Two degrees in, and reliably so
Libra opens at the September equinox, when the Sun crosses the celestial equator moving south. That crossing lands on the twenty second or twenty third, so by the twenty fifth the Sun has been in the sign for two or three days and stands between 1.5 and 2.9 degrees of it at noon.
The spread of that figure over the years from 1930 to 2035 comes to 1.4 degrees in total, and there is a mechanism behind the tightness. Each ordinary year the Sun reaches a given calendar date about six hours further along its orbit, then the leap day in February throws it back by roughly eighteen. The two effects trade off inside every four year cycle, so the position on a fixed date circles a small area instead of migrating.
The first decan of Libra
Anything from zero to ten degrees of Libra belongs to the first decan, so this date falls near its start in every year. The Chaldean sequence of planetary rulers, running through all thirty six decans in the old order of orbital speed, assigns this one to the Moon.
That gives a Venus ruled sign with a lunar sub ruler, a combination the tradition reads as the sign's concern with balance and relation expressed through mood and receptivity rather than through Venus alone. The older triplicity scheme labels the same band the Libra decan of Libra and keeps Venus as the sub ruler, which is a different reading of an identical position and a good illustration of how much of this is convention.
Neither scheme is a late invention and neither has displaced the other. The Chaldean order is the one most often quoted in modern decan writing, and the triplicity order is the older of the two in surviving Hellenistic material. Anyone quoting a decan ruler without naming the scheme has made a choice on the reader's behalf.
A cusp claim that does not survive checking
Because the date is close to the start of a sign, people born on it are sometimes told they are on the Virgo Libra cusp. The distance says otherwise. Two to three degrees of solar motion is two to three days, and no birth hour and no time zone moves the Sun that far.
The widget above puts a number on it. Given a year, an hour and a birthplace it reports the exact longitude and the gap to the nearest boundary in hours, and on this date that gap runs to sixty hours or more. A boundary that far away is not a boundary anyone needs to worry about.
What the date leaves undetermined
The Sun is the one placement a birthday can fix, because it moves slowly enough for a whole day of uncertainty to cost only a degree. Every faster body stays open. The Moon changes sign roughly every two and a quarter days, so this date offers two candidates in most years and the hour decides between them.
The Ascendant offers all twelve. It sweeps through the entire zodiac in a day, changing about every two hours, and with it go the houses that place a chart against ordinary life. The sun sign is the part of a chart that a birthday gives away, and it is also the part that a few million other people share exactly.
What a chart adds beyond signs is the houses, and those depend entirely on the Ascendant. Without a birth time they cannot be drawn, which is why an astrologer will ask for a birth certificate rather than a birthday. The date gets you the Sun and the rough position of the slow planets, and no further.
Questions people ask about this
Is 25 September Virgo or Libra?
Libra, in every year in the range calculated here. The equinox that begins Libra falls on the twenty second or twenty third, so by the twenty fifth the Sun is a full two days inside the sign.
What degree of Libra is 25 September?
Between 1.5 and 2.9 degrees at noon, depending on the year. Your own figure comes from your year, hour and birthplace, which the widget above uses.
Am I on the Virgo Libra cusp?
No. The Sun is two to three degrees past the boundary, which is two to three days of motion. Nothing about the birth hour or the time zone can close a gap that size.
Which decan is this?
The first decan of Libra, from zero to ten degrees. The Chaldean order gives it to the Moon, while the triplicity method calls it the Libra decan and keeps Venus.
Does an early degree mean a weaker sign?
No. A Sun at two degrees of Libra is as fully in Libra as one at twenty. The degree affects aspects and house placement, not whether the sign counts.
Why is the degree almost the same every year?
Because the leap year cycle keeps the calendar in step with the orbit. The drift accumulated over three ordinary years is undone by the extra day in the fourth, so the position on a date stays inside a narrow band.