The position
The sign is Aquarius, in every year of the modern calendar. The Capricorn boundary is more than two weeks behind and the Pisces one about eleven days ahead, which is far enough that no year, hour or time zone brings either into play.
At noon from 1930 to 2035 the Sun has stood between 17.6 and 19.0 degrees of Aquarius on this date. The narrowness is a product of the leap year rule, which returns the calendar to the orbit every four years and leaves only about a degree of drift per century.
The word knowable is doing real work in that sentence. A position is knowable because it is computed from an ephemeris and can be checked against any other ephemeris. A character claim drawn from a date is not knowable in the same way, and mixing the two in one paragraph is how birthday pages end up sounding more authoritative than they are.
The decan, and the line above it
Eighteen degrees falls in the second decan of Aquarius, the band from ten to twenty. Mercury rules it in the Chaldean order and the triplicity scheme gives it to Gemini, so both schemes name the same planet.
The third decan begins at twenty degrees, and on this date the Sun approaches without reaching it. In the latest years of the range the noon position is within a degree of the line, so a birth late in the day in one of those years sits close indeed. Above twenty the Chaldean ruler becomes the Moon and the triplicity scheme switches to Libra, so the two schemes stop agreeing there. The widget above will tell you which side your own position falls on.
The decan is also the first thing to check when a description of your sign feels wrong. Two people who share a sun sign can sit in different decans, and the tradition reads those quite differently, which accounts for a fair amount of the mismatch people report.
What tradition attaches, marked as tradition
Aquarius is fixed air, given to Saturn in the traditional rulership and to Uranus in modern practice. The tradition reads it as principled, systematic and observant rather than involved.
The second decan under Mercury is read as adding an interest in setting things out, comparing them and putting them into words. These readings are what the tradition says about the position. They are reported here because they are the material, not because a date on its own is evidence about a person.
Where the two rulership schemes disagree, as they do above twenty degrees, the honest thing is to say so rather than to pick one silently. That is the usual reason a decan ruler quoted in one place contradicts the same decan quoted in another.
The limits of a date
A birthday fixes the Sun, and with it the decan, and stops. The Moon changes sign every two and a half days or so and often does it partway through a day, so on 7 February it can be in either of two signs depending on the hour, and across years in any of the twelve.
The Ascendant is faster again, moving through the whole zodiac between one midnight and the next, so it depends on the minute of birth and on the latitude of the place. Every house placement follows from it. That is why this page stops where it does: past this point, the honest answer needs your time and your birthplace, and the widget above is where that goes.
Questions people ask about this
What sign is 7 February?
Aquarius, without exception. The date sits about eleven days short of the Pisces boundary, which no amount of year to year drift can close.
What is the solar degree on 7 February?
About 17.6 to 19.0 degrees of Aquarius at noon, across 1930 to 2035. Later years in that span sit at the upper end, since the position drifts slowly forward.
Could a 7 February Sun be in the third decan?
Not at noon in any year of that range, though the late years come within a degree of the twenty degree line. A birth late in the day in a recent year is worth checking with the widget above.
Who rules the decan on this date?
Mercury, both by the Chaldean order and, through Gemini, by the triplicity scheme. Above twenty degrees the schemes diverge, giving the Moon and Libra respectively.
Why does this page not describe my personality?
Because a date is shared with roughly one person in three hundred and sixty five, and a description that fits all of them fits none of them closely. The tradition's reading of the sign and decan is given, marked as tradition.
What would I need for a full chart?
The date, the time as precisely as you have it, and the birthplace. Those three give the Moon sign, the Ascendant, the houses and every aspect involving them.