A position that hardly moves
People assume a birthday's solar position drifts over a lifetime, let alone a century. It does not, and the reason is the leap year rule. The Sun's circuit takes about 365.2422 days while a common year runs 365, so each ordinary year the noon Sun ends up roughly a quarter of a degree further along.
Four of those quarters make close to a whole day, and the leap day removes it. The remaining pattern is a sawtooth about a degree and a half tall that repeats every four years, which is exactly the spread the figures for this date show.
Where four degrees sits in the sign
Aries begins at the March equinox, which in recent years falls on the twentieth. Five days later the Sun has covered four or five degrees of the sign, which is the early part of a thirty degree span.
That has practical consequences beyond the sign name. A Sun at four degrees makes its aspects to planets near four degrees of other signs, and in a house based chart it sits close to whichever cusp it follows. Neither of those has anything to do with the descriptions on a typical birthday page, and both are more use than another list of adjectives.
It also matters for anyone comparing two charts. Aspects are measured in degrees, so a Sun at four degrees of Aries makes a close square to a planet at four degrees of Cancer and no aspect at all to one at twenty. A sign name cannot express that, and a date range cannot produce it.
The first decan and its planet
Ten degree bands divide each sign into three. Anything under ten degrees of Aries falls in the first, which the Chaldean order assigns to Mars. Since Mars rules Aries as a whole, the decan repeats the sign's ruler rather than adding a second one.
The tradition reads that repetition as the sign expressed without mixture, and reserves the blended readings for the second and third bands where a different planet comes in. That is what the older texts say about the band. It is not a claim about anyone born in it, and a page that turns one into the other has quietly changed the subject.
The other reason the decan earns its place is that it narrows a month to about ten days. Everyone born across a whole sign shares the sign, whereas the band separates the opening third from the closing third, which is the only division inside a sign that most older sources use at all.
What still needs an hour
The Moon can be in either of two signs on a given 25 March, sometimes a third, because it crosses about thirteen degrees a day. A date alone will not tell you which.
The rising sign needs the clock and the birthplace, since all twelve signs come up over the horizon in a day and the rate depends on latitude. Nothing about the date constrains it.
Put together, that is why the sun sign is the least individual thing in a chart. It is shared by roughly a twelfth of everyone alive, while the placements that vary hour by hour are the ones a date can never reach. The widget above works out the ones it can once you supply a year, a time and a place.
Questions people ask about this
Is 25 March definitely Aries?
Yes. Every year from 1930 to 2035 puts the Sun several degrees into Aries at noon on this date, so no birth hour or time zone alters the sign.
How much does the degree vary between years?
By about a degree and a third in total. Years early in a leap cycle sit at the lower end of the range and years late in it at the upper end.
Which decan does this date fall in?
The first decan of Aries, zero to ten degrees, ruled by Mars in the Chaldean scheme. It is the band where the decan ruler and the sign ruler are the same planet.
Does the Sun move at a constant rate?
Almost, but not quite. The Earth's orbit is slightly elliptical, so the Sun's apparent daily motion varies between about 0.95 and 1.02 degrees across the year. Around late March it is close to a degree a day.
Why is the degree never published anywhere else?
Because most birthday pages are written from a table of date ranges rather than from a position, and a date range has no degree in it. Computing one needs an ephemeris.
What changes if I find my birth time?
Your solar degree becomes exact rather than approximate, the Moon resolves to one sign, and the Ascendant and houses become available. That is most of a chart.