The Autumnal Equinox – A Day of Balance

Around 22 September, day and night are approximately equal. The word ‘equinox’ comes from Latin aequus (equal) and nox (night). The Sun’s right ascension is 12h, declination 0°. The equinox is used as the reference point for the tropical year (the cycle of seasons). The precession of the equinoxes (discovered by Hipparchus) shifts the equinox point along the ecliptic over 26,000 years, changing the zodiac signs. The mathematics of precession involves torques from the Sun and Moon on Earth’s equatorial bulge – a problem in rotational dynamics.