The band and how the Sun reaches it
On 30 September the Sun stands between 6.4 and 7.8 degrees of Libra at noon, across all the years calculated. Ten degrees marks the end of the first decan, so this date sits in the last third of it and stays there in every year of the range.
The consistency comes from the leap year rule holding the calendar against the orbit. The Sun arrives at a fixed date roughly a quarter of a day further along each ordinary year and is pulled back when February gains its extra day, so the position wobbles within a small band rather than sliding out of it.
It is also worth marking that the end of a calendar month has no counterpart in the sky. The Sun crosses no line on the thirtieth or the first, and the lengths of the months come from Roman administration rather than from anything measurable. The only boundaries that exist here are the longitudes at which signs and decans change.
The Chaldean assignment
The Chaldean order lists the seven visible planets by apparent speed, slowest first: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. Decan rulers run that list continuously through the whole zodiac, beginning with Mars on the first decan of Aries and never resetting at sign boundaries.
Counted from there, the first decan of Libra falls to the Moon. So a chart with the Sun here has Venus ruling the sign and the Moon ruling the band inside it, a pairing older texts read as sociability with a fluctuating register, quick to reflect a room and slow to settle a position of its own.
The triplicity assignment
The competing method assigns each decan to a sign of the same element, taken in zodiacal order from the sign itself. The air signs run Libra, Aquarius, Gemini, so the first decan of Libra is the Libra decan, and Venus rules both the sign and the band.
That produces a reading of concentration rather than mixture, Libra's Venusian qualities without a second planet across them. Two systems, one position, opposite conclusions. Since no observation separates them, the honest description of the situation is that a practitioner chooses a scheme, and that anyone quoting one without saying so is presenting a preference as a fact.
What stays fixed underneath
The degrees do not care which scheme is used. The Sun was where it was, and the widget above will give that figure for a specific year, hour and birthplace along with the gap to the nearest sign boundary, which on this date is comfortably wide.
The rest of the chart is untouched by any of it. The Moon changes sign every couple of days, leaving two options for this date until the hour is known, and the Ascendant runs through all twelve signs daily. Those placements are where two people born on this same date will actually differ.
The distance to the second decan is another thing worth quantifying rather than guessing. From this position it runs to two or three degrees, which is two or three days of solar motion, so no birth hour on this date can carry the Sun past the ten degree line into the Saturn ruled band that follows.
Questions people ask about this
What decan is 30 September?
The first decan of Libra, the band from zero to ten degrees. The Sun sits between 6.4 and 7.8 degrees on this date, so it is late in that band but inside it in every year.
Is the decan ruler the Moon or Venus?
It depends on the scheme. The Chaldean order gives this band to the Moon and the triplicity method calls it the Libra decan under Venus. Both are traditional and they are not combined.
What is the Chaldean order?
It is the sequence of the seven visible planets arranged by apparent speed, from Saturn as slowest to the Moon as fastest. Decan rulerships cycle through it without restarting at each sign.
Is 30 September ever Virgo?
No. The Sun entered Libra at the equinox on the twenty second or twenty third, so by the thirtieth it is around seven degrees into the sign in every year calculated.
Does the decan appear in a birth chart?
Not as a drawn feature. It is read from the degree of a placement, so any chart that shows degrees contains the information without labelling it.
How far from the second decan is this date?
Between two and four degrees, which is two to four days of solar motion. No birth hour on this date can reach the ten degree mark that starts the next band.