Libra opens at the September equinox, the second of the year's two crossings of the celestial equator. Cardinal and air.
It is the only sign of the twelve whose symbol is an object rather than a creature — the scales — and the geometry supports it: this is the point where day and night are equal, and after which night begins to win.
Venus rules it as proportion and comparison. The traditional difficulty deciding alone is the equinox itself: a balance point is by definition unstable.
Traditionally attributed
Balance, comparison, charm, and difficulty deciding alone.
Listed because it is what the tradition says, not because this site can check it. Everything above this line is geometry; everything in it is convention.