Equinox to ten degrees
Each sign holds three faces of ten degrees, thirty six in all around the ecliptic. This one starts at the exact moment the Sun crosses the celestial equator heading south, which is also the start of Libra.
In most years the Sun is in these degrees from about 23 September to about 3 October. Neither edge is fixed. The equinox itself moves between 22 and 23 September depending on the year, and every decan boundary drifts by up to a day and a half for the same reason: a tropical year is not a whole number of days. Anyone born in the last days of September is in the middle of the decan and safe; the edges are where the guesswork starts.
Venus by triplicity, the Moon by Chaldean count
The triplicity scheme gives the faces of a sign to the signs of its element in order. Libra's air decans go to Libra, then Aquarius, then Gemini, so the first stays at home with Venus, the sign's own ruler.
The Chaldean order works from a repeating planetary sequence, Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, started at zero degrees of Aries and run continuously. Counting through to the nineteenth face brings up the Moon.
Venus doubled and the Moon are close enough in tone to be confused, which is arguably worse than an obvious clash. A source that says this decan is the most Venusian stretch of Libra is using triplicity; one that describes it as lunar is using the Chaldean order. Both are traditional, and the difference is only visible once the method is named.
What each is read as contributing
With no second sign mixed in, the triplicity reading treats this as Libra at its plainest: the concern with fairness, the habit of considering a situation from the other party's side, and the discomfort at leaving a disagreement open are all taken at full strength, with nothing in the decan to offset them. Older texts also note the corresponding weakness, an unwillingness to state a preference in case it unbalances things.
The Chaldean Moon is read as making that responsiveness immediate rather than considered. Where Venus is described as weighing, the Moon is described as reacting, so this face is often characterised as picking up the mood of a room before working out what to do about it. Traditional writing links the Moon here to changeable company and to judgements that shift with circumstance, which is a comment on tempo rather than on sincerity.
None of this is confined to the Sun. Decans are read on the Ascendant and on the Moon as a matter of course, and a lunar decan ruler on a natal Moon is exactly the sort of doubling practitioners look for. The technique descends from Egyptian star lists older than the twelve signs in their current form, so it has genuine standing, but it refines a chart rather than anchoring it.
Two boundaries, one calculation
Because this decan opens at the equinox, a birthday on 22 or 23 September raises the question of whether the Sun had left Virgo at all. At the far end, around 3 October, the question is simply which face of Libra the Sun had reached.
The check above resolves both at once by computing your actual solar longitude and reporting the degree with the ruler each scheme assigns. A published date range cannot do that for anyone born near an edge.
Questions people ask about this
What dates does the first decan of Libra cover?
Approximately 23 September to 3 October. The lower edge is the equinox, which itself moves between years, so a birthday on 22 or 23 September should be checked rather than assumed.
Is this decan ruled by Venus or by the Moon?
Venus under the triplicity scheme, since the face belongs to Libra itself, and the Moon under the Chaldean order. Both are shown here because the readings they produce are similar enough to be mistaken for one another.
Does a doubled Venus make someone more Libran?
That is the traditional argument for any first decan, since no second sign is blended in. It is an interpretation rather than something measurable, and the Moon, Ascendant and aspects can easily outweigh it.
Why does my birthday sometimes show as Virgo?
Because the equinox falls at a different clock time each year. If you were born within a day of the sign change, only a calculation from your date and time will settle which sign the Sun was in.
Do decans work for the rising sign?
Yes. Any point in a chart falls in one of the thirty six decans, and the Ascendant is a common place to apply the technique. The Sun is used on this page because it needs only a birth date.
How old is the decan system?
Older than the zodiac signs as we use them. It began with Egyptian star lists that marked the hours of the night, and planetary rulers were attached later, in the Hellenistic period.