The one thing the date does fix
The Sun on 15 September has stood between roughly twenty one and seven tenths degrees of Virgo and twenty three and a tenth at noon, taking every year from 1930 to 2035. That is the whole range in more than a century, and the reason it is so tight is the leap year correction, which keeps hauling the date back into step with the orbit.
From that follow two settled facts. The sign is Virgo, more than a week clear of the Libra boundary. The decan is the third, the band from twenty to thirty degrees, which the Chaldean order gives to Mercury and the triplicity scheme gives to Saturn. Both are positional statements, checkable against an ephemeris, and neither depends on the hour of birth.
The Moon has to be calculated
The Moon travels about thirteen degrees a day and spends around two and a half days in a sign. On any particular 15 September it will therefore be in one sign for part of the day and the next for the rest, and in some years a third comes into play once time zones are taken into account.
That single placement usually carries more of a traditional reading than the Sun does, because it is read as the responsive and habitual layer rather than the public one. The two candidates in a given year are often in different elements, which means the two possible readings are not variations of each other. There is no way to narrow it from the date, and any page that assigns a Moon sign to a birthday is either guessing or describing one particular year.
The rising sign has twelve candidates
The eastern horizon sweeps through the whole zodiac once a day, so each sign rises for roughly two hours somewhere in the twenty four. Every one of the twelve is available on this date, and only the clock time and the latitude of the birthplace decide.
Because the rising sign fixes the first house, it fixes all of them, and with them the area of life each planet is read as belonging to. The same late Virgo Sun can sit at the top of one chart and at the bottom of another born the same evening. If the birth time has been lost, the position can sometimes be narrowed from known events, but a date will never do it.
Why the sun sign is the small part
Roughly one person in twelve shares your sun sign, and everyone born in the same fortnight shares it exactly, decan included. That makes it the least distinguishing element of a chart, even though it is the only one most descriptions of a birthday mention.
The widget above takes your birth year, hour and place and returns the Sun's longitude for that moment rather than a range, together with the distance to the nearest sign boundary, which on this date will be comfortable. That figure is the honest limit of what a birthday can tell you on its own, and it is still more than a table of date ranges can offer. Everything past it needs details that only you hold.
Questions people ask about this
What sign is 15 September?
Virgo, with no qualification needed. The Sun does not reach Libra until around 22 or 23 September, and that boundary shifts only by hours between years.
Which decan does this birthday fall in?
The third decan of Virgo, twenty to thirty degrees. Chaldean rulership gives it to Mercury, the ruler of the sign, while the triplicity scheme names Saturn.
Can I work out my Moon sign from 15 September?
No. The Moon changes sign every two and a half days, so this date allows two possibilities in most years. The birth time is what settles it.
Is my rising sign related to my birthday?
Not at all. All twelve signs rise on every date, each for about two hours. Only the clock time and the birthplace decide which one was on the horizon.
What degree of Virgo is 15 September?
Between about twenty one and seven tenths and twenty three and a tenth degrees at noon over the last century. The widget above narrows that to one figure.
Why do birthday pages only mention the sun sign?
Because it is the only placement a date fixes on its own. Everything else needs the hour and the birthplace, which a page written for a whole date cannot assume.