EQUINOX

The equinox is the moment of the year when the Sun, in its apparent movement along the ecliptic, crosses the plane of the celestial equator, its declination becoming zero. Two equinoxes occur each year, around March 20 (March equinox, referred to as vernal in the northern hemisphere) and September 22-23 (September equinox): on these dates, the two terrestrial hemispheres are illuminated symmetrically with respect to the Sun, a consequence of the approximately 23.4° tilt of the Earth’s axis of rotation relative to the plane of its orbit. The traditional notion that the day and night would be strictly equal is only approximate: atmospheric refraction and the apparent diameter of the Sun actually lengthen the duration of the day by several minutes. In watchmaking, the equinox constitutes a reference point for astronomical calendar complications: the perpetual calendars, along with equation of time, seasons, or signs of the zodiac indicators, must account for the slight annual shift in its date — due to the difference between the calendar year and the tropical year — to remain faithful to celestial mechanics over several decades.