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