The sign is not in question
The Sun leaves Capricorn for Aquarius somewhere around the twentieth of January and leaves Aquarius for Pisces around the eighteenth of February. Twenty eight January sits in the middle of that stretch, roughly eight days past the entry and three weeks short of the exit.
That distance is what makes the date safe. Boundary dates move because the ingress moment drifts by about six hours a year and jumps back when February gains a leap day, but the drift only ever amounts to a day or so. A date sitting a week clear of the nearest boundary is never touched by it.
Where the Sun actually is
Taking noon in each year from 1930 to 2035, a 28 January Sun falls between 7.4 and 8.8 degrees of Aquarius. That is a spread of under a degree and a half across more than a century, which is a much tighter result than most people expect.
The reason is the leap year cycle. The tropical year runs about a quarter of a day longer than the calendar year, so the Sun slips a little later each year and then the extra February day pulls it back. Over four years the two effects almost cancel, and the calendar and the orbit stay in step. The residue is the slow century-scale drift you can see in the range above.
The widget above computes the figure for your own year, hour and birthplace rather than for noon, so it will give you a single degree instead of a band.
The practical upshot is that anyone born on this date can quote their degree to within a degree and a half without knowing anything about their year. That is unusual precision from a single line of information, and it is the reason a date can support a degree claim at all while it cannot support a Moon sign.
The first decan of Aquarius
Each sign divides into three bands of ten degrees, called decans. Eight degrees of Aquarius falls in the first of them, which runs from zero to ten. In the Chaldean order of rulers that decan belongs to Venus, and the triplicity scheme gives it to Aquarius itself and so to Saturn in the old rulership or Uranus in the modern one.
Tradition reads the first decan as the sign at its least modified, with the Venus assignment adding an interest in form and in how things are put together. Every other 28 January reading you will find describes Aquarius in general. The decan is the layer that separates your Aquarius birthday from one in the middle of February, and almost nothing published about birthdays mentions it.
What the date cannot tell you
The Sun is one body. On 28 January the Moon can be in any sign at all depending on the year, and within a single year it can change sign partway through the day, so two people born on the same 28 January can have different Moon signs. The rising sign runs through all twelve in roughly twenty four hours, so it depends on the hour and the latitude and not on the date in any useful way.
Put plainly, the sun sign is the part of a chart that a birthday fixes, and it is also the part that the fewest people share with the fewest others. Everything more specific needs the time and the place.
The one thing a date does buy you is a floor. Whatever else is unknown, the Sun is fixed, so an aspect between the Sun and a slow moving planet can be checked from the date alone. Aspects involving the Moon or the Ascendant cannot, and neither can anything about houses.
Questions people ask about this
Is 28 January always Aquarius?
Yes. The boundary with Capricorn falls around 20 January and the one with Pisces around 18 February, so this date sits comfortably clear of both. No year in the modern calendar puts it anywhere else.
What degree of Aquarius is 28 January?
Between about seven and nine degrees, measuring at noon across the last century. The widget above narrows that to a single figure once it has your year, hour and birthplace.
Which decan is a 28 January birthday?
The first decan of Aquarius, zero to ten degrees. Its Chaldean ruler is Venus. The triplicity scheme instead assigns the decan to Aquarius itself, so the two schemes disagree here.
Why does the degree hardly change between years?
Because the leap day corrects almost exactly for the quarter day the tropical year runs long. The Sun drifts later for three years, the leap day pulls it back, and the calendar stays locked to the orbit.
Does my birth time change my sun sign on this date?
No. The Sun moves about one degree a day, so a full day of birth time shifts the degree by one and never shifts the sign. Birth time matters here for the Moon and the Ascendant.
What can a birthday not tell me about my chart?
The Moon sign, the rising sign, the house positions and every aspect involving the Ascendant. Those need a time and a place. A date on its own fixes the Sun and very little else.