A line at twenty degrees
Each sign splits into three bands of ten degrees. In Aries the third band opens at twenty degrees, and in the Chaldean order it passes from the Sun, which holds the middle band, to Venus.
Nineteen degrees is the last full degree before that handover. At noon most years in the sample sit below the line, but the top of the range touches it, so the answer for an individual depends on the birth year and the hour rather than on the date by itself.
The band matters because it is the layer at which one Aries birthday stops reading like every other. The sign is shared with everyone born across a month; the ten degree band narrows that to about a third of them.
Why the year decides it
The Sun arrives at a given calendar date about a quarter of a degree further along in each ordinary year, and a leap day pulls it back by close to the amount the three previous years added. The result is a slow sawtooth within a window of roughly a degree and a quarter.
A birth in a leap year therefore sits near the bottom of that window and a birth in the third year after one sits near the top. On a date where the window brushes a band edge, that difference is the whole question. The hour matters too, since the Sun covers about a degree a day and a birth near midnight sits half a degree from the noon figure.
What the two bands are read as
The solar band is described as the Aries drive turned toward visibility and toward carrying something through in the open. The Venus band that follows is described as the same drive softened toward appetite, company and the enjoyment of what has been won.
Those descriptions are inherited convention and should be marked as such. The triplicity scheme, which builds bands from the rulers of a sign's element, assigns the last third of Aries to Jupiter instead of Venus, so even the ruler of the band ahead is disputed. Where two systems name different planets for the same ten degrees, the assignment is a convention rather than an observation.
Settling it, and everything else
The sign is never in question. Aries began at the equinox and continues to around the twentieth of April, so no year or hour places a 9 April birth outside it.
The band is a live question, and the widget above answers it by computing the exact solar longitude from a birth year, hour and place, then reporting the distance from the nearest boundary. It also fills in the placements the date cannot supply, since the Moon can be in either of two signs across this day and the rising sign can be any of the twelve.
Aries itself is cardinal fire under Mars, opening the year at the equinox, and that much is fixed for every birth in the sign. The rest of a chart is where the differences are.
Questions people ask about this
Which decan is a 9 April birthday in?
Usually the second, the band from ten to twenty degrees of Aries, but the noon position across the last century reaches as far as twenty degrees. Your own year and hour decide it, and the widget above will say which side you fell on.
What degree is the Sun on 9 April?
Between about eighteen and three quarters and twenty degrees of Aries at noon, depending on the year. Leap years sit at the low end and the third year after a leap year at the high end.
Does being near a decan line weaken anything?
No. A position at nineteen degrees is fully inside its band and one at twenty is fully inside the next. The tradition does not read a band edge as a blend, only as a boundary.
Who rules the third decan of Aries?
Venus in the Chaldean order and Jupiter under the triplicity scheme. This is one of the bands where the two systems in circulation genuinely disagree.
Is the sign itself ever in doubt on this date?
Never. The Sun does not reach Taurus until around the twentieth of April, so there is a fortnight of margin and no year in the modern calendar comes close.
Do I need my birth time for the decan?
It helps if your year puts you near the top of the range. Half a degree either side of noon is enough to matter when the line at twenty degrees is within a degree of your position.