A line that the date straddles
Measured at noon, the Sun on 12 September has fallen between about eighteen and four fifths degrees of Virgo and twenty and a fifth over the span from 1930 to 2035. The twenty degree mark sits inside that range, near its upper end.
So the honest statement is conditional. In most years of the last century this birthday belongs to the second decan, and in the years at the high end of the cycle it belongs to the third. Which applies depends on where your birth year sits in the leap cycle, and the hour matters too, since the Sun covers about a degree a day and an evening birth is measurably further along than a morning one. The widget above resolves it rather than guessing.
What lies on either side
The band from ten to twenty degrees is given to Venus in the Chaldean order, and tradition reads it as the sign's carefulness turned towards craft and arrangement. The band from twenty to thirty is given to Mercury in that same order, which doubles the ruler of the sign itself and is usually described as the most characteristically Virgoan stretch of Virgo.
Under the triplicity scheme the pair comes out as Mercury and then Saturn, because the earth signs are taken in order as Taurus, Virgo and Capricorn. So a reader with this birthday can meet four different planet names attached to their decan across a handful of sources, two from the ambiguity of the year and two from the disagreement between schemes.
The sign itself is settled
None of that touches the sun sign. Virgo runs from one hundred and fifty to one hundred and eighty degrees of longitude, and the Sun reaches the far boundary around 22 or 23 September. Twelve September is well clear of it, and the ingress moves by hours between years, not by a week and a half.
That is worth saying plainly, because a page describing something on this date as uncertain could easily be misread as hedging about the sign. It is not. Virgo is fixed here in the way a fortnight of margin makes things fixed. The uncertainty is entirely about a subdivision inside the sign.
Everything the date still leaves open
The Moon will be in one of two signs on any given 12 September, and three in the occasional year where a change of sign lands near the edges of the day. The rising sign is unrestricted, since the whole zodiac passes the horizon every day and each sign holds it for roughly two hours.
Because the rising sign sets the houses, it also decides which part of a chart the Virgo Sun is read as occupying. Two people born on this date twelve hours apart may not even agree about the decan, let alone about the rest. The sun sign is the placement shared with everyone born in a two week window, which is exactly why it carries so little on its own.
The order to work in is therefore the year first, then the hour and the place, and only then the interpretation. Reversing that order is how a conditional answer gets printed as a flat one.
Questions people ask about this
Which decan is 12 September in?
It depends on the year. The noon position straddles the twenty degree line, so most years fall in the second decan of Virgo and the higher years in the third.
Is the sign uncertain on this date too?
No. Virgo is settled by a margin of about a fortnight at both ends. Only the decan, a subdivision inside the sign, is year dependent here.
How do I find out which decan applies to me?
Use the widget above with your birth year, time and place. It computes the exact solar longitude, and the band follows from whether that figure is above or below twenty degrees.
Does the birth hour change the decan on this date?
It can, in the years where the noon figure sits close to twenty degrees. The Sun moves about a degree a day, so a morning and an evening birth are measurably apart.
Who rules the two bands involved?
In the Chaldean order the second decan of Virgo goes to Venus and the third to Mercury. The triplicity scheme names Mercury and Saturn for the same pair.
What does the third decan of Virgo mean in tradition?
It is usually read as the most concentrated form of the sign, because the Chaldean order gives it to Mercury, which already rules Virgo. That is an inherited description rather than a measured result.