This face comprises a disc representing the 12 signs of the zodiac. To explain its function, we first need to consider briefly the most important star: the Sun. We all know that our year is defined by the time it takes the Earth to make a complete journey around the Sun. But we could equally talk about the Sun’s apparent journey around the celestial sphere taking the Earth as our reference point. In astronomy, the tropical year begins at the vernal equinox point, which corresponds to the spring equinox, when day and night are of equal length.
The tropical year lasts some 365.242 mean solar days or, expressed in more conventional terms, 365 days, 5 hours and 49 minutes. This imaginary solar voyage follows what is called an ‘ecliptic’ course. It comprises 360 degrees and is divided into the twelve houses of the zodiac. In relation to the celestial equator, an imaginary extension of the terrestrial equator, the Sun’s path runs at an angle of 23 degrees and 27 minutes.
Under normal circumstances, the Sun enters the constellation of Pisces on 21 March. At this time, it finds itself directly above the terrestrial equator, such that the days and nights are of the same length. It then continues its journey in the direction of the northern hemisphere. At the beginning of Taurus, the Sun’s path crosses the celestial equator. Three months later, when it reaches the tropic of Cancer on 21 June, the Sun is at its maximum distance from the celestial equator. This point in the constellation of Gemini is called the summer solstice. Afterwards, the Sun directs itself again towards the south and the celestial equator. It crosses the latter on 23 September when it reaches Libra. The journey south in the direction of the tropic of Capricorn ends with the winter solstice on 21 December. At this time, the Sun enters into the house of Sagittarius before again setting out on its path towards the north and a new crossing of the celestial equator. The Sun reaches the latter around 21 March, when the spring equinox completes the tropical year. In astronomy there are 88 constellations, of which only twelve bear the names of the signs of the zodiac.
From an astrological point of view, it is important to distinguish between the constellations and the signs of the zodiac. The constellations are simply groups of stars which are moving slowly west of the signs of the zodiac as established by Ptolemy in 200 AD. Because of the precession, a minimal but constant shifting of the terrestrial axis, the Sun is now always to be found in the preceding sign of the zodiac. For instance, it is in Leo during the sign of Virgo.
Eighteen hundred years ago the situation was quite different. When the signs of the zodiac were first given their names, the Sun was indeed to be found in the sign of the zodiac corresponding to that of the constellation of the same name. Thanks to the gyroscopic movement of the terrestrial axis, this will again be the case in around 24,000 years.
Future owners of a Reverso grande complication à triptyque can afford to ignore these astronomical niceties, however. In fact, their watch indicates at one and the same time the zodiacal calendar around the circumference of the dial and the position of the constellations in the aperture of the celestial horizon.