Late in the sign, not near the edge
Capricorn opens when the Sun reaches two hundred and seventy degrees of ecliptic longitude, which is thirty degrees of Sagittarius. Taken at noon between 1930 and 2035, an 18 December Sun sits between about twenty five and a half and twenty seven degrees, so the shortfall is three degrees at the very least.
The Sun covers a degree a day, so that is three or four days of movement. No leap year adjustment comes close to it, since the entire year to year variation on a fixed date is around one degree.
Being late in a sign therefore carries no doubt about the sign. It affects the tone that tradition assigns to the position, and the arithmetic is not in question.
The confusion usually comes from date tables, which quote the end of Sagittarius as a range spanning 21 or 22 December and leave readers to guess how much slack that implies. The slack is hours, not days.
What the closing degrees are read as
Twenty six degrees is inside the third face of Sagittarius, the band from twenty to thirty. Saturn rules it in the Chaldean order, Leo in the triplicity scheme, and Jupiter rules the sign in both.
The last degrees of any sign have their own reputation in the tradition, which describes them as the sign in its most completed and least fresh form. For Sagittarius the language tends toward conviction that has already been argued out, and toward a Jupiter quality operating under Saturn's sense of limit.
None of that is a measurement, and none of it says anything about how a particular life will go. It is a description the tradition attaches to a region of the zodiac, and it changes if you consult a source using the other scheme.
The stability of the figure
The reason a single degree can be quoted for a date at all is the leap year cycle. Because the tropical year runs about a quarter of a day longer than a calendar year, the Sun's noon position on a fixed date advances slightly each ordinary year and drops back when February takes an extra day.
The cycle closes rather than accumulating, so a hundred years of births on this date are spread over a little more than one degree. A long term drift does exist, but the Gregorian rules about century years are there to keep it in hand, and it is invisible at this scale.
The widget above resolves the figure for your own year, hour and birthplace, and reports how far the Sun stood from the nearest sign boundary.
The parts of the chart still missing
A date settles the Sun and nothing else. The Moon travels between twelve and fifteen degrees a day, so on 18 December it may occupy either of two signs depending on the hour of birth.
The Ascendant is entirely a function of the clock and the birthplace, since every sign rises at some point in a day. The house framework is measured from it, so an unknown time leaves not one placement undetermined but a whole layer of them.
That is the ordinary shape of the problem with birthday material. The one placement it can supply is the one shared with the largest number of people, and the placements that distinguish two people born on the same day are exactly the ones a date cannot reach.
Questions people ask about this
Is 18 December Sagittarius or Capricorn?
Sagittarius, without exception. Capricorn begins around 21 December, and on this date the Sun is still three or four degrees short of the boundary.
What degree is the Sun on 18 December?
About twenty six degrees of Sagittarius at noon, with roughly a degree of movement between the earliest and latest years of the past century.
Which decan is 18 December in?
The third decan of Sagittarius. Its Chaldean ruler is Saturn and the triplicity scheme assigns it to Leo, while Jupiter rules the whole sign.
Do the last degrees of a sign mean anything special?
The tradition reads them as the sign in a finished rather than fresh state. That is an interpretation about a region of the zodiac and not a claim about a person.
Could my birth time push me into Capricorn?
No. A few hours moves the Sun by a fraction of a degree, and several degrees separate this date from the Capricorn boundary in every year.
What does the widget above add?
It computes your exact solar degree from your year, hour and birthplace and reports the distance to the nearest sign boundary, which a date range cannot do.