Crossing into the second band
Ten degrees is where the first decan of Aries ends. The noon figures for this date run from about 9.8 to 11.2 degrees across 1930 to 2035, so the great majority of years are past the line and a few at the very bottom of the range are not.
The Chaldean order gives the first band to Mars and the second to the Sun. Since Mars rules Aries itself, the change here is from a decan that repeats the sign ruler to one that introduces a planet the sign does not otherwise carry.
Ten degrees is about ten days of solar motion, which is why the band is the useful unit here. It separates the opening stretch of Aries from the middle of it, a distinction a thirty day sign name discards entirely and one that older sources take seriously.
What the tradition makes of it
Older writers read the second decan of Aries as the sign's material with the Sun's concern for standing and visibility laid over it, in contrast to the first band which they treat as undiluted. The reasoning is structural rather than observational, drawn from the planetary order used for the hours of the day.
It is a reading and should be labelled as one. The band is a measurable ten degrees of longitude that any ephemeris will confirm. The character attached to it comes from a chain of inherited texts and is not the sort of claim that can be checked, which is a reason to read it with interest rather than with authority.
Why your own year matters here
On a mid sign date the year barely matters, because the degree moves by only about a degree and a third across a century and there is nothing nearby for it to cross. On this date that same small variation sits astride a division.
The variation itself is arithmetic. A common year falls roughly a quarter of a day short of the Sun's circuit, so the noon Sun advances about a quarter of a degree annually, and February's extra day removes what four years build up. Where in that cycle your year falls is what decides the answer, and the widget above works it out from your year, hour and birthplace.
Underneath the four year pattern there is a slower correction, since the Gregorian calendar omits three leap days every four hundred years to keep the average year close to the Sun's circuit. It is far too small to affect a figure quoted to the nearest degree, and an ephemeris handles it without anyone needing to think about it.
What the last day of March does not decide
The sign is not in question. Aries runs to the third week of April, so no birth hour or time zone brings this date near a sign boundary.
Everything that varies within a day is still open. The Moon travels about thirteen degrees daily and can be in either of two signs on this date, sometimes a third. The Ascendant cycles through the whole zodiac in twenty four hours and depends on the clock time and the latitude of the birthplace.
Mercury and Venus are loosely constrained by their nearness to the Sun, which puts Mercury within about a sign of Aries here and Venus within two. Beyond that, a date has given everything it has, and the rest needs a time and a place.
Questions people ask about this
Which decan is a 31 March birthday in?
The second decan of Aries in most years, ruled by the Sun in the Chaldean order. Years at the very bottom of the range can still fall in the first decan under Mars, so it is worth checking your own.
What degree is the Sun on 31 March?
Between about ten and eleven degrees of Aries at noon. Your birth hour shifts that by up to half a degree either way, since the Sun covers roughly a degree a day.
Is the sun sign ever in doubt?
No. Aries runs until around 19 or 20 April, so this date sits well inside it in every year, at every hour and in every time zone.
Why does a decan matter?
It is the layer that distinguishes one birthday in a sign from another. Everyone born across a month shares the sign, while the ten degree band narrows that to about ten days.
Do all sources give the Sun for this band?
Those using the Chaldean order do. Other allocations exist, including the Egyptian decans, which are a separate system rather than a variant, so it is worth knowing which one a page is quoting.
What should I work out next?
The Moon sign and the rising sign. Both need a birth time, both change within a single day, and both carry more individual detail than the Sun does.