Near the edge of a band
Signs divide into three decans of ten degrees, and the first decan of Aries ends at ten. On this date the noon Sun has been between about 6.9 and 8.3 degrees in every year from 1930 to 2035, so it sits within two or three degrees of that inner boundary.
The sign itself is not remotely in doubt. Sign boundaries are thirty degrees apart and this one is a week of solar motion away in either direction. It is only the decan boundary that is close, and that is a subdivision rather than a change of sign.
The two planets on either side
In the Chaldean order the first decan of Aries goes to Mars and the second to the Sun. Because Mars also rules the sign, the first band repeats the sign ruler and the second brings in a planet that does not otherwise appear in Aries.
Traditional writers therefore read the two bands quite differently, the first as the sign undiluted and the second as the sign with an interest in standing and visibility added. Both of those are statements from the tradition about ten degree divisions, not findings about people, and it is worth carrying that distinction with you when you compare sources.
The order itself is worth knowing, since it explains why the assignments look arbitrary. Chaldean sequence runs Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, and it is applied without interruption to the thirty six bands starting from zero degrees of Aries. Mars falls first and the Sun fourth simply by position in that run.
How steady the figure is
Across the sampled century the position varies by about a degree and a third, and that is the whole of it. The mechanism is the leap year rule. A common year runs about a quarter of a day short of the Sun's circuit, so the noon Sun advances by roughly a quarter of a degree each year, and February's extra day subtracts nearly four years of that in one step.
The result oscillates in a four year pattern rather than drifting. If you were born late in a leap cycle your degree sits at the top of the range and might touch the ninth degree. The widget above settles it from your year, hour and birthplace.
A birth time changes the picture more than the year does. The Sun covers about a degree a day, so a birth near midnight sits roughly half a degree from the noon figure this page quotes, in one direction or the other depending on the time zone.
What a date will never tell you
The Moon crosses about thirteen degrees a day and will be in one of two signs on this date, sometimes one of three. Nothing in the calendar square narrows that further.
The Ascendant is decided by the birth hour and the latitude of the birthplace, since the whole zodiac rises in twenty four hours. It changes roughly every two hours, which is why two births on the same day can produce charts that look nothing alike.
Mercury and Venus stay near the Sun and so are loosely constrained by the date, Mercury within about a sign of Aries and Venus within two. That is the limit of what a date on its own can offer.
Questions people ask about this
Which decan is a 28 March birthday in?
The first decan of Aries in every sampled year, though it sits within two or three degrees of the second. In the Chaldean order the first band belongs to Mars and the second to the Sun.
Could my chart show the second decan instead?
It is unlikely on this date but worth checking if you were born late in a leap cycle and in the evening, since both push the degree higher. The widget above gives your exact figure.
Does a decan boundary matter as much as a sign boundary?
No. Crossing a sign boundary changes the sign, the element and the ruler. Crossing a decan boundary changes only which planet the ten degree band carries, and the sign stays the same.
Is the sign in any doubt on this date?
None. The nearest sign boundary is around a week of solar motion away, so no birth hour or time zone comes close to moving it out of Aries.
Why is the Sun the second decan ruler in Aries?
Because the Chaldean sequence runs Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon and repeats through all thirty six bands from the start of Aries. The Sun simply falls fourth in that run.
What is the next thing to look up?
The Moon sign and the Ascendant, since both need a birth time and both vary within a single day. They account for far more of a chart's individuality than the Sun does.