Clear of the first boundary
At noon between 1930 and 2035, the Sun on this date has been between about ten and two fifths and eleven and four fifths degrees of Sagittarius. The first face of the sign ends at ten degrees, so unlike the birthday immediately before it, this one is on one side of that line in every year checked.
The margin is not enormous. At the low end of the range a birth in the small hours of the morning comes back within half a degree of the boundary, so the position is worth calculating rather than assuming. It has still never crossed back at noon in the record.
The sign itself is not remotely in question. Eleven degrees is a third of the way through Sagittarius, with the nearest boundary in either direction more than a week away.
Why the Moon and Mars both have a claim
The Chaldean order assigns the thirty six faces in a fixed planetary sequence, Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, repeating from the start of Aries. Counted to the twenty sixth face, the middle decan of Sagittarius comes out as the Moon.
The triplicity scheme works by element instead. The fire decans run Aries, Leo and Sagittarius in order, and a sign's own faces begin with itself, so the middle face of Sagittarius belongs to Aries and therefore to Mars.
The Moon and Mars are about as far apart as the traditional planets get. This is consequently one of the faces where an unattributed description is least reliable, because the two schemes are not offering different emphases on one idea; they are offering different ideas.
The two readings
Through Aries, the tradition describes the Sagittarian interest in how things fit together turning into something acted on at once. The view becomes a position, the position gets defended, and both the argument and the journey start earlier than planning would advise.
Through the Moon, the same degrees are read almost in reverse. The Moon is taken to add sensitivity to the mood of a room and a need for familiar ground to come back to, which older sources set deliberately against the sign's restlessness. On that reading the travelling attaches to people rather than to places.
Both are interpretation and neither is a measurement. The measurable part is the degree, from which the face follows by division, and this page keeps the two apart on purpose.
What a birthday still cannot give you
Decans apply to every point in a chart, not only the Sun. An Ascendant at seventeen degrees of Sagittarius takes the same face, and since the rising degree changes by roughly a degree every four minutes, that is where the divisions separate two charts most sharply.
Which is the limit of what a date can do. The Ascendant needs a birth time, and the Moon, moving about thirteen degrees a day, can be in either of two signs on this date depending on the hour. Everyone born within roughly a month shares the sun sign, so it is the least individual thing in the chart even when it is stated precisely.
The widget above computes your solar degree from your own year and birthplace and tells you how far the position sits from the nearest sign boundary. On this date the answer is comfortable, and the degree it returns is what fixes the face.
Questions people ask about this
What decan is 3 December?
The second decan of Sagittarius, from ten to twenty degrees. The noon Sun has been past the ten degree mark in every year in the record, though not by a large margin at the low end.
Who rules the second decan of Sagittarius?
The Moon under the Chaldean order and Mars, through Aries, under the triplicity scheme. The two schemes disagree more here than almost anywhere else in the zodiac.
Could a 3 December birth be in the first decan?
It is possible in principle for a birth in the earliest hours of a year at the low end of the range, which is why the position is worth calculating rather than assumed.
Is the sun sign ever in doubt on this date?
No. Eleven degrees is a third of the way into Sagittarius and no calendar drift or time zone comes close to moving that.
What is a decan for?
It subdivides a sign into ten degree faces, each with a traditional ruler, so that two people with the same sun sign can be told apart. It shades a reading rather than replacing the sign.
Does the decan matter more than the sign?
No. It is a refinement laid over the sign, and a face that seems to contradict the sign should not be allowed to outrank it.