Past the line and staying there
Taken at noon between 1930 and 2035, a 13 December Sun runs from about twenty and a half to just under twenty two degrees of Sagittarius. The lowest figure in that span is still above twenty, which is where the third face begins, so the answer holds for the whole period.
The margin is not large. Roughly half a degree separates the earliest years from the boundary, which is about twelve hours of solar movement. It is enough, but only because the leap cycle keeps the yearly wobble to around one degree in total rather than letting it accumulate.
The immediately preceding date does not have this luxury. There the same wobble straddles twenty degrees, and the face genuinely varies by year, which is an unusual thing for a birthday page to have to admit.
Here no such admission is needed. The band can be stated once and it holds for everyone born on this date, whatever the year and whatever the hour.
Jupiter's sign under Saturn's face
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter and repeats from the start of Aries. Counting through the thirty six faces brings Saturn to the last ten degrees of Sagittarius, while Jupiter continues as ruler of the sign as a whole.
Older texts treat this as one of the more interesting mixtures, because the two planets are read as opposites in temperament. Jupiter is associated with widening and Saturn with narrowing, so a Saturn face inside a Jupiter sign is described as ambition tested against constraint, and as a Sagittarian outlook that has learned what it costs.
The triplicity scheme reaches a different pairing, giving this band to Leo and reading it as the more theatrical stretch. Neither reading is a measurement, and both are presented here as tradition rather than fact.
What the degree is for
A degree is not decoration. Aspects between two bodies are defined by the angle between them and allowed a small orb, so whether a transiting planet contacts your Sun at all is decided at this resolution. So is which house the Sun falls in when a chart is drawn.
A sign gives none of that. It covers thirty degrees and a month of births, which is why any statement built on the sign alone applies equally to somebody born three weeks earlier.
The widget above takes your year, hour and birthplace and returns the position exactly, along with the distance to the nearest sign boundary. On this date that distance is comfortable in both directions, so the result is a confirmation rather than a resolution.
Where the date runs out of information
The Sun is the only body a date fixes. The Moon moves twelve to fifteen degrees a day and may occupy either of two signs on 13 December depending on the hour, and it is read as the seat of need and habit rather than intention.
The Ascendant is free of the date entirely. All twelve signs rise in the course of a day, so the horizon and the house framework that hangs off it are determined by the clock and the birthplace alone.
Anybody comparing two people born on this date will find the Sun and its face identical and much of the rest unlike. That is the ordinary situation, and it is why the tradition never treated a birthday as a chart.
Questions people ask about this
What decan is 13 December?
The third decan of Sagittarius, from twenty to thirty degrees. The Sun is past twenty in every year on record for this date, so the answer does not depend on the year.
Who rules the last decan of Sagittarius?
Saturn in the Chaldean order and Leo in the triplicity scheme. Jupiter remains ruler of the sign itself under both.
What degree is the Sun on 13 December?
Around twenty one degrees of Sagittarius at noon, ranging from roughly twenty and a half to just under twenty two across the last century.
Is this date close to Capricorn?
Not close enough to matter. Capricorn begins near thirty degrees of the zodiac band, around 21 December, which is more than a week of solar movement away.
How is this date different from 12 December?
By about a degree, which happens to be the difference between a settled face and an unsettled one. The earlier date can fall either side of twenty degrees depending on the year.
Does the face change my sun sign reading?
It refines it rather than replacing it. The sign supplies the main description and the face is read as a modifier over the top of it.