The band, and why it is worth knowing
Decans are among the oldest subdivisions in the western tradition, older than most of the aspect technique in daily use. Thirty six of them cover the zodiac, three to a sign, and the Chaldean order assigns each one a planet drawn from the sequence used for the hours of the day.
The first decan of Aries covers zero to ten degrees and belongs to Mars. On this date the noon Sun has been between 2.9 and 4.3 degrees in every year from 1930 to 2035, which places the birthday near the start of that band with six degrees or so still ahead of it.
Ten days of solar motion is roughly what a decan covers, so the band is the difference between a birthday early in a sign and one at its close. That is the level at which one Aries birthday stops resembling another, and it is missing from nearly every page written from a table of date ranges.
What the tradition attaches to it
Mars rules Aries outright, so a Mars decan inside Aries is the one case where the band adds no second planet. Older sources describe it as the sign at its least diluted, and give the same reading to the sign and the band together.
The Egyptian decans and the Chaldean rulers are not the same system, and a modern source quoting a decan ruler is almost always using the Chaldean one. Knowing which scheme is in front of you matters more than choosing between readings, because the two allocate different planets to the same ten degrees. Either way, the ten degree band is a measured position and the character attached to it is interpretation.
Why the degree stays put
The range across a century is about a degree and a third, which is narrower than most people expect. The reason is arithmetic rather than astronomy. A common year of 365 days is roughly a quarter of a day shorter than the Sun's circuit, so the noon Sun gains about a quarter of a degree annually, and February's extra day gives back almost exactly what four years accumulate.
That leaves a small repeating sawtooth and no meaningful drift. The widget above places your own year inside it and reports the resulting degree along with the gap to the nearest sign boundary.
There is also a slow underlying change, because the tropical year is not exactly 365.2422 days and the Gregorian rule drops three leap days every four hundred years. Both effects are far too small to alter a figure quoted to the nearest degree, which is why an ephemeris rather than a rule of thumb should settle any exact position.
What the date leaves undecided
The Moon covers about thirteen degrees a day, so on this date it can be in either of two signs and sometimes a third. The date narrows the possibilities and no more.
The Ascendant is not narrowed at all. All twelve signs rise in the course of a day, which makes the rising sign a function of the birth hour and the latitude of the birthplace rather than of the calendar. It is the placement people most often expect a birthday to supply, and the one it never can.
Mercury and Venus are partly constrained, since both stay near the Sun. On this date that puts Mercury within a sign or so of Aries and Venus within two, which is useful without being an answer.
Questions people ask about this
What decan is a 24 March birthday?
The first decan of Aries, the band from zero to ten degrees. In the Chaldean allocation it is ruled by Mars, which is also the ruler of the sign itself.
What degree is the Sun on this date?
Between about three and four degrees of Aries at noon, in every year from 1930 to 2035. Your birth hour and birthplace shift it slightly within that range.
Could this date ever be Pisces?
No. The equinox falls on 20 or 21 March across the sampled century, so the Sun has been in Aries for three or four days by the time this date arrives.
Why do some sources give a different decan ruler?
Because more than one allocation exists. The Chaldean order gives Mars for this band, while some modern books substitute a planet from a different scheme. Compare readings only within a single system.
Does the decan change my sun sign reading?
It refines it rather than replacing it. The sign stays Aries and the decan says which third of the sign, which is the layer that distinguishes one Aries birthday from another.
What do I need to go further?
The hour and the place of birth. Those fix the Ascendant, the house cusps and the Moon's exact position, which is where most of an individual chart actually lives.