23 September to 22 October
Libra begins at a hundred and eighty degrees of ecliptic longitude, the point directly opposite the origin of the tropical zodiac, and the Sun reaches it at the September equinox. The season ends at two hundred and ten degrees, on 22 or 23 October.
The equinox falls on the 22nd or the 23rd of September according to the year's place in the leap cycle. A tropical year exceeds a calendar year by close to a quarter of a day, so the instant creeps later until February gains a day and resets it.
A cardinal season, and Saturn's exaltation
Cardinal signs open the four quarters, and Libra opens the autumn quarter in the northern hemisphere and the spring quarter in the southern. Through this month the Sun is south of the celestial equator and losing altitude at close to its fastest rate.
Venus rules the sign and Saturn is exalted in it at twenty one degrees, which lands in the middle of October. That pairing of a benefic ruler with a malefic exaltation is unusual, and it puts the sign's strongest structural point three weeks into the range rather than at either end.
The sidereal range and the constellation objection
Two separate points get confused here. The first is that sidereal astrology measures from the stars and so places these births largely in Virgo, the offset between the two zodiacs now standing at about twenty four degrees after two millennia of precession.
The second is that the constellation boundaries astronomers use are irregular and were fixed by agreement in the twentieth century. Neither zodiac claims to match them. Signs are equal thirty degree arcs, and the shared names are historical rather than positional.
Where the range runs out
A birth within roughly forty eight hours of either boundary cannot be settled by a table, because the tables round to whole days and the ingresses do not co-operate. Year, time and place run against an ephemeris give an exact longitude, and it falls in one sign only.
Inside the season the decans divide the month between the Moon, Saturn and Jupiter. Beyond that, a date fixes nothing else in a chart: the Moon moves about half a sign a day and the Ascendant covers all twelve in twenty four hours.
Questions people ask about this
What are the Libra dates?
Approximately 23 September to 22 October. The exact start falls on 22 or 23 September and the end on 22 or 23 October, varying with the year and with the time zone the birth is dated in.
Why do the dates change from year to year?
Because the tropical year is not a whole number of days. The ingress slips about six hours later each year and is pulled back by each leap day, so it oscillates across roughly a day and a half.
When is Libra season this year?
Within the range above, at a minute that has to be calculated for the specific year. Anyone born close to either edge needs the actual solar position rather than a table.
Why does Vedic astrology not make me Libra?
It measures from the fixed stars rather than from the equinox, and the two references have separated by about twenty four degrees. That moves every sidereal boundary later by a little over three weeks.
Does being born on the first day of Libra mean anything special?
It puts the Sun in the first degree of the sign, which matters for aspects and house placement. The tradition does not read it as a weaker or stronger version of the sign itself.