Why this date has two possible faces
Faces are ten degree bands, so the second decan of Sagittarius runs to twenty degrees and the third starts there. Taken at noon across the years from 1930 to 2035, a 12 December Sun falls between about nineteen and a half and just under twenty one degrees.
That range crosses the line. In some years the noon position is still under twenty and the face is the second one, and in others it has passed twenty and the face is the third. The hour of birth moves it further, since the Sun advances a degree a day and roughly two and a half minutes of arc an hour.
So the honest answer for this date is that the decan depends on the year and the time, and there is no single figure that covers everybody born on it.
What changes when the boundary is crossed
The sign does not change. Sagittarius is settled either way, and Jupiter rules the whole span. What changes is the face ruler in the Chaldean order, which is the Moon below twenty degrees and Saturn above it.
Those two are read very differently. A lunar face is taken to turn a Jupiter sign inward, toward habit and private conclusion. A Saturn face is read as the same sign brought up against limits, structure and the passage of time, and it is often described as the most sober stretch of Sagittarius.
Under the triplicity scheme the pairing is Aries below the line and Leo above it, which is a different contrast again. Whichever scheme is used, the two answers are not interchangeable, so the boundary is worth resolving rather than glossing.
How to settle it for yourself
The calculation needs three things: the year, the hour and the birthplace. The year sets the base position, the hour adds the fraction of a degree the Sun has covered since midnight, and the birthplace supplies the time zone that turns a local clock reading into a universal one.
Time zone is where most published sun sign answers go wrong. A single instant is one hour in London and a different calendar hour in Sydney, so an uncorrected local time can shift a borderline result to the wrong side.
The widget above does that correction and returns the degree along with the distance to the nearest boundary. On a date like this one, the distance figure is the useful part, because it tells you whether the answer is comfortable or marginal.
The rest of the chart, as ever
Resolving the face settles a detail of one placement. The Moon on this date may sit in either of two signs depending on the hour, and the rising sign can be any of the twelve since the whole zodiac crosses the horizon daily.
That comparison is worth keeping in view. The sun sign is shared with everyone born across four weeks, the face with everyone born across ten days, and the Ascendant with a much smaller group born nearby at nearly the same moment.
None of which makes the solar degree unimportant. It decides which transits contact your Sun and by what margin, and on this date it also decides which of two traditional rulers applies to you.
Questions people ask about this
Which decan is 12 December?
It depends on the year and the hour. The Sun sits near twenty degrees of Sagittarius, which is the exact line between the second face and the third, so both answers occur.
Is 12 December Sagittarius or Capricorn?
Sagittarius, with no ambiguity. The sign boundary is around 21 December and only the internal face boundary is in question on this date.
What rules the face on either side of the line?
Below twenty degrees the Chaldean ruler is the Moon and above it Saturn. The triplicity scheme gives Aries below the line and Leo above it.
How do I find out which side I am on?
Enter your year, hour and birthplace in the widget above. It returns the exact solar degree, which is the only thing that decides the question.
Does a birth time matter here?
For the face, yes, if the year already places you close to twenty degrees. For the sign it makes no difference at all, since the sign is settled in every case.
Does being near a face boundary weaken the reading?
No. A position at twenty degrees is fully inside the third decan in the same way a position at twenty five is. Nearness to a line affects certainty about the data, not the placement itself.