A degree instead of a band
Naming a sun sign locates the Sun somewhere within thirty degrees. Naming the degree locates it within one, and on 15 December the noon position across the years from 1930 to 2035 runs from about twenty two and a half to twenty four degrees of Sagittarius.
Nothing exotic keeps that range narrow. The calendar year is close to the orbital year but not equal to it, so the position advances by a fraction of a degree each ordinary year and is pulled back when February gains a day. The cycle repeats every four years and returns to nearly the same place.
The consequence is that the figure quoted here is close to right for anybody born on this date in living memory, without needing to know which year.
Where twenty three degrees falls
Faces divide a sign into three parts of ten degrees, so twenty three degrees is in the third face of Sagittarius, roughly at its midpoint. The Chaldean order gives that band to Saturn, and the triplicity scheme gives it to Leo, while Jupiter rules the sign in both.
Sitting at the middle of a face is worth noting for a plain reason: it is as far from either boundary as a position can be. Neither the year of birth nor the hour can move a 15 December Sun into a neighbouring band, so the face answer needs no calculation to be safe.
Dates a few days earlier do not share that comfort, and their face has to be worked out year by year. The difference between a settled band and an unsettled one can be as little as a degree of solar movement, which is why a calculated position is worth more than a table of date ranges.
How the tradition reads this stretch
The final third of Sagittarius is usually described as the sign after its first enthusiasm, with the Saturn face read as conviction that has been through something. Older writers associate it with endurance, with a preference for the long route, and with the point where a broad outlook is asked to hold up in practice.
Under the Leo pairing from the other scheme the same band comes out differently, weighted toward performance and visible confidence. The schemes have coexisted for centuries and neither has displaced the other.
What the older material almost never does is attach a temperament to a calendar date. It attaches readings to positions, and the date is only a convenient way of arriving at one.
Both are interpretation. Nothing here predicts anything about a reader, and the position itself is the only part of the page that can be checked against an ephemeris.
The wide open half of the chart
A date settles the Sun and leaves the fast moving parts alone. The Moon covers between twelve and fifteen degrees a day, so on 15 December it may occupy either of two signs depending on the hour of birth, and occasionally a third.
The Ascendant is decided by the clock and the birthplace rather than the date, since the whole zodiac crosses the horizon in a day. Because the houses are counted from it, the area of life every planet is read into moves with the birth time as well.
The widget above will compute the exact solar degree from your year, hour and birthplace and report the distance to the nearest sign boundary. Anything beyond the Sun needs a full chart rather than a date.
Questions people ask about this
What sign is 15 December?
Sagittarius, settled in every year. The sign runs until around 21 December, so this date sits well inside it with no boundary question.
What degree of Sagittarius is 15 December?
About twenty three degrees at noon, with about a degree and a half of variation between one year and another.
Which decan is this date in?
The third decan of Sagittarius, near its midpoint. Saturn rules the band in the Chaldean order and Leo does in the triplicity scheme.
Could the year of birth change the decan?
Not on this date. The position is far enough from both edges of the band that no year and no hour of birth can carry it across.
Is the Sun the most important part of a chart?
The tradition does not say so. It is simply the placement a date can supply on its own, which is why it dominates published material.
What does the third decan mean for a Sagittarius Sun?
It is read as the sign under Saturn, meaning breadth tempered by limit and duration. That is a traditional reading rather than a measured property.