Autumn

The September equinox marks the sun's crossing of the celestial equator moving southward, bringing day and night back to approximately equal length worldwide and opening the astrological season of autumn, which the tropical zodiac reckons as running from that equinox to the December solstice. The season opens with Libra, the cardinal sign whose beginning is defined by the equinox itself and whose symbol, the scales, is often read as reflecting the day and night balance the equinox itself produces, moves through the fixed sign Scorpio, and closes with the mutable sign Sagittarius as the season transitions toward winter, completing the same cardinal, fixed, and mutable pattern the tropical zodiac repeats across all four seasonal quarters. Libra is the only sign of the tropical zodiac represented by an inanimate object rather than a person, animal, or hybrid figure, a distinction some historians of astrology connect to the sign's comparatively late addition to the zodiac as a separate figure, since Babylonian tradition originally read the same stars as the claws of the neighboring scorpion figure before Hellenistic astronomy split them into an independent sign. As with the spring equinox, the September equinox's approximately equal day and night length is a genuine worldwide astronomical event, distinct from the sign boundaries the tropical zodiac assigns to it, which track the equinox point rather than the visible constellation Libra's actual current position along the ecliptic.

Sources TetrabiblosClaudius Ptolemy with Hellenistic Astrology: The Study of Fate and FortuneChris Brennan

The autumn equinox falls at the height of harvest season across much of the temperate northern hemisphere, and a wide range of independent cultural traditions mark the period with thanksgiving observances tied more to the practical agricultural calendar than to the equinox's precise astronomical date. Modern Wiccan and broader neopagan practice observes the equinox as Mabon, one of the eight sabbats of the Wheel of the Year, themed around the second of the tradition's three harvest festivals and around balance and gratitude as day and night reach equal length before winter's darkness begins to dominate; the name itself is a comparatively recent addition to modern Pagan practice, adopted in the later twentieth century from Welsh mythology rather than continuously transmitted from any older seasonal observance under that specific name.

Chinese tradition's own Mid Autumn Festival, discussed on its own entry, falls near but not fixed to the same astronomical point, timed instead to the full moon of the eighth lunar month, illustrating how a lunisolar calendar and a purely solar one can each produce an autumn harvest observance that lands close to, without being defined by, the same equinox. Harvest festivals of this general kind, whatever their specific date or origin, tend to share a common underlying logic regardless of the tradition producing them: a season's agricultural labor reaches its conclusion at broadly the same point on the solar calendar wherever a community depends on a temperate growing season, so independent cultures repeatedly arrive at similar autumn thanksgiving customs without needing to have influenced one another.