Where it falls
Ten degrees to twenty degrees of Libra, the second of the sign's three equal divisions. The Sun takes roughly ten days to pass through, typically between about 3 October and about 13 October.
Those dates carry the usual caveat. The tropical year is a fraction longer than three hundred and sixty five days, so every ingress creeps through the calendar and the boundaries can sit a day and a half either side of where they fell last year. A birthday on 3 or 13 October tells you very little on its own.
Two routes to a similar place
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter on repeat from zero degrees of Aries through all thirty six decans. Followed through, the middle decan of Libra arrives at Saturn.
The triplicity scheme instead gives each decan to a sign sharing its element, in order. The air signs go Gemini, Libra, Aquarius, so the second air decan of Libra belongs to Aquarius. Aquarius is ruled by Saturn in traditional practice and by Uranus in modern practice, which is where the two readings start to separate again.
So the schemes agree more than they usually do, but only if the modern ruler is set aside. Read with Uranus, the triplicity account of this decan is considerably less settled than the Chaldean one. As ever, most popular writing names a ruler without naming the method, and here that means Saturn and Uranus are both quoted for the same ten degrees.
Aquarius over Libra, and Saturn's contribution
Libra shaded by Aquarius is read as the sign's concern with fairness widening past the people actually in the room. The comparison is less between two individuals and more between an individual and a rule, so the tradition attaches to these degrees an interest in principle, in how a group is arranged, and in what a person will do when courtesy and consistency ask for different things. Read less kindly, that is deciding a case in the abstract while the person in front of you waits.
Saturn is taken to add weight and delay to the same picture. The older material on this face speaks of terms, obligations and arrangements that hold over time rather than agreements made on the spot. Saturn is also traditionally exalted in Libra, which is unusual for a planet with a difficult reputation, and the readings for these degrees lean on that: judgement that is slow and heavy rather than harsh.
Not only the Sun, and what to expect of decans
Every placement occupies a decan. An Ascendant at sixteen degrees of Libra takes this one, and so does a Moon at eleven, and traditional practice read the Ascendant's decan at least as closely as the Sun's. The rulers described above apply to whichever point sits in the degrees.
It is worth keeping the technique in proportion. The decans reached astrology from Egyptian star lists used to mark the hours of the night, and the planetary rulers were laid over them centuries later. The antiquity is real. The role is still a supporting one: sign, house and aspect carry a reading, and a decan qualifies it. Where popular writing goes wrong is not in using decans but in presenting a ten degree band as though it settled more than the sign around it.
The check at the top of the page computes your actual solar degree from your birth date and reports both rulers for it, which is what a fixed date range cannot do for a birth near either edge.
Questions people ask about this
What dates does the second decan of Libra cover?
Approximately 3 to 13 October in most years. The boundaries move by up to a day and a half between years, so a birthday at either end needs the degree rather than the date.
Is the ruler Saturn or Uranus?
Saturn under the Chaldean order, and also under triplicity if you use the traditional ruler of Aquarius. Modern practice substitutes Uranus for Aquarius, which is how both names end up attached to the same ten degrees.
Why is Aquarius involved here?
The triplicity scheme distributes a sign's decans among the signs of its own element in order. Gemini, Libra and Aquarius are the air signs, so the middle air decan of Libra falls to Aquarius.
Does Saturn's exaltation in Libra matter for this decan?
The tradition reads it as supporting the decan's association with considered judgement rather than severity. It is an interpretive link rather than a rule, and it applies to the sign as a whole, not just these degrees.
Do decans apply to the Moon or Ascendant?
Yes. Any point in a chart falls in a decan, and the Ascendant's decan was read closely in older practice. The Sun receives the attention now mainly because it is the placement most people know.
Where does the decan system come from?
From Egyptian star lists that tracked which groups of stars rose during each hour of the night. Astrology later matched them to thirty six ten degree divisions and attached planetary rulers, which is the layer this page describes.