The September equinox sets this boundary
A hundred and eighty degrees of ecliptic longitude is the point opposite the one the whole tropical zodiac is measured from. The Sun arrives there at the September equinox, crossing the celestial equator southward, and Libra begins at that instant by definition.
Only four sign boundaries are fixed to an astronomical event of that kind, and this is one of them. The date is 22 or 23 September and moves for the ordinary reason: the Sun's circuit runs about six hours longer than a calendar year and the surplus is cleared out each leap day.
Mutable earth handing over to cardinal air
Mercury's evening sign gives way to Venus's day sign here, and a mutable season ends as a cardinal one begins. Cardinal signs open the four quarters of the year, so Libra does not merely follow Virgo; it starts a new quarter, which the older texts treat as a meaningful position in itself.
The idea that a birth near the equinox belongs to both signs at once has no support in any of that structure. The Sun's longitude is one value, and one value cannot straddle two arcs.
What a cusp is, properly
House cusps are the boundaries of the twelve houses, generated from the horizon and the meridian of the birthplace. They depend on latitude and on the minute of birth, and near the poles they become extremely uneven. Signs are thirty degree divisions of the ecliptic with none of that dependence.
Someone born on 23 September may well have a house cusp worth attention. It will not be this line.
Reading the two sides
Late Virgo remains Virgo entirely, and its closing decan belongs to Venus, which softens the band in a way texts describe without ever borrowing from Libra to explain it. Mercury is exalted in Virgo at fifteen degrees, so a late Virgo Sun is past the exaltation point rather than approaching it.
Early Libra sits under the Moon in the Chaldean decans, and Saturn is exalted at twenty one degrees of Libra, well past the boundary. The most distinctive degrees of Libra are nowhere near its opening.
Establishing a birth on 22 or 23 September
Find the equinox instant for the birth year, translate it into the local time where the birth happened, and compare the two. The comparison settles the sign completely.
Because the instant is global and the calendar date is local, the same equinox is the evening of one date in some countries and the morning of the next in others. That accounts for most of the disagreement between published tables.
Questions people ask about this
Am I a Virgo Libra cusp?
No, because that placement does not exist. The Sun was either short of a hundred and eighty degrees at your birth or past it, which makes you Virgo or Libra.
What date does the Sun move from Virgo to Libra?
22 or 23 September, depending on the year and on the time zone the birth is dated in. The instant is the same worldwide; only the local calendar date differs.
Do I have traits of both signs?
Popular astrology says so and sells the blend as a type. The chart does not show it: one Sun, one longitude, one sign. Other placements can of course sit in either sign.
What is a cusp in real astrology?
A house boundary. Houses are divided using the horizon and the meridian of the birthplace, and their cusps are genuine dividing lines within a chart.
How close to the boundary do I need to be to bother checking?
About forty eight hours either side. Inside that window the year and the birth time decide the answer; outside it, any date range gives the same result.