A decan ruled by the sign's own planet
Signs divide into three decans of ten degrees each. The Sun on 19 September stands between 25.6 and 27.0 degrees of Virgo, which puts it in the last of the three, covering 20 to 30 degrees.
Decan rulers in the Chaldean scheme follow the old speed order of the visible planets, Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, cycled continuously through all thirty six decans starting from the first decan of Aries. Counted through, the third decan of Virgo lands on Mercury. Since Mercury is also the traditional ruler of Virgo, this band is one of the places where a sign and its sub division answer to the same planet.
The spread across the whole period from 1930 to 2035 runs from 25.6 to 27.0 degrees, which is narrow enough that a grandparent and a grandchild born on this date differ by little more than a single degree. The leap day is doing the work: three ordinary years push the Sun about a quarter of a day further along each time, and February's extra day pulls the accumulated lead back.
What the tradition makes of the repetition
Where a decan ruler differs from the sign ruler, the older texts read the pair as a blend, the sign's nature inflected by a second planet. Where they agree, the reading given is one of concentration rather than mixture. For late Virgo that means the analytical, sorting, language handling associations of Mercury described without a competing influence.
Worth saying plainly: this is inherited interpretation, not a measurement. The measurable claim on this page is the longitude. What tradition attaches to that longitude is a separate layer, and a reader is entitled to take the first and leave the second.
Late Virgo also sits in the part of the year where the Sun is picking up speed on its way to perihelion in early January. The autumn signs are therefore crossed slightly faster than the summer ones, which is why the boundaries in this stretch of the calendar move a little more freely against the dates than the ones in June.
The other decan scheme
There is a second and older way of dividing the same bands, sometimes called the triplicity or sign based method. It assigns each decan to a sign of the same element in order, so the decans of Virgo go to Virgo, then Capricorn, then Taurus. Under that scheme this date reads as the Taurus decan of Virgo and the sub ruler becomes Venus rather than Mercury.
Both schemes are in current use and neither has displaced the other. The degrees do not change between them, only the label attached to the band, which is a useful reminder of how much of astrological practice is convention layered on top of an unambiguous position.
Everything the date leaves open
The date fixes the Sun to inside a 1.4 degree window. Nothing else is fixed at all. The Moon travels through a sign in a little over two days, so a birth in the small hours and a birth at night on this same date can carry different Moon signs. The Ascendant turns over completely in a day, which leaves all twelve possibilities live until the birth time and place are supplied.
The widget above closes those gaps. Enter the year, the hour and the town and it returns the Sun's exact degree for that moment together with the distance to the nearest sign boundary, which on this date is a wide one in every year.
None of that is a shortcoming of the date. It is simply what a single number can carry. A birthday identifies the slowest of the personal placements and leaves the two fastest open, which is the reverse of the impression a sun sign column gives.
Questions people ask about this
What decan is 19 September?
The third decan of Virgo, from 20 to 30 degrees. The Sun sits between 25.6 and 27.0 degrees on this date, so the decan is the same in every year of the range.
Why is Mercury listed twice for this birthday?
Because Mercury rules Virgo in the traditional scheme and the Chaldean order also gives Mercury the third decan of Virgo. The sign ruler and the decan ruler coincide.
Which decan scheme should I use?
Both are legitimate and they disagree on the sub ruler, giving Mercury under the Chaldean order and Venus under the triplicity method. Practitioners choose one and keep to it rather than merging them.
Is 19 September ever Libra?
No. Libra begins at the September equinox, which arrives on the twenty second or twenty third, so this date has been Virgo in every year calculated.
Does the decan matter more than the sign?
Neither ranks above the other. The decan is simply finer grained, which is why it separates birthdays that a sign name treats as identical.
How close to the end of Virgo is this?
Around three to four degrees from the boundary, which is three or four days of solar motion. That is far enough that no reasonable birth time could put the Sun in the next sign.