Position in the sign and in the year
These are the final ten degrees of the cardinal earth sign, ending where Aquarius begins. In traditional practice the last degrees of a sign are read as already turning towards the next, which for Capricorn means towards a more detached and collective register.
The Sun crosses in about ten days, roughly 10 January to 19 January. That window moves by up to a day and a half between years, so the closing boundary in particular is unreliable for anyone born on it.
The mid January boundary at the start of this face is a decan boundary rather than a seasonal one, so it moves with the calendar alone. The closing boundary is the start of Aquarius, which is quoted inconsistently between publishers even for a single year.
Solar face, Virgo decan
Under the Chaldean order, the sequence Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter repeats from the start of Aries across all thirty six faces. Counted out to the thirtieth, this decan of Capricorn takes the Sun.
The triplicity scheme runs by element rather than by sequence. Earth decans are given to Taurus, Virgo and Capricorn in order, so the closing decan of Capricorn belongs to Virgo, and Virgo is ruled by Mercury.
That is a genuine split, and it is the ordinary state of affairs rather than an exception. Because most writing on decans names one ruler and no method, a reader comparing sources on these degrees will find the Sun and Mercury both asserted with equal confidence. Both are given here so the reason is visible.
What each addition is taken to mean
The Virgo colouring is read as Capricorn's ambition narrowed onto the working detail. Capricorn on its own is described in terms of position and long structures; with Virgo mixed in, the tradition describes attention moving to how the thing is actually done, so process, correction and standards recur. The cost named alongside it is a difficulty delegating, since the standard is easier to meet personally than to explain.
The solar face is read differently. The Sun is associated with holding a single line and with being identified with the work, so sources describe a placement where the role and the person become hard to separate. That is credited with persistence and blamed for taking setbacks in the job as verdicts on the self.
These are readings, not findings. Where the two schemes disagree, the sensible course is to hold both loosely and let the rest of the chart decide.
Faces also carried a technical role. A planet in its own face was counted as holding the weakest of the essential dignities, which is a narrow claim about strength rather than the character reading modern articles build from the same divisions.
Beyond the Sun, and the degree check
Decans apply to any placement. A Moon at twenty four degrees of Capricorn sits in this face, as does an Ascendant at twenty nine degrees. The Ascendant is the sharpest use of the technique, because it advances roughly a degree every four minutes and therefore distinguishes people born hours apart on the same date.
The honest framing is that decan work is old, descending from Egyptian star lists that once marked the hours of the night, and it is a real part of the tradition. It is nonetheless a refinement layered on top of the chart, subordinate to sign, house and aspect, and it should not be asked to carry more than that.
The check above works out your actual solar degree rather than matching a date range, which is the only reliable answer near 10 January or the cusp of Aquarius. A birth at the very end may in fact land in early Aquarius.
Questions people ask about this
What dates does the third decan of Capricorn cover?
Approximately 10 January to 19 January. Both ends move by as much as a day and a half depending on the year, so the range should not be trusted for a birth right on a boundary.
Which planet rules the last decan of Capricorn?
The Sun in the Chaldean order, Mercury under the triplicity scheme, since that scheme assigns these degrees to Virgo. Neither source is wrong; they are answering with different methods.
What does the Virgo decan add to Capricorn?
The tradition reads it as shifting attention from position to process, so standards, correction and technical competence come forward. Delegation is described as the usual difficulty, because the standard is personal rather than written down.
Is a decan the same as a face?
Yes. Face is the older term in traditional texts and decan the more common modern one. Both refer to a ten degree division, of which each sign holds three and the zodiac holds thirty six.
Can decans be applied to the Ascendant?
They can, and it is arguably the best use of them. The rising degree changes fastest of any point in the chart, so a ten degree division separates charts that would otherwise look almost identical.
Are decans a major technique?
No. They are an old and legitimate refinement, but a chart reading rests on sign, house and aspect, and the decan schemes disagree on enough faces that no single decan ruler can be treated as settled.