The mismatch and the correction
The Earth takes about three hundred and sixty five and a quarter days to go round the Sun, and the calendar year is three hundred and sixty five. Without a correction, the Sun's position on a fixed date would slip backwards by about a quarter of a degree each year, which over a century would come to twenty five degrees, most of a sign.
The leap day is that correction. Adding a day every fourth February pushes the calendar forward by roughly one degree of solar motion, cancelling four years of accumulated slippage in one step. The result on any fixed date is a sawtooth: three years drifting one way, then a jump back.
What the sawtooth looks like on this date
The full spread on the seventeenth of August comes to under one and a half degrees, and the position in a leap year sits near the bottom of it while the third year after a leap year sits near the top. There is also a very slow secular drift from the century rules in the Gregorian calendar, but on this timescale it is small compared with the sawtooth.
The practical consequence is that the sign and the decan on this date are the same for everyone. Both edges of the range sit well inside the closing band of Leo, and the Virgo boundary is still five or six days off, so nothing here is year dependent.
Worth adding is that the ranges quoted here are noon positions. A birth at midnight sits about half a degree below the noon figure for that year and a birth late in the evening about half a degree above it, so the practical spread for real births is slightly wider than the noon spread. On this date the extra half degree changes nothing, since both edges remain far from any line.
The band and its ruler
Twenty four degrees puts the Sun in the third decan of Leo, the stretch from twenty degrees to the Virgo boundary. Mars rules it under the Chaldean count, which orders the planets by apparent speed from the opening of Aries, and Mars rules it under the triplicity scheme as well, which reaches the same answer through Aries as the first of the fire signs.
The tradition reads Leo as fixed fire under the Sun and reads this closing band as its more assertive part. Both are interpretations from the written record rather than conclusions from data, and they are worth using only with that label attached.
Stability is not the same as completeness
A steady solar position tells you the sign and the band and nothing more. The Moon moves up to fifteen degrees a day and can change sign during any given seventeenth of August, so two people born on the same date can have it in different signs. The Ascendant covers the whole zodiac in a day and depends on the latitude as well as the hour of birth.
The widget above works from the year, the time and the place rather than from a table, which is what makes the answer specific instead of typical. It gives the exact solar longitude, the distance to the nearest sign boundary, and the placements that a date on its own leaves entirely open.
Questions people ask about this
What sign is 17 August?
Leo, without qualification. The Virgo boundary falls around the twenty second, so this date sits several days clear of it in every year.
Why does the solar degree change from year to year?
Because the calendar year is a little shorter than the orbital year. The leap day corrects the difference every four years, which produces a small sawtooth rather than a steady slide.
How large is that variation?
Under a degree and a half in total on this date. That is enough to matter when a decan edge falls inside the range, which is not the case here.
What decan does 17 August fall in?
The third decan of Leo, ruled by Mars in both schemes still in common use. Every noon position for this date over the last century sits inside that band.
Will the date drift into Virgo eventually?
Not within any timeframe that matters to a birth chart. The Gregorian rules hold the calendar against the seasons to within a day over many centuries.
Does a stable sun sign make the rest of the chart predictable?
No. The Moon and the rising sign depend on the hour and the birthplace, and neither is constrained by the date in any useful way.