The band and where this date falls in it
Capricorn runs from zero to thirty degrees of longitude and splits into three ten degree decans. Measured at noon over the years from 1930 to 2035, the Sun on 27 December holds between about four point eight and six point two degrees, which places it in the middle of the first band with room on either side.
That means the answer is the same whatever year you were born in. There is no boundary nearby for the century's slow drift to carry the position across, so the first decan applies to every 27 December birth without a caveat attached. Dates a few days later in the calendar do not have that luxury, and naming a band for them without asking the birth year gives some readers the wrong one.
The Chaldean ruler and what it adds
Decan rulers come from an old planetary sequence applied in order across all thirty six bands, and the first decan of Capricorn draws Jupiter. Saturn rules Capricorn itself, so a reading of this position has two planets in it rather than one, and traditional sources describe the combination rather than either alone.
The usual description sets Saturn's caution and long time horizon against Jupiter's appetite for scale, and reads the result as ambition that wants a large frame to work inside. That is tradition speaking, not measurement, and it is offered here as what the sources say rather than as a claim about anyone born on the date. Nothing in the position is favourable or unfavourable in itself, and sources that treat it that way are adding something the arithmetic does not contain.
Why almost nothing you read mentions this
Popular sign writing works from date ranges. A date range gets you to a sign and no further, because the ranges are built from ingress dates rather than from positions. Once a page has produced a sign name it has used up everything its method can supply, so it fills the remaining space with adjectives.
Decans need a degree, and a degree needs a calculation. It is not a difficult one, but it is a step beyond looking up a table, and that step is where most descriptions of a birthday stop. The result is that a genuinely distinguishing layer of the tradition goes unmentioned on nearly every page about this date, and readers are left with a description that fits a whole month of birthdays equally well.
How far the decan gets you, and no further
Sun, degree and decan together are three real facts about a 27 December birth, and they are the same three for everyone born on it. That is the limit of what a date can give. The Moon changes sign roughly every two and a quarter days, so this date narrows it to a pair and the hour picks one. The rising sign is not narrowed at all, since the whole zodiac crosses the horizon in a day.
The widget above computes your own solar longitude from your year, hour and birthplace, and reports how far that put you from the nearest boundary. From there the useful next step is a birth time, because it is the only thing that opens up the placements a calendar date can never reach.
Questions people ask about this
What decan is 27 December?
The first decan of Capricorn, the band from zero to ten degrees. The Sun on this date sits near five degrees, safely inside it in every year from 1930 to 2035.
Which planet rules that decan?
Jupiter, in the Chaldean sequence of decan rulers. Saturn continues to rule Capricorn as a whole, so both appear in a traditional reading of a Sun in this band.
Is 27 December always Capricorn?
Yes. The solstice ingress falls several days earlier, on the twenty first or twenty second, so the Sun is already well inside the sign by this date in every modern year.
Does knowing my decan replace knowing my chart?
No. It refines one placement. The Moon, the Ascendant and the planets are untouched by it, and in practice they carry more of a chart reading than the Sun's decan does.
Why do decans have their own rulers?
The scheme comes from an old sequence of the seven visible planets applied in order across the thirty six bands starting at Aries. It is arithmetic rather than observation, and it has been used in this form for a very long time.
Should I read the first decan description instead of Capricorn?
Read both. The sign is the wider frame and the decan is the qualification inside it, and traditional sources have always used them together rather than choosing between them.