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THE INDICATION OF SEASONS, EQUINOXES, SOLSTICES & SIGNS OF THE ZODIAC

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1. Solar Astronomy and Watchmaking: An Ancient Tradition

Astronomical indications related to the sun are among the oldest represented on mechanical clocks. As early as the 14th century, large monumental clocks displayed the position of the sun in the zodiac, the Seasons, and the equinoxes.

These indications reflect watchmakers’ desire to represent not only the time, but also the position of the Earth in its annual orbit. They place the watch within a cosmic Time, far beyond seconds and hours.

The Seasons, the equinoxes, the solstices, and the signs of the zodiac share a common origin: they all describe the same astronomical reality, seen from different angles. They describe the Earth’s revolution around the Sun and its effects on sunlight and the Seasons.

In watchmaking, these indications are grouped under the generic term solar astronomical complications. They are distinguished from lunar complications — Moon phase, moon age — which describe the movement of the moon around the Earth.

Their integration into a Movement mechanical relies on a common principle: a disc or a Hand completing one full revolution in a tropical year, driven by the base movement with an appropriate reduction ratio.

2. The tropical year: a common astronomical basis

The tropical year is the duration separating two successive passages of the Sun at the spring Equinox. Its average duration is 365 Days, 5 hours, 48 minutes and 45 seconds, or approximately 365.2422 Days.

It is this duration that defines the complete cycle of the Seasons. It is slightly shorter than the Gregorian civil year of 365.2425 days. This tiny difference is the origin of the progressive drift of the Julian calendar, corrected by the Gregorian reform of 1582.

In watchmaking, the tropical year is the reference cycle for all solar astronomical displays. A solar indication disc must make exactly one revolution in one tropical year to remain in agreement with the civil calendar. Many mechanisms inaccurately drive solar astronomical indications through a perpetual calendar, inducing an error of a quarter of a Day per year (compensated during the leap year). Paradoxically, the use of an annual calendar is more appropriate, as the error correction is performed every year.

The non-coincidence between the tropical year and the civil year of 365 days explains why the Equinoxes and Solstices do not fall on exactly the same date each year. The spring Equinox falls between March 19 and 21 depending on the year.

Horological mechanisms for solar indication are generally set to the Gregorian civil year, whose average duration is very close to that of the tropical year. This approximation only generates a negligible error over the course of a human lifetime.

3. Equinoxes and solstices

3.1 Astronomical definitions

Equinoxes and solstices are the four moments of the year that mark the transitions between seasons. They define the cardinal points of the Earth’s orbit relative to the Sun.

The spring (vernal) Equinox occurs around March 20 in the northern hemisphere. It corresponds to the moment when the sun crosses the celestial equator heading north. The length of Day is then equal to that of night at every point on Earth.

The summer Solstice occurs around June 21. It corresponds to the moment when the sun reaches its maximum declination toward the north. It is the longest day of the year in the northern hemisphere, and the shortest in the southern hemisphere.

The autumn Equinox occurs around September 22. The sun crosses the celestial equator again heading south. As in spring, day and night have the same duration. The winter Solstice, around December 21, marks the shortest Day of the year in the northern hemisphere and the longest in the southern hemisphere.

3.2 Positions in the year and irregularities

The four solstitial and equinoctial points do not divide the year into four equal parts. The Earth does not move at a constant speed along its elliptical orbit. It is faster at perihelion, in January, and slower at aphelion, in July.

This results in Seasons of unequal duration. In the northern hemisphere, spring and summer are slightly longer than autumn and winter. This irregularity is one of the technical difficulties in the mechanical representation of the Seasons.

Simple horological mechanisms ignore this irregularity and represent the four Seasons as equal quarters of the year. The rare mechanisms ofEquation of time include this correction in their programs.

4. Signs of the zodiac

4.1 Definition and origin

The zodiac is a celestial band 18 degrees wide, centered on the ecliptic — the plane of Earth’s orbit projected onto the celestial sphere. It is divided into twelve equal sectors of 30 degrees each, called Signs of the zodiac.

The twelve signs — Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces — correspond to twelve constellations of the ecliptic. Their delineation dates back to Babylonian antiquity.

The first sign, Aries, begins at the spring Equinox. The sun enters each new sign approximately every 30 days and 10 hours. It travels through the entire zodiac in one complete tropical year.

It is important to distinguish the signs of the zodiac from the constellations of the same name. Due to the precession of the equinoxes, the signs and the constellations have no longer coincided for several centuries. In horology, it is always the position of the sun within the signs — and not within the actual constellations — that is represented.

4.2 Relationship with the seasons

The twelve signs of the zodiac are directly linked to the seasons and the equinoxes. Each season corresponds to three consecutive signs. Spring covers Aries, Taurus, and Gemini. Summer covers Cancer, Leo, and Virgo.

Autumn covers Libra, Scorpio, and Sagittarius. Winter covers Capricorn, Aquarius, and Pisces. The equinoxes and solstices mark the transitions between the seasons, namely the entries into Aries, Cancer, Libra, and Capricorn.

This correspondence between the signs and the Seasons is fundamental to understanding the logic of astronomical watchmaking displays. A sundial simultaneously displaying the Seasons, Equinoxes, Solstices and Signs of the zodiac actually represents one and the same reality: the position of the sun on the ecliptic throughout the year.

5. Mechanical principle of the Display

5.1 The annual Disc

The basic mechanical principle is common to all these indications. A Cam makes one complete revolution in one calendar year, driven by the base Movement with an appropriate reduction ratio.

This Disc carries the graphic representations or inscriptions corresponding to the Seasons, the Signs of the zodiac, the Equinoxes and the Solstices. Its visible face, in the Aperture / Window or under the reading Hand, indicates the current value.

The required reduction ratio is considerable: the base Movement produces 260,000 oscillations per Day for a Balance regulated at 2.5 Hz. This Frequency must be brought down to one Lathe per year, i.e. approximately 1/1,080,740 of the Train (wheel train) per Oscillation.

This ratio is achieved through a series of reduction wheels interposed between the finishing Train (wheel train) and the annual Cam. Each reduction wheel divides the Frequency by a calculated factor in order to precisely match the length of the calendar year.

Today, some watches use a Cam rotating over a sidereal year, i.e. 365.2425 Days, which remarkably increases the accuracy of the indications.

5.2 Drive from the Date / Calendar

In many multi-complication Calibres, the annual Disc is not directly driven by the base Train (wheel train). It receives its Impulse from the Date / Calendar mechanism, which advances by one step per Day.

The annual Disc is then driven by a Finger integral with the Date / Calendar Disc. With each daily advance of the Date / Calendar, the annual Disc advances by one step corresponding to a Day over the 365-position cycle.

This architecture considerably simplifies the drive mechanism. It avoids the multiplication of reduction Trains (wheel trains). However, it creates a functional dependency between the Date / Calendar and the solar indications.

A manual correction of the Date / Calendar automatically causes a corresponding advance of the solar indications. During the initial time setting, the annual Disc must be positioned in accordance with the actual Date and the current season.

6. The Display of Seasons

6.1 The Disc of the Four Seasons

The Display of the Seasons is most often presented in the form of a Disc divided into four sectors, bearing the names or graphic representations of the four Seasons. This Disc completes one full Lathe per year.

Each sector covers approximately a quarter of the disc, or around 91 days. This distribution is an approximation, as astronomical seasons are not strictly of the same duration. The simplification is nonetheless acceptable for everyday use.

The seasons Disc appears in an Aperture / Window cut into the Dial, or beneath a reading Hand. The transition from one season to the next corresponds to entering the zodiac sign associated with the Equinox or Solstice concerned.

Some Calibres depict the four Seasons through allegorical illustrations — nature scenes, mythological allegories — painted, enamelled or engraved on the Disc. These representations make the Disc a decorative object in its own right, on a par with the Moon phase Disc.

6.2 Hemispheric Representation

As with the Moon phase, the Display of the Seasons raises the question of the reference hemisphere. The Seasons are reversed between the northern hemisphere and the southern hemisphere. The boreal spring corresponds to the austral autumn.

Season Discs are traditionally designed for the northern hemisphere, where European watchmaking production has historically been concentrated. Versions for the southern hemisphere exist, but are notably rarer.

Some high-end watchmaking pieces feature a reversible Disc or two separate Displays, one for each hemisphere. This technically complex solution is reserved for grand complications.

7. Displaying Equinoxes and Solstices

Equinoxes and Solstices can be indicated in several ways on a Dial. The most common representation consists of marking their approximate dates on the annual Disc or on a fixed reference Dial.

An annual Hand rotates on a circular Dial bearing the dates of the four significant solar events. When the Hand points to the date of an Equinox or a Solstice, the event is underway.

Another approach consists of integrating transition indicators on the Seasons disc. Graphic markers — astronomical symbols or inscriptions — indicate the precise moments of the Equinox and the Solstice in the sector of each season.

The traditional symbol for the spring Equinox is the sign of Aries (♈). The autumn Equinox is represented by Libra (♎). The summer Solstice bears the symbol of Cancer (♋) and the winter Solstice that of Capricorn (♑). These zodiacal symbols are universally recognized in the horological astronomical tradition.

Certain monumental clocks and grand complications represent the position of the sun on the ecliptic by a solar Hand that moves across a zodiacal Dial. This representation, more complex, allows a precise reading of the solar position at any time of the year.

8. The Display of the signs of the zodiac

8.1 The zodiacal Disc

The Display of the Signs of the zodiac in watchmaking is based on a Disc with twelve positions corresponding to the twelve signs. This Disc completes a full Lathe in one year. Each sign occupies about 30.4 Days on this Disc.

The twelve signs are represented by their traditional graphic symbols, by their full or abbreviated names, or by allegorical illustrations. The current sign is visible in an Aperture / Window or under a reading Hand.

The sign change occurs approximately every four weeks. Depending on the construction, it can be dragging, semi-instantaneous, or instantaneous.

In some Calibres, the zodiac Disc is combined with the Month Disc. The twelve positions then correspond both to the Month of the civil calendar and to the sign of the zodiac currently in progress. This dual function simplifies the architecture, but assumes an approximate correspondence between the two cycles.

8.2 The solar hand on zodiac Dial

The most faithful and complex representation of the signs of the zodiac is the solar hand on the zodiac Dial. A dedicated Hand rotates on a circular Dial bearing the twelve signs and their graphic representations.

This Hand completes a full revolution in one year. Its position indicates at any moment which sign the sun is in. The reading is continuous and precise, allowing the date within the current sign to be located with a resolution of a few days.

Some zodiac Dials also indicate the approximate dates of the Sun’s entry into each sign. The user can thus simultaneously read the current sign and the proximity of the next zodiac transition.

The solar hand on the zodiac Dial is the direct heir to the great medieval astronomical clocks. It is found notably on monumental clocks such as the Prague Astronomical Clock or the Strasbourg Cathedral Clock.

9. The annual drive mechanism

9.1 The annual reduction Train (wheel train)

As mentioned in chapter 5, driving a Disc or a Hand that completes one turn per year requires a very high reduction ratio compared to that of the finishing Train (wheel train). This ratio is achieved through a series of toothed wheels interposed between the finishing Train (wheel train) and the Display component.

The 24-hour wheel, which completes one turn per Day, is often the starting point of the annual reduction chain. This Frequency must still be divided by 365.25 to obtain one turn per year.

This ratio of 365.25 is approximated by a combination of wheels whose number of teeth is chosen to minimize the error. For example, a 73-tooth wheel driving a 20-tooth wheel gives a ratio of 1/3.65. Combined with a second reduction stage, it provides the desired approximation.

The minimum number of reduction stages is generally two or three, depending on the desired precision. Each additional stage improves precision but increases the size and energy consumption of the mechanism.

9.2 Continuous drive and step-by-step drive

Two training philosophies compete regarding annual discs. Continuous drive rotates the disc permanently, without interruption. The indicated position is therefore accurate at any moment, but reading can prove difficult during transitions.

Step drive advances the disc by a fixed position each day or each week. The reading remains clear and stable between two advances. The value change occurs in a jerky manner, once per day or per week depending on the chosen resolution.

Daily step drive is the most widespread. It is mechanically simple and offers perfect readability. Continuous drive is reserved for exceptional pieces where the accuracy of the solar position takes precedence.

10. Accuracy and correction

The accuracy of annual solar indications directly depends on the reduction ratio of the drive mechanism. The gap between the tropical year of 365.2422 days and the civil year of 365.25 days is around 11 minutes per year.

This difference is very small. It generates an error of about one day every 128 years for a mechanism calibrated to the Gregorian civil year. This discrepancy is negligible for the vast majority of practical uses.

In fine watchmaking pieces, the reduction ratio can be optimized to more closely match the actual tropical year. These corrections make it possible to reduce the error to a few hours per century.

Solar indication mechanisms generally do not require regular manual corrections. The user must perform a single initial adjustment during commissioning, then, at each rewinding after a long period of inactivity.

The initial adjustment consists of positioning the annual disc or hand on the season and sign of the zodiac corresponding to the current date. This operation is carried out via a dedicated Corrector or by the Crown, depending on the calibre.

11. Association with other complications

Annual solar indications are naturally associated with other astronomical and calendar complications. They frequently appear alongside the perpetual Date / Calendar, the Moon phase and the Equation of time in grand complications.

The association with the Equation of time is particularly logical. The Equation of time describes the discrepancy between true solar time and mean civil time. This discrepancy depends directly on the Earth’s position on its elliptical orbit, that is, on the current season.

The combination of the Seasons and the Equation of time allows for a coherent and complete representation of solar time. It mechanically reproduces what a precision sundial naturally indicates: the position of the sun at the given moment.

The association with sunrise and sunset is also common in great astronomical complications. Since the length of the day varies according to the season and latitude, the indication of the seasons is essential for mechanically calculating the times of sunrise and sunset.

In certain fine watchmaking pieces incorporating a planetarium or an armillary sphere, the indications of seasons, equinoxes, solstices and signs of the zodiac are part of a coherent whole representing the solar system in miniature.

12. Graphic representations and aesthetics

The indications of seasons, equinoxes, solstices and signs of the zodiac offer the decorator and the dial maker a particularly rich field of aesthetic expression. Their symbolic and mythological content inspires representations of great diversity.

The traditional zodiac symbols — ♈ ♉ ♊ ♋ ♌ ♍ ♎ ♏ ♐ ♑ ♒ ♓ — are the most compact and universal representations. They are used when the space available on the dial is limited.

Allegorical representations of the Seasons draw upon a centuries-old iconographic repertoire. Spring is associated with flowers and young shoots. Summer evokes heat, harvests, and abundance. Autumn symbolises grape harvesting and falling leaves. Winter symbolises cold, snow, and nature’s rest.

Since Antiquity, the Signs of the zodiac have inspired figurative representations of great richness: Aries with its horns, the powerful Bull, the entwined Twins, the Crab of Cancer, the majestic Lion, the Virgin holding an ear of wheat.

These representations appear on the Dials of grand complications in the form of painted miniatures, cloisonné enamels, relief engravings, or inlays of precious Metals.

13. Place in the History of Watchmaking

Solar astronomical indications are among the oldest in mechanical watchmaking. The great monumental Clocks of the Middle Ages — Prague, Strasbourg, Bern, Lyon — display the position of the sun in the zodiac, the Moon phases, and the Seasons on astronomical Dials of great complexity.

These public clocks had both a practical function and a symbolic function. They demonstrated the community’s mastery of time and represented cosmic order in all its complexity. Their astronomical Dial was as much a display of knowledge as of craftsmanship.

With the progressive miniaturization of Movements, these indications migrated from longcase Clocks to the prestigious pocket watches of the 17th and 18th centuries.

In the 19th century, the great pocket complications — known as “ultra-complications” — combined perpetual Date / Calendar, Equation of time, striking mechanism and complete astronomical indications.

Today, indications of Seasons, Equinoxes, Solstices, and Signs of the zodiac remain present in the exceptional pieces of the great manufactures. They bear witness to the continuity of a watchmaking tradition that sees in the watch much more than a simple tool for measuring Time: a living link between man and the cosmos.