Where the Sun actually stands
Sign membership is a coarse measurement. It tells you the Sun was somewhere within a thirty degree arc, which is a stretch the Sun takes about a month to cross. The finer statement is the longitude itself, and on 1 April that has fallen between roughly ten and three quarters and twelve and a quarter degrees of Aries in every year from 1930 to 2035.
That is a window of about a degree and a half across more than a century. The reason is the leap year cycle. Each ordinary year the Sun arrives at a given calendar date about a quarter of a degree further along, and each leap day pushes it back by very nearly the amount three ordinary years added. The calendar and the orbit stay in step, so the drift never accumulates.
Just inside the middle band
The oldest surviving subdivision of the zodiac splits each sign into three bands of ten degrees. Eleven degrees of Aries is therefore in the second band, close enough to the opening line that a birth in a leap year sits nearer the start of it than one three years later.
In the Chaldean sequence, which orders the planets by apparent speed and runs that order continuously from the first degree of Aries, the second band of Aries belongs to the Sun. The sign itself is ruled by Mars, so the tradition reads this stretch as Mars with a solar qualification: the drive of the sign turned toward being seen and toward holding a position rather than simply starting one.
What the tradition attaches, and what it does not
Aries is cardinal fire. Cardinal signs open a season, and the reading built on that is initiative, appetite for a fresh start, and impatience with the middle of a process. The solar band is described as adding warmth and a wish for the effort to be acknowledged.
All of that is inherited interpretation rather than measurement. The degree can be checked against an ephemeris by anyone. The character attached to the band cannot be checked at all, and different schools attach different characters to the same ten degrees. Keeping the checkable part separate from the interpretive part is the honest way to read any of it.
It is also worth saying what a band cannot do. It cannot make one Aries birthday better placed than another, and no part of the older material treats a decan as fortunate or unfortunate in itself. It narrows a description, and that is the whole of its job.
Everything this date leaves undecided
The Moon moves between twelve and fifteen degrees a day, so on 1 April it usually occupies two signs across the twenty four hours and occasionally touches a third. Two people with this birthday can therefore have genuinely different lunar placements, decided by the hour rather than the date.
The rising sign is wide open, since it turns through all twelve signs in a day and depends on the minute of birth and the latitude of the place. Every house cusp follows from it. The widget above takes a year, an hour and a birthplace, returns the exact solar degree rather than the century wide window, and reports how far the birth fell from the nearest sign boundary.
None of that weakens the sun sign. It puts it in proportion as one measured position among the several a full chart records.
Questions people ask about this
Is 1 April always Aries?
Yes. The Sun enters Aries at the March equinox and leaves it around 19 or 20 April, so this date sits well inside the sign in every year. No birth time or birthplace can change the answer.
What degree is the Sun on 1 April?
A little past eleven degrees of Aries, varying by about a degree and a half across the last century. Leap years sit at the low end of that window and the third year after a leap year sits at the top.
Which decan is a 1 April birthday in?
The second, which covers ten to twenty degrees of Aries. Because eleven degrees is near the opening of the band, the position is early in the decan rather than in the middle of it.
Who rules the second decan of Aries?
The Sun, under the Chaldean order. The triplicity scheme reaches the same answer for this band, since the second third of a fire sign is handed to the Sun in that system as well.
Does the decan override the sign?
No. The tradition treats the sign as the main statement and the decan as a qualification on it. A solar band inside Aries is still read as Aries, described more narrowly.
Can two people born on 1 April have very different charts?
Easily. Their Suns will agree to within a degree, but the Moon can differ by a whole sign and the rising sign can be any of the twelve. The Sun is the least individual part of a chart, not the most.