Autumnal Equinox

Definition

The autumnal equinox is the instant when the Sun, moving along the ecliptic, crosses the celestial equator from north to south. Its apparent geocentric ecliptic longitude is then exactly 180° and its declination changes from positive to negative. It falls on 22 or 23 September, which is why the neutral name September equinox is often preferred: in the southern hemisphere this is the start of spring, not autumn.

The word also names the point of the sky where the crossing happens, diametrically opposite the vernal equinox. Like it, the autumnal equinox is an intersection of the ecliptic and the celestial equator, and it moves slowly westward with precession.

Dates of the September equinox

Each year the equinox comes about 5 h 49 min later than the previous year, the excess of the tropical year over 365 days. Leap years pull it back by a day, and the Gregorian rule of skipping three leap years every four centuries keeps it from drifting over the long term.

Unequal seasons

From the March equinox to the September equinox, the northern spring and summer, the Sun takes about 186 days. From September back to March it takes only about 179 days. In 2026–2027, for instance, the September equinox on 23 September at 00:05 UT is followed by the March equinox on 20 March 2027 around 20:25 UT: 178.8 days.

The reason is the eccentricity of the Earth's orbit. The Earth passes perihelion in early January and moves faster then (Kepler's second law), so it covers the half of its orbit between the September and March equinoxes more quickly. Northern autumn and winter are therefore the shortest seasons, by about a week.

Day length near the equinox

At the equinox the Sun rises almost due east and sets almost due west everywhere on Earth, and the day lasts about twelve hours. It is not exactly twelve: refraction lifts the Sun by about 34′ at the horizon, and sunrise and sunset are timed on its upper edge, so the day is several minutes longer than the night. The day with equal day and night, sometimes called the equilux, comes a few days after the September equinox in the northern hemisphere (around 25–26 September at mid-latitudes).

Around the equinoxes the day length changes fastest, because the Sun's declination changes fastest, about 0.39° per day:

The first point of Libra

About 2,000 years ago the September equinox lay in the constellation Libra, and it is still called the first point of Libra. Because of precession it has moved about 28° westward since then and now lies in Virgo, near the star Beta Virginis. It will enter Leo in the 2400s.

In the tropical zodiac used in Western astrology, the sign of Libra still begins, by definition, at the September equinox, whatever the constellation behind the Sun.

Computing the instant

Meeus gives a two-step method. A mean instant JDE0 comes from a polynomial in the year, then 24 periodic terms correct it for the perturbations of the Earth's motion by the Moon and the planets:

Y = (year − 2000) / 1000JDE0 = 2451810.21715 + 365242.01767 Y − 0.11575 Y² + 0.00337 Y³ + 0.00078 Y⁴JDE = JDE0 + 0.00001 S / Δλ

The result is in Dynamical Time; subtract ΔT (about 69 s) to get UT. For the highest accuracy, Meeus suggests iterating instead: compute the Sun's apparent longitude with the full VSOP87 theory and correct the date by 58 sin(180° − λ) days until λ = 180°. This site's planetary engine finds the 2026 equinox within a few minutes of the official value.

Explore it on AstroAlgos

• Vernal equinox — the March crossing
• Heliocentric view — the Earth at the equinoxes
• Ecliptic — the Sun's yearly path
• Night sky — the September sky