The middle third of the sign
This is the central face of Sagittarius, sitting comfortably away from both sign boundaries. That makes the sign itself unambiguous for anyone placed here, even though the decan boundaries at either end can still be close.
The Sun takes about ten days to cross, roughly 2 December to 12 December. Those dates move by up to a day and a half from year to year, so a birthday on either edge does not settle the question by itself.
Early December sits well away from the solstice, so nothing in the seasonal frame pins these boundaries. They drift with the calendar in the ordinary way, resetting a little at each leap day.
A clear disagreement between the schemes
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter and repeats from zero degrees of Aries. Counted through to the twenty sixth face, the middle decan of Sagittarius falls to the Moon.
The triplicity scheme works by element instead. Fire decans go to Aries, Leo and Sagittarius in order, so the middle decan of Sagittarius belongs to Aries, whose ruler is Mars.
The Moon and Mars are not close in meaning, so this is one of the faces where quoting a scheme matters most. A reader who finds these degrees called lunar in one place and martial in another has not found an error, only sources that did not name their method.
Two readings, pulling different ways
The Aries colouring is read as making the Sagittarian interest in the wider world into something acted on immediately. Sagittarius alone is described as forming views about how things fit together; with Aries mixed in, the tradition describes the same views taken up as positions and defended, so the argument and the journey both start sooner than planning would allow.
The Chaldean lunar face is described almost in reverse. The Moon is taken to add responsiveness to the mood of a room and a need for familiar ground to return to, which the older sources set against the sign's restlessness. Read that way, the same degrees produce someone whose travelling is tied to people rather than to distance, and whose opinions shift as the company changes.
Since the schemes conflict, there is no single line to report. Both accounts are interpretation, and the actual chart would settle which, if either, describes anything.
There is also a technical use behind both readings. In traditional practice a planet in its own face counted as holding a minor dignity, the weakest of the recognised ones, and the faces survived in the tradition largely for that purpose rather than as personality descriptions.
Any placement, and where the range fails
Every point in a chart falls in a decan, so this face applies to an Ascendant at seventeen degrees of Sagittarius or a Moon at twelve degrees just as much as to the Sun. The rising degree changes by about a degree every four minutes, which makes the Ascendant the placement where decan divisions separate charts most sharply.
As for how much weight to give any of it, decans descend from Egyptian star lists that marked hours of the night, so the technique is ancient and legitimately part of the tradition. It is still a refinement laid over a reading rather than the base of one, and a decan that contradicts the sign should not be allowed to outrank it.
The check above computes your solar degree from your birth date instead of matching a table. That is the only way to answer the question for someone born around 2 December or 12 December, since the ranges themselves shift with the year.
Questions people ask about this
What dates are the second decan of Sagittarius?
Roughly 2 December to 12 December. Both boundaries drift by up to a day and a half across the leap cycle, so births near either date should be checked with a calculated degree.
Is this decan ruled by the Moon or by Mars?
The Moon in the Chaldean order and Mars under the triplicity scheme, which assigns these degrees to Aries. The schemes disagree sharply here, so it is worth knowing which one a source is using.
What does the Aries decan add to Sagittarius?
The tradition reads it as turning broad interest into immediate action and stated positions. Travel, argument and undertakings are described as beginning before the preparation is complete, which sources frame as decisiveness or as haste.
Which scheme is older?
The Chaldean faces are the older assignment in surviving astrological texts, while the triplicity arrangement is the one most modern writing uses. The decan divisions themselves are older than either, coming from Egyptian star lists.
Do decans matter for the Ascendant?
Yes, and arguably more than for the Sun. The Ascendant moves through a degree every four minutes, so it is the placement where a ten degree division actually separates one chart from another.
How much should a decan change a reading?
Not much on its own. It is a second-order refinement that adjusts the description of a placement already established by sign, house and aspect, and the schemes disagree often enough that certainty is not available.