The band the Sun is standing in
Each sign splits into three faces of ten degrees. A Sun near eighteen degrees of Sagittarius is in the second, which runs from ten to twenty and is comfortably clear of both edges on this date. Nothing about the year or the hour of birth can push a 10 December Sun out of it.
The Chaldean order assigns rulers to the thirty six faces in a repeating sequence beginning at Aries, and this one comes out as the Moon. The triplicity scheme, which walks through the signs of the same element in turn, gives it to Aries instead. Popular articles often quote one without naming which.
Jupiter rules the sign throughout, so neither scheme replaces the sign ruler. The face is read as a modifier laid over it, narrowing a thirty degree description to a ten degree one.
That narrowing is the main thing a birthday page can offer beyond the sign. It does not make the reading more reliable, but it does make it apply to a smaller group of people, which is a different and more honest kind of progress.
What the tradition reads into the combination
Jupiter under a lunar face is described in older texts as expansion turned toward the interior. Sagittarius is usually characterised by distance, travel and the search for a larger frame, while the Moon governs habit, memory, the household and the things done from feeling rather than argument.
The combination is therefore read as someone whose reach is real but exercised in private, with conclusions arrived at before they are announced. Under the triplicity reading, with Aries in place of the Moon, the same band comes out faster and more combative instead.
These are two traditions disagreeing, not two measurements conflicting. The degree is checkable, the ruler depends on which scheme you adopt, and what either implies about a person is interpretation.
How stable the position is
Measured at noon in every year from 1930 to 2035, a 10 December Sun ranges from about seventeen and a half to just under nineteen degrees. The whole century of variation is narrower than the width of the face by a wide margin, which is why the decan answer for this date is safe without a calculation.
The mechanism is the leap cycle. Three ordinary years push the noon position forward by around a quarter of a degree each, and the leap day pulls it back roughly three quarters of a degree at a stroke. The pattern closes on itself rather than accumulating.
Dates that sit near twenty degrees do not get this reassurance, because the same small wobble can carry them across a face boundary and change the ruler.
Where the date stops being informative
The face refines the Sun. It says nothing about the rest of the chart, and the rest of the chart is where most of the variation between two people born on the same day lives.
The Moon covers roughly half a sign in a day, so on this date it may occupy either of two signs and occasionally a third, depending on the hour. The rising sign is unconstrained altogether, since the full zodiac passes the horizon in twenty four hours, and the house framework follows it.
The widget above resolves the solar figure exactly from your year, hour and birthplace, and reports how far the Sun was from the nearest sign boundary. For the placements the date cannot reach, a full chart calculation is the only route.
Questions people ask about this
What decan is 10 December?
The second decan of Sagittarius, from ten to twenty degrees. The Sun sits near eighteen degrees on this date, so the answer holds in every year regardless of the hour.
Who rules the second decan of Sagittarius?
The Moon under the Chaldean order and Aries under the triplicity scheme. Jupiter rules the whole sign in both cases, so the face ruler is an additional layer.
Is 10 December ever Capricorn?
No. Capricorn begins around 21 December, more than ten days later, so this date has never fallen outside Sagittarius in any modern year.
What degree of Sagittarius is this date?
About eighteen degrees at noon, varying by roughly one degree across the last century. Your own figure depends on year, hour and birthplace.
Does the decan matter more than the sign?
It is finer, not more authoritative. The sign locates the Sun within thirty degrees and the face within ten, so the face simply carries more information.
Can two people born on 10 December have very different charts?
Yes, substantially. They share a Sun and a face and can still differ in Moon sign, rising sign and the house position of every planet.