A line that runs through the date
Leo is not in question on the twelfth of August. The Virgo boundary is still ten days away and the Cancer boundary three weeks behind, so no year, hour or longitude has ever moved the sign. That part can be stated flatly.
The band is another matter. The ten degree divisions inside a sign have edges of their own, and the one at twenty degrees passes straight through this date's range. In most years the noon Sun is still short of it and the birthday belongs to the middle band. In the highest years, and in births late in the day, it has already crossed and the birthday belongs to the last band instead. Anyone writing that this date is flatly one decan or the other is guessing.
What changes when the line is crossed
Below twenty degrees the band is Jupiter's, in both the Chaldean count and the triplicity scheme, which reach it independently and agree. Above twenty degrees the band passes to Mars, and again the two schemes agree, the Chaldean order by planetary speed and the triplicity scheme by way of Aries.
The traditional readings differ in temperature more than in kind. The Jupiter stretch of Leo is described as the open handed and expansive part of the sign; the Mars stretch is described as the sharper and more combative part, quicker to contest something. Both are inherited interpretations rather than findings, and they qualify a description of Leo that stays the same either way.
It is also worth being clear that the boundary is a convention about where to divide the circle, not a physical feature. Nothing happens to the Sun at twenty degrees of Leo. The line matters because a scheme puts it there and readings are built on which side of it a position falls, which is precisely why the position has to be calculated rather than guessed from a table.
Why the position barely moves and yet still matters
The solar position on a fixed calendar date is remarkably stable. Leap years pull it back by nearly a day of motion and the three following years push it forward again, so the whole hundred year spread comes to well under two degrees. On most dates that stability means the band never changes.
This date is the exception, because the line happens to sit inside the narrow window rather than outside it. A small spread is enough to straddle a boundary if the boundary is in the wrong place, which is why the answer here has to be worked out from a specific year rather than looked up in a table.
The rest of the chart is untouched by any of this
None of the above says anything about the Moon, which moves between twelve and fifteen degrees a day and regularly changes sign during a single date. Nor about the rising sign, which cycles through all twelve signs every twenty four hours and depends on the latitude of the birthplace as well as the hour.
The widget above takes the year, the time and the place, returns the solar longitude to the degree, and reports how far the position sat from the nearest boundary, which on this date is the useful number. A description built on the sun sign alone is describing a twelfth of everyone born, and that is not really a description of anybody.
Questions people ask about this
Is 12 August in the second or third decan of Leo?
It depends on the year and the hour. The noon position across the last century runs from just under nineteen degrees to just over twenty, and the third decan begins at twenty, so most years fall in the second and some in the third.
What sign is 12 August?
Leo in every year. The nearest sign boundary is more than a week away, so nothing about the birth time or place can change it.
Which ruler applies to my band?
Jupiter below twenty degrees and Mars above it. Both schemes in general use agree on each side, so the only question is which side of the line your birth falls on.
How do I find out which side I am on?
Enter your birth year, hour and place in the widget above. It computes the solar longitude directly rather than reading it off a date table, which is the only way to settle a case this close.
Does being near a decan edge weaken anything?
No. A position at nineteen and a half degrees is fully inside its band and so is one at twenty and a half. Nearness to an edge is a fact about the number, not a dilution of the reading.
Why do other pages state one decan for this date?
Because they work from a fixed date table rather than a calculation. A table cannot represent a boundary that falls inside the date's own range of positions.