The settled part, stated once
The Sun crosses into Capricorn at the December solstice, which lands on 21 or 22 December in the modern calendar and only rarely strays. By the twenty third it is past, in every year from 1930 to 2035 and comfortably so. Nobody born on this date needs to consult a table to learn their sun sign.
What the table does add is the position. At noon the Sun sits between roughly zero point seven and two point one degrees of Capricorn, which places this birthday in the opening stretch of the sign and in the first decan, the band from zero to ten degrees. In the Chaldean assignment that decan belongs to Jupiter, so the tradition describes Saturn's sign with its most expansive and least austere third.
The spread across that whole century is a little over a degree, which is small enough to be worth explaining. The calendar runs slightly fast against the orbit and slips back a fraction each ordinary year, and the leap day restores it in one step, so the two effects nearly cancel and the position on a fixed date stays put.
The Moon is genuinely uncertain
The Moon travels through a sign in about two and a quarter days, which means that on any given 23 December it occupies one of two signs and occasionally clips a third. Two people born on this date twelve hours apart can hold different lunar signs, and in traditional practice that difference is read as a difference in temperament, appetite and habit rather than a subtlety.
No page keyed to a calendar date can resolve this, and any page that tells you what the Moon does for people born on 23 December is describing one particular year and quietly generalising. The honest answer is that the date narrows the Moon to a pair of candidates and the birth hour picks between them.
The rising sign is not narrowed at all
The Ascendant is the degree of the ecliptic climbing over the eastern horizon at the moment of birth, and the Earth's rotation carries the whole zodiac past that point once every twenty four hours. A birth on 23 December can therefore rise in any of the twelve signs, with no bias from the date whatsoever.
Latitude complicates it further. The signs do not rise at even speeds, and the further from the equator the birthplace, the more unequal the intervals become, so the same clock time gives different Ascendants in Edinburgh and in Nairobi. If you know your birth time to within a few minutes this is recoverable. If you do not, it is the one part of a chart worth the effort of finding out.
What to take from the date, then
A birthday tells you the Sun's sign with certainty, its degree to within a fraction, and its decan. That is a real amount of information and more than most descriptions of this date supply. It is also, in a whole chart, one placement among ten or so, and the tradition has never treated it as the whole picture.
The reason the Sun gets all the attention is that it is the only placement a date alone can reach. That is a fact about the convenience of the lookup rather than about the importance of the placement, and it is worth keeping the two apart.
The widget above computes the Sun's exact longitude for your year, hour and birthplace, along with how far you were born from the Sagittarius boundary behind you. Feed it a birth time and it can go on to the placements this page has deliberately refused to guess at.
Questions people ask about this
Is 23 December Capricorn or Sagittarius?
Capricorn, in every year on record. The solstice ingress happens on 21 or 22 December in the modern calendar, so by the twenty third the Sun has already moved on and the date carries no ambiguity.
How far into Capricorn is the Sun on 23 December?
Between about zero point seven and two point one degrees at noon, taking the years from 1930 to 2035. That is the very start of the sign, well inside the first decan.
Which Moon sign goes with this birthday?
There is no single answer. The Moon changes sign every two and a bit days, so any 23 December offers two possibilities and the birth hour decides. The widget above can settle it once you supply a time.
Can I find my rising sign without a birth time?
Not exactly, but you can narrow it. The rising sign turns through all twelve in a day, so an approximate hour gives an approximate answer and a known hospital or city helps more than people expect.
Does being born just after the solstice mean anything?
The tradition reads the opening degrees of a sign as its most undiluted expression, but it does not attach fortune to them. Nothing about the position is favourable or unfavourable in itself.
Which decan ruler applies here?
Jupiter, under the Chaldean order, for the first ten degrees of Capricorn. Saturn still rules the sign as a whole, so the reading is usually described as Saturn's structure with a broader and more optimistic cast.