The problem with a sign as an answer
A sun sign is a thirty degree band the Sun takes about a month to cross. Naming it tells you what you have in common with everyone born in that month and nothing about what separates you from them. That is why descriptions of this date and of one three weeks earlier tend to be indistinguishable.
The tradition itself has always worked at finer resolution. Ancient practice divided each sign into ten degree faces, and before that Egyptian star lists divided the whole circle into thirty six of them. Degrees, faces and terms were part of the ordinary vocabulary long before sun sign columns existed.
None of that makes the sign wrong. It makes it coarse, in the way a country is a correct but unhelpful answer to where somebody lives.
What the position actually is
Taken at noon and measured across every year from 1930 to 2035, a 9 December Sun falls between about sixteen and a half and just under eighteen degrees of Sagittarius. The variation is a little over one degree, which is smaller than most people expect from a calendar that adds a day every fourth February.
The reason is that the leap rule is built to cancel exactly the drift it would otherwise create. Each ordinary year the noon position advances by roughly a quarter of a degree, and the extra February day pulls it back again. The pattern repeats rather than accumulating.
The practical upshot is that the degree given here is close to right for anyone born on this date in living memory, and the widget above refines it for your particular year, hour and birthplace.
The face inside the sign
Seventeen degrees puts the Sun in the second decan of Sagittarius, which covers ten to twenty degrees. Under the Chaldean sequence of rulers that face belongs to the Moon, while Jupiter continues to rule the sign around it.
Traditionally this pairing is read as the reflective part of a sign otherwise described as outward and declarative. The Moon governs habit, memory and the domestic, so a Jupiter sign coloured by it is taken to want the same horizons but to process them quietly rather than in company.
The competing triplicity scheme gives the band to Aries and reaches a rather different conclusion. Where two schemes disagree the honest thing is to show both and treat neither as settled fact, which is what happens on the decan pages here.
The information a birthday cannot carry
Two placements that most readers care about are untouched by the date. The Moon changes sign every two and a half days or so, and on any given date it may cross a boundary at some hour, so the lunar sign for 9 December has more than one possible answer until a time of birth is supplied.
The Ascendant is entirely a function of time and place. It moves through the full zodiac in a day, which means two people born on this date in the same hospital an hour apart can have different rising signs and a completely different house layout.
Set against that, the sun sign is the piece of a chart that varies least between people. It is worth knowing, and it is the beginning of the description rather than the whole of it.
Questions people ask about this
Is 9 December always Sagittarius?
Yes. The sign runs to around 21 December, so this date is well clear of both boundaries and has never fallen outside Sagittarius in any modern year.
What is the solar degree on 9 December?
Roughly seventeen degrees of Sagittarius at noon, with a spread of about one degree across the last century. The widget above computes yours precisely.
Which decan does 9 December fall in?
The second, from ten to twenty degrees of Sagittarius. Its Chaldean ruler is the Moon and the triplicity scheme assigns it to Aries instead.
Why does every 9 December horoscope read alike?
They are all written from the sign, which four weeks of birthdays share. Nothing in a sign description can distinguish one date within it from another.
Do I need a birth time for this date?
Not for the sun sign, which is certain. You need one for the Moon, the Ascendant and the houses, none of which the date alone can fix.
Is the decan more accurate than the sign?
It is more specific, which is not the same thing. The band is a measurable fact about the position, while what the tradition reads into it remains interpretation.