Ninety two years, and fourteen that differ
Fourteen exceptions in a run of one hundred and six is a minority no fixed calendar range will mention, and a very significant one if your birth is among them. Those are the years in which the solstice had not yet arrived by noon on 22 December.
The event moves for a mundane reason. A tropical year is longer than a calendar year by close to a quarter of a day, so the instant the Sun reaches two hundred and seventy degrees of ecliptic longitude arrives about six hours later with each year, then jumps back roughly eighteen hours when February gains a twenty ninth day. Follow the solstice across a decade and it travels over about a day and a half. Lately the drift has been pulling it earlier into 21 December, which is why the Sagittarius years on this date are not spread evenly through the period.
Which day the solstice belongs to depends on where you are
A solstice is one instant for the whole planet. What changes from place to place is the date and time written next to it. A solstice at 22:00 UTC on 21 December is five in the afternoon on the twenty first in New York and seven the following morning in Tokyo, so a Japanese birth on the morning of the twenty second is Capricorn while an American birth at the same absolute moment is on the twenty first.
This is the ordinary route to a wrongly stated sun sign. A date is read off a certificate and looked up in a table that assumed one time zone and was never adjusted for the reader's. Once the birthplace is in hand the ambiguity has nowhere to go. The widget above works from your year, hour and place, and it reports how many hours from the solstice your birth actually fell.
The cusp is a missing fact
People born on this date are commonly assigned to a Sagittarius Capricorn cusp and handed a description mixing optimism with discipline. The Sun cannot deliver a mixture. It holds one ecliptic longitude at any instant, either below two hundred and seventy degrees or above, so a birth two hours before the solstice is wholly Sagittarius and one two hours after is wholly Capricorn.
The word is doing administrative work rather than astrological work. It records that the year, the hour or the place was never established, and it becomes empty as soon as they are. Being minutes from the change makes the resulting placement no less complete.
Jupiter beside Saturn
Sagittarius is mutable fire under Jupiter; Capricorn is cardinal earth under Saturn. No ruler is shared, but the pair is governed by the two outermost planets visible to the naked eye, and the tradition has always treated the step from one to the other as the sharpest change of tone in the wheel, from expansion to limit.
The received descriptions still push it too far. Both signs are read as long range in their thinking and unusually interested in what a life adds up to, one through belief and the other through record. Both are also described as poor at pretending to agree with something they do not.
If the answer is not the one you expected, it changes a label rather than a chart. The Sun is one placement, and the Moon, the Ascendant, the planets and the house divisions are unaffected by which side of the solstice your birth landed on.
Questions people ask about this
Is 22 December Sagittarius or Capricorn?
Usually Capricorn. At noon between 1930 and 2035 the date gave Capricorn in ninety two years and Sagittarius in fourteen, so about one birthday in eight on this day is Sagittarius.
Is the December solstice the start of Capricorn?
Yes, they are the same moment. The Sun reaching two hundred and seventy degrees of ecliptic longitude is both the solstice and the ingress into Capricorn.
Which years on this date come out Sagittarius?
Mainly the earlier ones in the period, because the solstice has been drifting back into 21 December. Rather than reading a trend, the widget above computes the position for the year you enter.
Am I on the Sagittarius Capricorn cusp?
No. A cusp is not something the Sun can occupy. It sits at one longitude, within one sign, at every instant, and the term describes information that was never supplied.
Does my birth hour matter?
It matters in the years where the solstice falls during 22 December itself. In those years a birth in the morning and one in the evening on the same date give different signs.
Is a solstice birth read as significant?
The tradition treats a Sun at zero degrees of a cardinal sign as strongly placed, not as fortunate or unfortunate. What would change that reading is the rest of the chart, not the date itself.