The equinox is the start, not a date near it
Three of the four cardinal signs are pinned to a turning point of the year, and Libra takes the September one. When the Sun crosses the celestial equator going south, day and night are close to equal across the planet and the Sun's ecliptic longitude reads exactly 180 degrees. That instant is the beginning of Libra, by definition rather than by convention.
So a Libra start date of 22 or 23 September is not a rounding someone chose. It is the calendar date on which the equinox happens to fall in a given year, and that varies because the tropical year runs about 365.2422 days rather than a whole number. Each ordinary year pushes the equinox roughly six hours later on the clock, and each leap day pulls it back by nearly a full day. The table above resolves it to the minute for whichever year you pick.
Where the season ends
Libra runs until the Sun reaches 210 degrees and passes into Scorpio, which usually happens on 22 or 23 October. The span between the two boundaries comes to a little over thirty days in the current era.
That length is not arbitrary either. The zodiac is twelve equal arcs of thirty degrees, but the Earth's orbit is an ellipse and the planet moves at different speeds along it. In late September and October the Earth is on the way in towards perihelion in early January, so it is speeding up and the signs are gradually shortening. Libra is a shade shorter than Virgo before it and a shade longer than Scorpio after it.
What the phrase actually means
Calling it a season is a recent habit. The wording spread through astrology accounts on social media over the past decade or so, and it has stuck because it is genuinely useful shorthand: the month in which the Sun occupies a sign, shared by everyone regardless of their own chart.
In traditional terms this is simply the Sun's transit through Libra. It is not the same thing as being a Libra. Anyone born in the range has the Sun in Libra natally, but the transit affects every chart, and where it lands depends on which house Libra covers for you. Libra is cardinal air, ruled by Venus, concerned with comparison and proportion rather than with decision.
Cusp birthdays and the southern hemisphere
If your birthday falls on 22 or 23 September, or on the Scorpio boundary in late October, no blending is involved. The Sun sits at one longitude at any instant, and that longitude is inside one sign. What settles the question is your birth time together with the time zone actually in force where you were born, which for much of the twentieth century differed from the one in force there today.
The dates are also identical everywhere on Earth, because an ingress is a single moment for the whole planet. The season is what reverses. In Australia, Chile or South Africa, Libra season begins at the spring equinox and the days are lengthening rather than shortening. The seasonal imagery attached to the sign in the northern tradition simply does not transfer, though the arithmetic does.
Questions people ask about this
Does Libra season always begin on 23 September?
No. It begins at the September equinox, which in most recent years falls on 22 September and in some years on the 23rd. The drift comes from the tropical year not being a whole number of days, so the equinox moves within a window of roughly a day and a half. The table above gives the exact instant for a chosen year.
How long is Libra season?
A little over thirty days. Because Earth is accelerating towards its January perihelion during autumn, each successive sign in this stretch of the year is slightly shorter than the last, and Libra sits near the middle of that decline.
I was born on 23 October. Am I Libra or Scorpio?
Whichever the Sun's longitude says at the minute you were born. The Libra to Scorpio boundary can fall on either the 22nd or the 23rd depending on the year and can land at any hour. Look up your year in the table, convert to your birth location's clock, and the answer is unambiguous.
Is Libra season the same in the southern hemisphere?
The dates are the same to the minute, because the Sun enters Libra at a single moment for everyone. The season is the reverse: it opens with the spring equinox rather than the autumn one, and daylight is increasing rather than shrinking.
Which planet rules Libra?
Venus, which Libra shares with Taurus. The Taurus expression is sensory and possessive, the Libra one is social and proportional. Libra is also a cardinal sign and an air sign, so it initiates through comparison and conversation rather than through action.