Where the Sun actually sits on this date
Measured at noon in each year from 1930 to 2035, the Sun on 18 September occupies somewhere between 24.6 and 26.0 degrees of Virgo. That is the whole spread over more than a hundred years, and it is a narrower window than most people expect once they have been told that calendars and orbits do not match.
The reason is the leap year rule doing its job. A tropical year runs about a quarter of a day longer than a common calendar year, so the Sun arrives at any given calendar date a little later each year, and then a leap day pulls it back. The correction and the drift cancel out over each four year cycle, which keeps the position on a fixed date pinned inside a narrow band rather than sliding away from it.
The third decan of Virgo
Each sign divides into three bands of ten degrees called decans. Anything from 20 to 30 degrees of Virgo falls in the third, and this date sits comfortably inside it with no ambiguity about which band applies.
In the Chaldean order, which runs the visible planets from slowest to fastest and hands the next decan to the next planet in the sequence, the third decan of Virgo is assigned to Mercury. Mercury also rules Virgo itself, so the sign ruler and the decan ruler are the same body. Tradition reads that repetition as the Mercurial qualities of the sign showing up undiluted rather than being coloured by a second planet, which is the sort of claim worth marking clearly as interpretation.
The alternative division, sometimes called the triplicity method, sends the decans of a sign to the signs sharing its element in order. For Virgo that gives Virgo, Capricorn and Taurus, so this same band becomes the Taurus decan and takes Venus rather than Mercury. The degrees do not move between the schemes, only the label and the planet attached to them.
What a date cannot decide
The Sun is the slowest moving of the personal placements, which is exactly why a date can fix it. Everything faster remains open. The Moon covers roughly thirteen degrees a day and changes sign every two and a bit days, so on any 18 September it is in one sign for part of the day and often a different one by evening. Knowing the date alone leaves the choice between two.
The rising sign is worse served still. The whole zodiac comes up over the horizon in a day, so the Ascendant passes through all twelve signs in twenty four hours and changes roughly every two hours. A birth on this date could have any of the twelve rising, and the date narrows that not at all. In practical terms the sun sign is the placement a birthday reveals and also the least individual one.
Using the widget rather than a table
The widget above takes your year, hour and birthplace and returns the Sun's longitude for that specific moment instead of an average. It also reports the distance to the nearest sign boundary, which on this date will come back as a comfortable margin of several days rather than hours.
That margin is worth seeing written down. People born in the second half of a sign are sometimes told they are near a cusp because the date looks close to the end of a month, and month ends have nothing to do with sign boundaries. The Sun leaves Virgo when it reaches a specific longitude, and on this date it still has a few degrees to travel.
Cusp language survives because date tables are written to the day while the actual boundary is an instant that shifts by hours from one year to the next. In the middle of a sign the rounding costs nothing, and this date sits far enough inside Virgo that the table and the calculation agree in every year.
Questions people ask about this
Is 18 September always Virgo?
Yes, in every year covered by the calculations here. The Sun does not leave Virgo until the September equinox, which falls several days later, so there is no year in this range where the date reads as anything else.
What degree of Virgo is 18 September?
Between 24.6 and 26.0 degrees, depending on the year and the hour. The widget above will give the figure for your own birth rather than the range.
Which decan does this date fall in?
The third decan of Virgo, the band from 20 to 30 degrees. Every year in the range lands inside it, so there is no year dependence to worry about here.
Why does the degree hardly change from year to year?
Because the leap day cancels most of the drift. The Sun reaches a given calendar date roughly six hours later each ordinary year, then February gains a day and pulls it back, so the position oscillates inside a small band instead of wandering.
Does the decan change my sun sign?
No. A decan is a subdivision inside a sign, not an alternative to it. The sun sign stays Virgo and the decan describes which third of Virgo the Sun occupied.
Can I know my Moon sign from the date alone?
Not reliably. The Moon changes sign every two days or so, which means any calendar date covers part of one sign and often part of the next. The birth hour is what settles it.