Astride the middle
At noon between 1930 and 2035, the Sun on this date has fallen between about fourteen and a half and fifteen and four fifths degrees of Sagittarius. Fifteen degrees is the halfway mark of the sign, so this birthday sits on it rather than beside it.
The spread comes from the leap year cycle. Each ordinary year the Sun arrives at a given longitude about six hours later against the calendar, and each leap day pulls it back by close to a whole day, so a fixed date oscillates across a band a little over a degree wide instead of drifting away.
Nothing about the sign is affected. Fifteen degrees is as far from either boundary of Sagittarius as it is possible to be, and no hour of birth or time zone brings one within reach.
Why the middle of a sign has a status
Fifteen degrees of the fixed signs, Taurus, Leo, Scorpio and Aquarius, has a name in the tradition. Those points sit close to the old positions of four bright stars and were treated as structurally significant, which is where the fixed cross of much older imagery comes from.
Sagittarius is mutable rather than fixed, so it does not belong to that group, but the halfway degree still does something in ordinary chart work. It is the point where a placement is maximally distant from both ingresses, which the tradition reads as the sign expressed without the qualification that boundary degrees carry.
Whether that qualification is real is a matter of judgement, and it is marked here as interpretation. What is not interpretive is the geometry: a Sun at fifteen degrees is exactly opposite fifteen degrees of Gemini and exactly square the same degree of Virgo and Pisces.
The face and its rival rulers
Fifteen degrees is in the middle of the second decan of Sagittarius, which runs from ten to twenty and has the widest margin of any face here. The Chaldean order assigns it to the Moon; the triplicity scheme assigns it to Aries and therefore to Mars.
Those two are not variations on a theme. The Aries reading describes the sign's opinions turning immediately into positions to defend, with the journey and the argument both starting sooner than planning would advise. The lunar reading describes the same restlessness tied to people and to somewhere familiar to return to.
A description of this face found without an attached scheme could be built on either. That is worth knowing before treating two sources as contradicting each other.
What the date does not settle
A solar degree is one number in a chart. The Moon moves around thirteen degrees a day and can be in either of two signs on 7 December depending on the hour, and the Ascendant runs through all twelve signs in a day, changing by roughly a degree every four minutes.
Since everyone born within about a month shares the sun sign, it is the layer of a chart that says the least about a particular person, however precisely it is stated. The layers that do say something require a birth time.
The widget above computes your own solar degree from your year and birthplace and reports how far you were born from the nearest sign boundary. On this date it will confirm the largest possible distance, and it will also tell you which side of the halfway mark you fell on.
Questions people ask about this
Is 7 December always Sagittarius?
Yes. The Sun is at the halfway point of the sign, which is the furthest it ever gets from a boundary, so there is no year or hour in which the answer changes.
What is special about fifteen degrees?
It is the midpoint of a sign, the position furthest from both ingresses. In the fixed signs it also marks points the tradition treated as structurally important.
Which side of fifteen degrees am I on?
That depends on your year and hour. The noon figure straddles the mark across the last century, and the widget above computes the exact value for your details.
What decan does this birthday fall in?
The second decan of Sagittarius, at its centre. Its ruler is the Moon in the Chaldean order and Mars under the triplicity scheme.
Does being at the midpoint strengthen the sign?
The tradition reads it that way, on the grounds that no boundary is nearby to qualify it. That is an interpretation rather than a measurement.
What aspects does a Sun here make?
A Sun at fifteen degrees of Sagittarius is exactly opposite the same degree of Gemini and exactly square Virgo and Pisces. Whether any of those matter depends on what else is in the chart.