The part the date settles
The Sun is the slowest moving thing a calendar date can pin down, covering just under a degree a day. That is why the sun sign survives having no birth time attached, and why a date is enough to state it without hedging.
For this date the noon figure has stayed inside 1.9 to 2.3 degrees at the low end and 3.3 at the high end across the sampled century. Two or three degrees into Aries is early in the sign, a couple of days past the equinox, and comfortably clear of the Pisces boundary behind it.
Why the figure holds across a hundred years
The calendar and the orbit stay in step through the leap cycle. Each ordinary year the Sun ends up about a quarter of a degree further along at the same clock time, because the year is a quarter of a day short of the circuit, and the leap day then subtracts nearly all of the accumulated surplus at once.
The visible effect is a sawtooth roughly a degree and a half tall that repeats every four years. Nothing about it drifts over a century in a way you would notice on a birthday page. Your own place in the cycle is what the widget above computes, together with the distance in degrees from your Sun to the nearest boundary.
There is a slower change underneath, since the length of the tropical year is not perfectly constant and the Gregorian rule skips three leap days every four centuries. Neither effect is large enough to show on a page like this, and both are handled properly by any ephemeris rather than by a rule of thumb.
The band inside the sign
Decans cut each sign into three ten degree pieces. Anything below ten degrees of Aries falls in the first, and the Chaldean order gives that band to Mars, the sign's own ruler.
Where another sign's first decan is read as a blend of two planets, this one is read as a single influence at full strength. That is the tradition's reasoning, and the tradition is where it stops. The band is measurable and public, the reading is inherited and untestable, and a birthday page is more useful when it says which is which instead of running them together.
The decan is still worth having because it narrows a month down to about ten days. Every birthday in a thirty day stretch shares a sign, while a ten degree band separates the people born at the start of that stretch from the people born at the end, which is the distinction a sign name throws away.
The parts still missing
Ask what the Moon was doing and the date cannot answer. It moves about thirteen degrees a day and will be in one of two signs on this date, occasionally one of three, with the hour of birth deciding.
The rising sign is entirely a function of the clock and the birthplace. Every sign rises in the course of a day, so someone born at dawn and someone born at dusk on the same 23 March share this Sun and differ in nearly every angle of the chart.
This is the honest reason a sun sign describes so little. It is the one placement that a twelfth of the population shares with you, worked out from the least specific piece of information you have.
Questions people ask about this
Is 23 March always Aries?
Yes, in every year sampled at noon from 1930 to 2035. The equinox falls on 20 or 21 March across that range, so this date is always at least two days past the start of the sign.
How far into Aries is this birthday?
Between about two and three degrees at noon, which is the third day of the sign. The exact figure depends on your year and hour, and the widget above computes it.
Which decan ruler applies?
Mars, for the first decan of Aries. Because Mars also rules the whole sign, sign ruler and decan ruler coincide here rather than combining two different planets.
Does my birth time change my sun sign?
Not on this date. The Sun moves under a degree a day and the nearest boundary is more than two degrees behind, so no hour of the day puts it in Pisces.
Why do birthday pages all say the same three adjectives?
Because they are describing the sign rather than the date, and every date in a thirty day stretch shares the sign. The degree and the decan are what separate one Aries birthday from another.
What should I look up next?
The Moon sign and the Ascendant, both of which need a birth time. They change within a single day and account for most of what people expect a chart to say.