Where the Sun actually is
Taking noon positions from 1930 to 2035, an 11 January birth places the Sun between about 20.1 and 21.5 degrees of Capricorn. The lower edge of that band is a fraction above the twenty degree decan boundary, so a birth in the early hours of a low year comes close to the line without much room to spare.
The narrowness of the band is worth noticing on its own. Over more than a century the position varies by under a degree and a half, because the leap year cycle cancels the six hour annual slip of the calendar against the orbit almost exactly.
That stability is why the decan holds. A birthday sitting a degree above a boundary would be in genuine doubt if the yearly variation ran to several degrees. It does not, so the answer is the same for a birth in the 1930s and one in the 2030s.
The Sun in a Saturn sign
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter through the thirty six faces starting at zero degrees of Aries. Counting through, the third decan of Capricorn falls to the Sun.
The tradition finds this combination interesting because the Sun and Saturn are treated as opposed in temperament, one warm and expressive, the other cold and restraining. A solar decan inside a Saturnine sign is read as brightness working under constraint, or as authority that has been earned rather than assumed. That is the inherited reading and this page marks it as one.
The triplicity scheme reaches a different answer for the same band. Capricorn's third earth decan goes to Virgo, and so to Mercury, which is read instead as care, method and attention to detail inside the same Saturnine framework.
The sign is not in doubt
None of this is a boundary question in the sign sense. Aquarius begins when the Sun completes thirty degrees of Capricorn, around 19 or 20 January. On 11 January the Sun is eight or nine degrees short, which is more than a week of travel, so the sun sign holds in every year and at every hour.
Only the decan is close enough to be worth calculating, and even that is settled in most years by a comfortable margin.
Capricorn itself is cardinal earth, beginning at the winter solstice, and the tradition reads it as structural and patient regardless of which decan a placement falls in. The decan modifies that description rather than replacing it.
What the birthday still cannot tell you
A solar degree and a decan describe one placement out of many. The Moon travels about thirteen degrees a day, so an 11 January birth may have it in either of two signs according to the hour. The rising sign changes roughly every two hours and depends on latitude, so it can be any of the twelve.
The widget above computes your own solar degree from your year and birthplace rather than a noon default, names the decan, and reports how far your Sun sits from the nearest sign boundary.
That is the honest limit of a birthday page. It can give the Sun exactly, name the band it falls in and set out what the tradition attaches to both, and it cannot say anything about the reader beyond that.
Questions people ask about this
What decan is 11 January?
The third decan of Capricorn, twenty to thirty degrees. The Sun sits just past the boundary, so births at the very start of the day in some years are worth checking against the widget above.
Why is the third decan of Capricorn ruled by the Sun?
Because of where it falls in the Chaldean sequence, which runs the seven visible planets in a fixed order through all thirty six decans starting from Aries. The count simply lands the Sun there.
Is that considered a strong placement?
The tradition finds it notable rather than strong, since the Sun and Saturn are read as opposite in nature. The usual description is warmth operating under restraint, which is an interpretation and not a measurement.
What sign is 11 January?
Capricorn, in every year. The Aquarius boundary is more than a week away, so no hour or time zone brings it within reach.
What degree is the Sun on 11 January?
Roughly 20.1 to 21.5 degrees of Capricorn at noon across 1930 to 2035. Your own value shifts within that band according to your birth hour and time zone.
Does the triplicity scheme agree?
No. It gives this band to Virgo and therefore to Mercury. The schemes disagree about most decans, and articles quoting a ruler rarely say which one they have used.