Where the decan sits
Ten degrees to a decan, three to a sign. This is the last of Libra's, running from twenty degrees to the end of the sign at thirty.
The Sun covers it in about ten days, roughly 13 October to 23 October, ending when it moves into Scorpio. Both ends move. The tropical year runs a fraction over three hundred and sixty five days, so ingresses creep forward and then reset at each leap year, and a decan edge can sit as much as a day and a half from where a fixed table puts it. That is enough to change the answer for a birthday in the last few days of October.
Mercury by one route, Jupiter by the other
Under the triplicity method the air decans are taken in the order Gemini, Libra, Aquarius. Libra is the second air sign, so counting from Libra itself the third decan comes round to Gemini, which is ruled by Mercury.
The Chaldean order does not consult the element at all. It repeats Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter from zero degrees of Aries, and the twenty first decan in that rotation is Jupiter's.
Mercury and Jupiter are traditionally read as opposites of scale, the particular against the general, so the two schemes produce noticeably different portraits of the same ten degrees. Anyone quoting a single ruler for this decan without naming the method is leaving out the part that explains the disagreement.
What the layers are taken to mean
Libra shaded by Gemini is read as the sign at its most verbal. Libra is described as weighing and comparing, and the Gemini overlay is taken to move that weighing into conversation: options stated aloud, positions tried out, agreement reached by talking rather than by waiting. The tradition also reads it as the least settled part of Libra, since Gemini is not given to closing a question.
Jupiter, from the Chaldean side, is read as adding scale. The associations are breadth, generosity of judgement and a preference for the principle over the instance, and here they are taken to lift Libra's comparing from personal preference towards something more like a stated standard. Where the Gemini reading gives fluency, the Jupiter reading gives the wish to be fair in general and not only in the case at hand.
The usual caution applies. Decans come from Egyptian lists of stars used to mark the hours of the night and are a legitimate part of the tradition, but they are a refinement placed over the signs, not the ground the chart stands on.
One further contrast is worth naming. Libra is cardinal and Gemini is mutable, so the triplicity colouring loosens a sign that otherwise likes to set terms, while the Chaldean planet pushes in the opposite direction by supplying a standard to appeal to. Readings of this decan often sound contradictory for that reason alone, and the contradiction is in the schemes rather than in the person.
Other placements, and getting the degree right
The technique was never confined to the Sun. The Ascendant is read by decan constantly, since the rising degree is the most precisely determined point in a chart, and the Moon nearly as often. Any planet in the last ten degrees of Libra belongs to this decan.
The check at the top of the page computes your solar degree from your own birth date and reports which decan it falls in. For birthdays around 22 or 23 October, where the question is not only which decan but which sign, that calculation is the only thing that settles it.
Questions people ask about this
What dates are the third decan of Libra?
Approximately 13 to 23 October. The closing date is the Sun's entry into Scorpio, which varies by up to a day and a half between years, so late October birthdays need a calculated degree.
Is the third decan of Libra ruled by Mercury or Jupiter?
Mercury under the triplicity scheme, by way of Gemini. Jupiter under the Chaldean rotation. Both schemes are standard and they simply answer on different principles.
What does Gemini bring to Libra?
The tradition reads it as fluency and a talking through of choices, with less inclination to settle them. Libra supplies the weighing, Gemini supplies the words and the reluctance to conclude.
Why does Jupiter appear here at all?
Because the Chaldean order is a fixed rotation counted from the start of Aries, not a scheme built on elements. It reaches Jupiter at the twenty first decan regardless of which sign that falls in.
Can I read my rising sign by decan?
Yes, and it is one of the better uses of the technique. The Ascendant degree is derived from your exact birth time and place, so it can be placed in a decan with confidence.
How much should a decan change my reading?
It should adjust the emphasis rather than replace the description. The sign does most of the work, the decan narrows it, and anything derived from the decan alone rests on very little.