The position, not just the sign
Checked at noon in every year from 1930 to 2035, the Sun on this date stood between roughly twenty one and twenty two and a half degrees of Libra. There is no year in that span where it fell outside those bounds, and no year where the sign was in any doubt. The Scorpio boundary is still a week away and the Virgo boundary is three weeks behind.
So the sign is settled and the degree is nearly as settled. That second figure is the one almost nobody quotes, even though it is more specific and easier to check than the date ranges that vary between sources by a day in either direction.
Why the figure holds still
A common calendar year runs about a quarter of a day short of the orbit, so the Sun reaches any given longitude roughly six hours later by the clock than it did twelve months earlier. On its own that creep would march the date steadily through the sign over a lifetime.
The leap day stops it. Every fourth February gains a date and the whole reckoning jumps back by close to a full day, which undoes the accumulated creep in one step. What survives is a small residual swing of a little over one degree, and the rule that skips the leap day in 1900 and 2100 keeps even that from building up across centuries.
The closing face of Libra
Each sign divides into three faces of ten degrees, and this position sits in the last of them, the band running from twenty to thirty. The Chaldean order of the planets assigns that band to Jupiter. The triplicity scheme takes a different route, associating the band with Gemini and so with Mercury, and either attribution is defensible depending on which tradition a source is following.
This layer is what separates one Libra birthday from another. Someone born in the opening days of the sign and someone born here share a sun sign and very little of the older material written about their positions. A page that names only the sign has thrown that distinction away before it started.
What the tradition reads into the band is a separate matter from where the band is. Libra material centres on weighing, comparison and the settling of a judgement, and the closing face is treated as that work carried through to a conclusion. Those are attributions rather than measurements, and they should be read as such. The division into faces is old, older than most of the Greek writing that carries the surviving interpretation, but age is not evidence and the position is the only part of this that can be checked against an ephemeris.
What a date can and cannot fix
The Sun is the only body a calendar date pins down. The Moon crosses the whole zodiac in under a month, so on this date it can sit in any sign at all depending on the year, and within a single 15 October it can move from one sign to the next in the space of an afternoon. The rising sign turns through all twelve in a day and depends entirely on the hour and the latitude of birth.
That is why the sun sign is the least personal element of a chart rather than the most. It is shared with everyone born in the same month, in any year, anywhere on earth. The widget above takes your year, your hour and your birthplace and returns the exact degree along with how far that placement stood from the nearest sign boundary.
Questions people ask about this
What sign is 15 October?
Libra, without qualification. At noon in every year from 1930 to 2035 the Sun stood well inside the sign, several days clear of the change into Scorpio and weeks past the change out of Virgo.
What degree of Libra is a 15 October birthday?
Between about twenty one and twenty two and a half degrees at noon across that century. Your own figure depends on your year, your birth hour and your birthplace, which is what the widget above works out.
Which decan does this fall in?
The third decan of Libra, the band from twenty to thirty degrees. The Chaldean count gives it to Jupiter, while the triplicity scheme routes it through Gemini to Mercury.
Does the decan change the sun sign?
No. The decan is a subdivision inside the sign, so a position in the third face of Libra is fully Libra. The decan adds detail rather than moving the reading elsewhere.
Can 15 October ever be Scorpio?
Not in the sampled century. The change into Scorpio happens around 23 October and shifts by up to a day and a half between years, which leaves a comfortable margin.
Why does the birth time still matter?
Because the Sun advances about a degree over the course of a day, and because the Moon and the ascendant depend on the hour outright. The date alone leaves both of those open.