How those pages are made
Nearly every birthday page online is generated from a table of twelve sign date ranges joined to a stock paragraph. Feed in 25 January, the table returns Aquarius, and the paragraph served is the same one given to everyone born across a full month.
Nothing in that process computes a position. It cannot report a degree, it cannot name a decan, and it has no way to say how close to a boundary a given birthday falls. That is why the pages are interchangeable, and it is a limit of the method rather than of the subject.
The sign descriptions themselves are usually a long way downstream of any source. They are paraphrases of paraphrases, which is why the same four adjectives recur across sites that share no other text, and why they rarely mention modality, element or ruler in any detail.
What the calculation returns
On 25 January the Sun has been between about 4.4 and 5.8 degrees of Aquarius at noon in every year from 1930 to 2035. That range is barely more than a degree and a half wide across more than a century, held in place by the leap year cycle keeping the calendar in step with the orbit.
From that figure the decan follows at once. Five degrees is in the first band of Aquarius, zero to ten, which the Chaldean order assigns to Venus while Saturn and Uranus rule the sign itself. The tradition reads that band as the most sociable third of a reserved sign, which is inherited interpretation rather than measurement and worth marking as such.
The margin to the boundary
The Sun entered Aquarius on 19, 20 or 21 January depending on the year, so this birthday sits three to six days past the ingress. That is far more than an unknown birth hour or a time zone difference could ever bridge.
The margin is genuinely useful information. On the handful of dates that fall within a degree of a boundary, an unknown birth time leaves the sign itself unsettled and the honest answer is that the details are needed. On this date the answer holds regardless, and the widget above reports the margin as a number rather than a reassurance.
The same calculation constrains Mercury and Venus a little, since neither strays far from the Sun. On this date Mercury is generally within about a sign of Aquarius and Venus within two, which is a real narrowing even though it stops well short of an answer.
What is left for a chart to answer
The Moon changes sign every two and a half days, so on any 25 January it may be in one of two or three signs depending on the year and hour. The ascendant runs through all twelve in a day and answers to the clock and the latitude of the birthplace.
Those are the placements that differ between two people who share this birthday. Mercury and Venus stay near the Sun and so are loosely constrained by the date, Mercury within about a sign and Venus within two, which narrows without answering.
Jupiter and Saturn are not constrained by the date at all, since both depend on the birth year rather than the day of it.
Questions people ask about this
What sign is 25 January?
Aquarius, with several days of margin past the boundary in every year. No birth hour or time zone puts it in Capricorn.
What is the Sun's degree on 25 January?
Around four to six degrees of Aquarius, set precisely by your year and hour. The widget above computes it and reports the distance to the sign boundary.
Which decan is this birthday in?
The first decan of Aquarius, zero to ten degrees, assigned to Venus in the Chaldean order while Saturn and Uranus rule the sign.
Why does every page say the same thing about this date?
Because they all read a sign from a date table and print a stock description. That method never calculates a position, so nothing more specific can come out of it.
Is there anything wrong with the standard description?
Not as tradition. Aquarius as fixed air under Saturn and Uranus is coherent material. The problem is that it describes a month and is presented as though it described a person.
What should I look up next?
The Moon and the ascendant, since those need a year, an hour and a birthplace and are the placements that actually differ between people born on this date.