Two rulers, both still used
Before Uranus was found in 1781 the planetary rulerships covered the twelve signs with the seven visible bodies, and Saturn was given both Capricorn and Aquarius. When Uranus was discovered it was assigned to Aquarius, and most modern practice has used it since.
Neither assignment was withdrawn. Traditional astrologers work with Saturn and treat Uranus as an additional factor at most, while modern astrologers usually lead with Uranus and keep Saturn as the older ruler. The two readings pull in opposite directions, Saturn toward structure and restraint, Uranus toward disruption and distance from convention.
The same pattern applies elsewhere. Scorpio was Mars before Pluto and Pisces was Jupiter before Neptune, and in both cases the older ruler stayed in use alongside the newer one. Aquarius is the case where the two readings sit furthest apart, which is why the question comes up here more than anywhere else.
Where the decan comes in
On 27 January the Sun has been between about 6.4 and 7.8 degrees of Aquarius at noon in every year from 1930 to 2035. That places the birthday in the first decan, the band from zero to ten degrees, which the Chaldean order assigns to Venus.
So the tradition puts Venus over a sign already claimed by two planets. The older texts read the first third of Aquarius as its most companionable stretch, where the sign's detachment is softened without its fixity being touched. That is inherited interpretation and worth marking as such. What is measurable is only the longitude and the band it falls in.
The position itself is not in doubt
The Sun entered Aquarius on 19, 20 or 21 January depending on the year, so this date sits six to eight days past the ingress and roughly twenty two degrees short of the Pisces boundary. Neither edge is reachable by an unknown birth hour or a time zone shift.
The range is also narrow because the leap year cycle keeps the calendar and the orbit in step. The position slips about a quarter of a degree forward through each ordinary year and the leap day pulls it back, so it circles a fixed value rather than drifting. The widget above returns your own figure and the measured margin to the nearest boundary.
What the date cannot supply
The Moon moves around thirteen degrees a day and can be in two or three signs on a given 27 January depending on the year and the hour. The ascendant cycles the whole zodiac daily and is decided by the clock time and the latitude of the birthplace.
If you want to see the ruler question in practice, the place to look is where Saturn and Uranus actually sit in your own chart, since a ruler is only as informative as its position and its aspects. That needs a full calculation rather than a birth date.
The date does put loose limits on Mercury and Venus, since neither strays far from the Sun. On this birthday Mercury is generally within about a sign of Aquarius and Venus within two, which narrows the field without settling it.
Questions people ask about this
Who rules Aquarius, Saturn or Uranus?
Both, depending on the scheme. Saturn is the traditional ruler and was never withdrawn, and Uranus has been assigned to the sign since its discovery in 1781. Most modern practice leads with Uranus and keeps Saturn as a secondary.
Is 27 January definitely Aquarius?
Yes. The Sun is six to eight degrees into the sign on this date in every year, which no birth hour or time zone can change.
What degree is the Sun on this date?
Roughly six to eight degrees of Aquarius, depending on the year and hour. That falls in the first decan, the band the Chaldean order gives to Venus.
Does having two rulers make the reading contradictory?
It makes it layered rather than contradictory. Saturn describes the sign's fixity and restraint and Uranus its distance from convention, and most practitioners read one as primary and the other as background.
Does the decan ruler outrank the sign ruler?
No. The sign ruler applies across all thirty degrees and the decan ruler refines the ten degrees it covers. The decan is a modifier rather than a replacement.
How do I find where Saturn and Uranus sit in my chart?
That needs your year, hour and birthplace. Both are slow moving, so the year alone fixes them roughly, but the house they fall in depends on the birth time.