The calendar and the orbit stay in step
A tropical year is a little under three hundred and sixty five and a quarter days. A common calendar year is three hundred and sixty five, so the Sun's position on a given date creeps forward by about a quarter of a degree each year as the calendar falls slightly behind the sky.
The leap day corrects it. Adding a twenty ninth of February pulls the date back by very nearly a full day, which returns the Sun to roughly where it stood four years earlier. The result is a sawtooth that repeats indefinitely instead of drifting away, and it keeps this birthday anchored to the same stretch of Aquarius.
The correction is not perfect, which is why the century rule skips the leap day in 1900 and 2100 while keeping it in 2000. Over the span covered here that refinement barely shows, and the window stays under two degrees wide.
Where in the sign that puts you
Around twenty four degrees is four fifths of the way through Aquarius. The Sun entered the sign about three and a half weeks before this date and will leave it in about five days, so the position is late in the sign without being anywhere near the boundary.
That places the date in the third decan, the band from twenty degrees to thirty. In the Chaldean sequence of planetary rulers that band belongs to the Moon, and tradition reads it as the reflective and inward facing part of an otherwise outward looking sign. The band is arithmetic and the reading attached to it is convention, which is a distinction worth keeping.
Being late in a sign is sometimes described as being on the way out of it, which is a figure of speech rather than a technique. The Sun occupies one longitude and that longitude is inside Aquarius, with no fading at the edges.
Why the degree is worth knowing at all
In practice the degree is what lets a chart be built. Aspects are measured in degrees, so whether a planet forms a square or a trine to your Sun depends on the exact figure rather than on the sign. Two people who are both Aquarius but twenty degrees apart will have entirely different aspect patterns from the same set of planets.
It also matters for progressions and returns, both of which count forward from a precise starting longitude. Round the Sun to the nearest sign and none of that work can be done.
Solar returns are the clearest case. A return chart is drawn for the moment the Sun comes back to its natal longitude, which can fall on the day before or the day after the birthday depending on the year, and that moment cannot be found without the degree.
What the date still cannot tell you
The Moon moves about thirteen degrees a day and changes sign every two and a half days or so, which means an 13 February birth falls in one of two signs depending on the hour. The rising sign is completely open, since it turns through the whole zodiac every twenty four hours and depends on the latitude of the birthplace as well as the clock time.
The widget above calculates all of this from your year, time and place, gives the solar degree to the minute and reports how far your birth fell from the nearest sign boundary. On this date that will come back at roughly five days, which is a firm margin.
Questions people ask about this
What sign is 13 February?
Aquarius. The Sun is around twenty four degrees into the sign and leaves it about five days later, so the answer does not vary by year.
Why does the degree stay so stable?
Because the leap year cycle keeps the calendar aligned with the orbit. The position creeps forward about a quarter of a degree per year, then the added leap day pulls it back, so it oscillates instead of drifting.
How wide is the variation?
Under a degree and a half at noon across the period from 1930 to 2035. Your birth hour can move it by another half degree within the day.
Does an Aquarius born late in the sign differ from one born early?
In measurable terms yes, since the Sun is a long way apart and forms different aspects. In terms of the sign itself, both are equally Aquarius.
Which decan does this date fall in?
The third decan of Aquarius, twenty to thirty degrees, given to the Moon in the Chaldean order of rulers.
Do I need a birth time?
Not to establish the sign, which is settled. You do need one for the rising sign, the house layout and often the Moon.