The sign settles quickly
Aquarius, in every year. The Sun crossed out of Capricorn around ten days earlier and will not reach Pisces for another three weeks, so nothing about the year, the hour or the birthplace can move this date to another sign.
That is the whole of what most pages about this date have to offer. It is true, and it is also the least individual fact in anyone's chart, since it is shared with everybody born across a month.
A boundary that is real on this date
Measured at noon across 1930 to 2035, a 30 January Sun has fallen between 9.5 and 10.8 degrees of Aquarius. The decan line sits at exactly ten degrees, so this date straddles it. In earlier years of that span the Sun is usually still in the first decan; in recent and coming years it is usually past the line and into the second.
The reason for the slow crawl is that the leap day slightly overcorrects for the quarter day the tropical year runs long, so a calendar date creeps forward against the orbit by roughly a degree a century. Within any four year block the position wobbles by about three quarters of a degree, and on this date that wobble is enough to move the answer.
Hour of birth matters here too, in a way it does not on most dates. The Sun covers a degree a day, so a birth in the early morning and one late at night can land on opposite sides of the line in the same year. The widget above uses your own time and place instead of assuming noon.
What the two decans are given
Decans are ten degree divisions, three to a sign and thirty six in all, and they carry their own rulers. In the Chaldean order the first decan of Aquarius belongs to Venus and the second to Mercury. The triplicity scheme gives the first to Aquarius itself and the second to Gemini, which also points at Mercury.
Tradition reads the first decan as the sign undiluted and the second as the sign inflected by an interest in language, exchange and the handling of information. Whether that shading deserves much weight is a judgement, and this page marks it as one rather than stating it as fact.
Worth noting that the disagreement between schemes is not a defect in either. They were built for different purposes at different times, and both are in current use. What matters when reading about a decan elsewhere is knowing which one the writer used, because a ruler quoted with no scheme named is ambiguous.
The parts a date cannot reach
Even with the decan settled, a birthday fixes one placement. The Moon on 30 January can be in either of two signs within a single year, since it changes sign roughly every two and a half days, and across the century it has been in all twelve. The rising sign depends on the hour and on how far north or south you were born, and it turns through the full zodiac in a day.
Anyone telling you what a 30 January birth means is describing a sun sign held by a twelfth of the world.
What the widget above can add, beyond the decan, is the distance to the nearest sign boundary. On this date that distance is a comfortable ten degrees or so, which is a useful thing to have stated rather than assumed.
Questions people ask about this
Is 30 January Aquarius?
Yes, in every year and at every hour. The nearest sign boundaries are around 20 January and 18 February, both far enough away that no year brings them near this date.
Which Aquarius decan is 30 January?
It depends on the year. The noon degree runs from about 9.5 to 10.8 across the last century, and the decan line sits at ten, so this date falls in the first decan in some years and the second in others.
Does my birth time change my decan here?
It can. The Sun moves about a degree a day, so in a year where noon puts you close to ten degrees, a morning birth and an evening birth land on opposite sides. The widget above takes your hour into account.
Who rules the two decans involved?
Venus rules the first decan of Aquarius in the Chaldean order and Mercury rules the second. Under the triplicity scheme the first goes to Aquarius and the second to Gemini, which again points to Mercury.
Does the decan matter as much as the sign?
It is treated as a refinement rather than a foundation. The technique is old and genuinely part of the tradition, but it modifies a reading of the sign rather than replacing it.
Why is my degree slightly different from a friend's born the same date?
Different years put the Sun at slightly different points, and different birth hours shift it further. A gap of under a degree between two people born on the same calendar date is entirely normal.