Distance from both edges
Libra occupies thirty degrees of longitude, and this date sits about fifteen of them in. That leaves around a fortnight of solar motion behind it to the equinox crossing and a fortnight ahead to the Scorpio one. No combination of year, hour and birthplace can move a position that far.
It is a genuinely different situation from a date in the third week of the month, where a few hours of time zone difference can put the birth on the other side of a crossing. Here the answer holds in every case, which is worth stating once rather than hedged. The widget above will report your distance from the nearest boundary and it will be a large number.
The middle of the middle decan
The three ten degree bands of Libra are divided at ten and twenty degrees. Fifteen sits at the centre of the second, so this birthday is as securely inside its decan as it is inside its sign. The Chaldean order gives that band to Saturn, working beneath Venus as the ruler of Libra itself.
Tradition reads the sign as concerned with proportion and the middle band as the stretch where that concern acquires a standard and holds to it. The opening band, ruled by Venus twice over, is described as the sign at its most Venusian, and the closing band under Jupiter as its most expansive. These are attributions from a very old scheme, offered as tradition rather than as measurement.
Why the degree stays put across a century
The Earth takes about three hundred and sixty five and a quarter days to complete an orbit, and the calendar keeps whole days. Every common year the Sun therefore reaches a given date about a quarter of a day short, and every fourth year February's extra day pushes it back the other way. The two corrections trade against each other and the noon position on any fixed date stays inside a band of roughly one and a third degrees.
That is what makes a birthday a workable coordinate. Someone born on this date in 1938 and someone born on it in 2028 have Suns within a degree and a half of each other, close enough that they occupy the same part of the sign by any reading. The same is true of the decan, which is fixed for this date rather than shifting with the year as it does for birthdays sitting on a ten degree division.
What still needs a birth time
The Moon moves about thirteen degrees a day and can change sign at any hour, so the date narrows it to two possibilities at best. The rising sign is not narrowed at all, since every sign passes the horizon during a full rotation and only the hour and the latitude of the place pick out the one that was up.
Between them those two placements shape most of a reading, and neither follows from the calendar. A settled sun sign on this date is a real fact and a small one. It is shared with everyone born across roughly a month, and the decan narrows that to about ten days, which is as fine as a calendar square can cut. PLACEHOLDER_NOTUSED
Questions people ask about this
Is 8 October definitely Libra?
Yes. The date sits near the fifteenth degree of the sign, about two weeks from each boundary, so no year between 1930 and 2035 places it anywhere else regardless of the hour or the birthplace.
What does being in the middle of a sign mean?
Technically it means the Sun is far from both ingress points, so the sign is unambiguous. Some traditions also read the middle degrees of a sign as its most characteristic stretch, which is an interpretation rather than a calculation.
Which decan is 8 October?
The second decan of Libra, from ten to twenty degrees, with the Sun near its centre. The Chaldean sub ruler for that band is Saturn, sitting under the Venus rulership of the sign.
Could a different time zone change my sign?
Not on this date. Time zone shifts amount to at most a day of solar motion, which is about one degree, and this position has a fortnight of margin in each direction.
How exact is the degree for my own birth?
The Sun moves close to one degree per day, so the hour of birth shifts the figure by a fraction of a degree. The widget above applies the hour and the birthplace to give the value rather than the noon approximation.
Does a mid sign Sun make the sign stronger?
Not in any technical sense. A Sun at one degree of Libra is as much in Libra as one at fifteen. The degree matters for aspects and house placement, not for whether the sign applies.