The figure and its spread
At noon on 27 September, in every year from 1930 to 2035, the Sun has stood between 3.4 and 4.8 degrees of Libra. Grandparent and grandchild born on this date carry solar positions barely a degree apart, which is closer than most people would guess.
The mechanism is simple arithmetic. A tropical year is about three hundred and sixty five days and a quarter, so in an ordinary year the calendar finishes early and the Sun is caught roughly six hours further on when the date next comes round. After three such years the accumulated lead is close to a full day, and the leap day in February absorbs it. The position therefore oscillates rather than drifts.
What is left over
The match is not perfect, which is why the range has any width at all. A tropical year is a little short of three hundred and sixty five and a quarter days, so the Gregorian rule skips the leap day in three century years out of four to compensate. Nineteen hundred was skipped, two thousand was not, and that detail is visible in the slow tilt of the range across the span covered here.
None of this is enough to move the sign or the decan. The Sun on this date has been in Libra and in the sign's first decan in every year of the range, and the residual drift shows up in the first decimal place of the degree rather than anywhere a reader would notice.
There is a second reason the figure is not identical from year to year, which is the four year cycle itself. Within it the position steps later through the three ordinary years and falls back after the leap day, so a birth in 1936 and one in 1939 sit at opposite ends of that small swing while both remain firmly inside Libra.
Where that leaves the reading
The first decan of Libra runs from zero to ten degrees and takes the Moon as its Chaldean sub ruler, under Venus as ruler of the sign. The triplicity scheme labels the same band for Libra itself and keeps Venus in both roles. Whichever is preferred, the band is fixed for this date and is not one of the things that varies from year to year.
What tradition attaches to the position, whether lunar responsiveness or doubled Venus, is interpretation and is offered as such. The position itself is a computed quantity that anyone can check against an ephemeris.
The parts that do vary between people
Stability is a property of the Sun, not of charts. The Moon moves thirteen degrees a day and changes sign every couple of days, so the Moon sign for this date is one of two possibilities in most years and depends entirely on the hour of birth.
The Ascendant varies faster still, running through all twelve signs during a single rotation of the Earth. A birthday tells you nothing about it. Enter the year, hour and place in the widget above and it returns the solar degree for that exact moment along with the distance to the nearest sign boundary, which on this date is several days in either direction.
It is worth stating what this stability is not. It does not mean people born on the same date share a chart, and it does not make the date special. It means the Sun is a poor tool for telling two of them apart, and that everything doing the distinguishing lies in the placements a birth time supplies.
Questions people ask about this
Do people born on the same date in different years share a sun position?
Very nearly. On this date the noon figure ranges only from 3.4 to 4.8 degrees of Libra across more than a century, because the leap year cycle keeps the calendar aligned with the orbit.
Is 27 September always Libra?
Yes, in every year calculated. The Sun entered Libra at the equinox several days earlier, so there is no year in the range where this date reads otherwise.
Why is there any variation at all?
Because the correction is approximate. The leap day slightly overshoots, which is why three century years in four skip it, and the leftover shows up as the small spread in the figure.
What decan does 27 September fall in?
The first decan of Libra, zero to ten degrees, in every year of the range. Its Chaldean sub ruler is the Moon.
Does the birth hour change the degree much?
By half a degree at most, since the Sun covers about a degree in a full day. It matters for precision and for aspects, not for the sign or the decan on this date.
How far is this date from the nearest sign boundary?
Around four degrees from the start of Libra and around twenty five from the start of Scorpio, so several days on the near side. The widget above converts that to hours for your own birth.