

The calendar and the computus in the center	The computus for year 2003

These are located under the astrolabe. The Computus in the centre is established for 66 years and was extended by Ch. Morat for the years 2020 to 2085. It is surrounded by the circle of the calendar. On the right you can see the 15 indications of the Computus : the year, the number of Sundays after Epiphany, the dates of Septuagesima, of Ash-Wednesday, of Eastern, of Ascension Day, of Whitsunday, Corpus Christi and several other holydays.

Mechanism of the calendar	Indications of the calendar

Figure on the left illustrates the mechanism of the yearly drop: at each revolution of wheel V292 (making a full revolution per year), the pin sigma on one of its arms lifts the lever beta. As a result, lever beta pushes a pin on the ratchet-wheel rho forcing it to rotate 1/6 of a turn. The wheel affixed on rho has 24 teeth ; it acts on wheel U264 having 264 teeth. The ratio Q/P being 1/11, each revolution of wheel V292 enduces the rotation of wheel U264, which carries the Computus, by 1/66 turn.

Photo on the right shows the six indications given by the calendar : the month (Augustus), the lunar calendar (in 2003 the new moon was on the 6th), the Dominical Letter (E for 2003), the Roman calendar (VIIth day before the Calends of February), the date (26), the Saint of the day (Saint-Polycarpe). The most interesting indication is that of the Dominical Letter given each year by the Computus (the first seven days of the week being designated by the seven first letter of the alphabet). In 2003 the Dominical letter is E, which will correspond to all Sundays of the year. Augustus 8 (E) is a Sunday, Augustus 9 (D) a Monday, ... Augustus 13 (A) a Friday ... etc.

The going train

Drawing of the main train	Dial of minutes

 The drawing on the right shows the main train of the clock. The pendulum is 2m715 long; its period is 3.3 seconds. Hour keeping is remarkable for a mechanism designed in the XVIIIth century : deviations do not exceed one minute per month.

 The drawing on the left represents the working mechanism of the retractable minute hand on the oval dial. The hand is fixed on pinion B. The rod C is attached to pinion A which rotates in the opposite direction. The two ends of the hand D and D', locked to the rod C via arms F and G, follow an elliptic path.