Two hundred and ten degrees, on 22 or 23 October
Venus's air sign ends and Mars's water sign begins at two hundred and ten degrees of ecliptic longitude, a point the Sun reaches on 22 or 23 October depending on the year. The two rulers are the traditional pair, the lesser benefic and the lesser malefic, which makes this the sharpest contrast of rulership on the wheel.
The wobble in the date has one cause: the Sun takes about a quarter of a day more than the calendar year to come back to any given longitude, so the crossing slides later for three years and is pulled back by the leap day.
Where the constellation argument comes in
These dates attract the Ophiuchus objection more than any others, since the constellation boundaries drawn by astronomers put the Sun in Scorpius for only about a week and in Ophiuchus for rather longer. The objection is sound about the constellations and beside the point about the signs.
Signs are equal thirty degree arcs measured from the equinox. Constellations are irregular patches of sky of very different sizes, and their modern borders were fixed by committee in the nineteen twenties. A sign named after a constellation is not a claim to be sitting in it.
No blended placement
The Sun has one longitude at a time. A moment before the crossing it is wholly in Libra, a moment after wholly in Scorpio, and neither the classical dignities nor any modern technique defines something in between. In astrology the word cusp names a house boundary — the line between one house and the next, drawn from the birthplace's horizon and meridian — and those boundaries have nothing to do with the thirty degree divisions of the ecliptic.
Late Libra, early Scorpio
The last decan of Libra belongs to Jupiter, the broadest band of the sign and the one least concerned with keeping the peace at close range. Saturn's exaltation at twenty one degrees of Libra sits inside that stretch, so a late Libra Sun is past the sign's strongest structural point.
Scorpio then opens under Mars itself, which makes the earliest Scorpio degrees the most undiluted expression of the sign rather than a residue of the one before. Anyone reading the boundary as a fade has the decans backwards.
Checking a birth against the ingress
Year, clock time, place. Those three run against an ephemeris give the solar longitude to a fraction of a degree, and it is short of two hundred and ten degrees or past it.
Inside roughly two days of the mark no printed range can be trusted, since ranges are rounded and the crossing is not. Outside that window every table agrees and the calculation is unnecessary.
Questions people ask about this
Am I a Libra Scorpio cusp?
No, because that placement does not exist. The Sun was either short of two hundred and ten degrees at your birth or past it, which makes you Libra or Scorpio.
What date does the Sun move from Libra to Scorpio?
22 or 23 October, 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.