An equinox that will not sit still
The Sun entering Libra at one hundred and eighty degrees of ecliptic longitude is the same event as the autumn equinox in the northern hemisphere. The equinox is widely announced each year, and it lands on 22, 23 or occasionally 24 September, which is a fair warning that the sign boundary does the same.
The cause is arithmetic. A tropical year exceeds the calendar year by close to a quarter of a day, so the equinox arrives about six hours later with each passing year and then jumps back by roughly eighteen hours when February takes on a twenty ninth day. Across a decade the moment shifts over about a day and a half. In recent decades the drift has been carrying it steadily earlier into the twenty second, which is why the Virgo minority on this date is concentrated rather than spread evenly through the century.
One instant, many local dates
An equinox is a single moment for everyone alive, and the only thing that varies is the time on the wall. If it falls at 01:00 UTC on 23 September, that is nine in the evening on the twenty second in Buenos Aires and mid morning on the twenty third in Sydney. A birth on the evening of the twenty second in South America can therefore be Libra while a birth at the same absolute instant in Britain is still Virgo.
This is where most misreported sun signs come from. A date is taken from a certificate and matched to a range that was calculated for somebody else's meridian, and the birthplace is never brought into it. The widget above uses your year, your hour and your place, and it also states how many hours from the equinox the birth actually was. That figure is what tells you whether the answer is comfortable or borderline.
The blend that is not there
Descriptions of a Virgo Libra cusp tend to promise carefulness paired with charm. The Sun is incapable of supplying that. At any instant it has one ecliptic longitude, either below one hundred and eighty degrees or above, and a birth two hours before the equinox is wholly Virgo while one two hours after is wholly Libra.
The honest use of the word cusp is as a marker of an incomplete record. It records that year, hour or place were not available to whoever gave the reading, and it ceases to apply the moment they are. Being close to the change makes a placement no less definite.
Mercury against Venus
Virgo is mutable earth ruled by Mercury; Libra is cardinal air ruled by Venus. There is no ruler in common, and the classical readings pull in different directions, one towards analysis and correction, the other towards proportion and agreement.
The popular sketches still overstate the distance. Both signs are read as strongly concerned with what is due and with getting the arrangement right, one at the level of the work and the other at the level of the people, and both are described as reluctant to commit until they have weighed the case. Neither is a comfortable fit with the idea of acting first and reviewing later.
If the answer comes back as the sign you were not expecting, the change is narrower than it feels. The Sun is one placement out of many, and the Moon, the Ascendant, the planets and the houses stand where they stood regardless of which side of the equinox your birth fell on.
Questions people ask about this
Is 23 September Virgo or Libra?
Libra in most years and Virgo in a substantial minority. At noon from 1930 to 2035 the tally was seventy nine Libra to twenty seven Virgo, so roughly a quarter of birthdays on this date are Virgo.
Is the equinox the same thing as the start of Libra?
Yes. The Sun reaching one hundred and eighty degrees of ecliptic longitude is both the September equinox and the ingress into Libra. They are one event with two names.
Which years on this date are Virgo?
Mostly the later ones, because the equinox has been drifting earlier into 22 September. The widget above computes your specific year rather than leaving you to guess from a trend.
Am I on the Virgo Libra cusp?
No. The Sun holds a single longitude at any instant and that longitude lies inside one sign. Cusp is a description of missing information, not of a mixed placement.
Does my birth time matter here?
It does in years where the equinox falls during 23 September itself. In those years a birth in the morning and one in the evening on the same date give opposite answers.
Does an equinox birth mean anything special?
The tradition reads it as the Sun at zero degrees of a cardinal sign, which is treated as a strong placement rather than a fortunate or unfortunate one. Nothing in the technique treats it as luck.