How close the boundary is
Across noon positions from 1930 to 2035 the Sun on 10 March falls between about 19.0 and 20.4 degrees of Pisces. The decan boundary sits at twenty, which means the upper end of that range has already crossed it.
So this is one of the few birthdays where the year and the hour genuinely change something. Later positions in the leap cycle and later hours of the day push the Sun past twenty and into the final decan of the sign. Earlier ones leave it in the middle decan. A date range cannot resolve that; a calculated position can.
The mechanism behind the shift is the leap cycle. A calendar year is about a quarter of a day shorter than the orbit, so each year the Sun on a given date sits a fraction further on, and every fourth February resets it. That is why the range has a width at all rather than a single value.
What sits on either side of the line
Below twenty degrees is the middle face of Pisces, given to Jupiter by the Chaldean order and to Cancer, and so the Moon, by the triplicity scheme. Above twenty is the final face, which both schemes hand to Mars: the Chaldean sequence closes on it, and triplicity reaches it through Scorpio.
Those are not small differences in tone. Tradition reads the middle band as absorbing and the last as pointed, and a birth a few hours apart on this date can sit under either description.
Both schemes are in ordinary use and neither is more correct than the other. What matters is knowing which one a description is built on, because a decan ruler quoted without its scheme is impossible to check against anything.
Why the sign itself is safe
None of this touches the sun sign. Pisces runs from zero to thirty degrees, and twenty is squarely inside it. The equinox, where Aries begins, is still about ten days away in every year on record.
It is worth separating the two questions. Sign boundaries produce the arguments people have about cusps. Decan boundaries produce a quieter kind of uncertainty that hardly anyone checks, because hardly anyone knows the boundary is there.
Settling it, and what stays open
The widget above computes the Sun's actual longitude for your year, hour and birthplace, so it will report which side of twenty degrees you fall on rather than leaving it to a range. It also gives your distance from the nearest sign boundary, which here is large.
Beyond that the date settles little. The Moon can be in either of two signs depending on the hour, and the rising sign can be any of the twelve, since the horizon turns through the whole zodiac in a day. The Sun is the placement shared by everyone born this week, not the one that distinguishes you.
Nor does the decan question change how the sign is read. Someone who lands at nineteen point eight degrees and someone at twenty point two are both Pisces, both in the same sign by every measure, and differ only in which ten degree band their Sun happens to occupy.
Questions people ask about this
Which decan is 10 March in?
It depends on the year and the hour. The Sun on this date ranges from about nineteen to twenty and a half degrees of Pisces, and the decan boundary sits at twenty, so both answers are possible.
What changes if I am on the other side of twenty degrees?
The ruling planet of the decan. Below the line the Chaldean ruler is Jupiter, above it the ruler is Mars, and the triplicity scheme moves from Cancer to Scorpio at the same point.
Does that affect my sun sign?
Not at all. Pisces occupies the full thirty degrees and twenty is in the middle of it. Only the face within the sign is in question.
Is 10 March a cusp birthday?
Not in the usual sense, since that word is used for sign boundaries and the nearest one is over a week away. It is a decan boundary date, which is a different and much less discussed thing.
How do I find out which side I am on?
Use the widget above with your birth year, hour and place. It returns the solar longitude to a fraction of a degree, which is what the question turns on.
Why does the degree shift between years at all?
Because the calendar year and the orbital year are not the same length. The date creeps later by about six hours a year and the leap day resets it, so the same date lands within a narrow band rather than on a fixed point.