How close is close
Across noon positions from 1930 to 2035, a 9 January birth places the Sun between roughly 18.1 and 19.5 degrees of Capricorn. The upper end of that band is half a degree short of the twenty degree line.
Half a degree is about twelve hours of solar motion. A birth late in the evening in a high year therefore comes closer to the boundary than the noon figure suggests, though it still does not cross it in the years covered here. This is the one date in the early part of January where the decan answer is worth calculating rather than assuming.
What the boundary separates
The second decan of Capricorn runs from ten to twenty degrees. Its Chaldean ruler is Mars and its triplicity ruler is Venus, through Taurus. The third decan runs from twenty to thirty. Its Chaldean ruler is the Sun and its triplicity ruler is Mercury, through Virgo.
So the two bands are read quite differently. One is Saturn's structure with force applied to it, the other is Saturn's structure turned towards discrimination and detail. Crossing that line changes the modifier, though not the sign, which stays Capricorn on both sides.
It is worth being clear that neither band is better than the other. The tradition attaches different imagery to each and says nothing about one being fortunate and the other not. What would change a reading is the position, and that is a matter of arithmetic rather than of judgement.
Why the sign itself is not in question
The Sun leaves Capricorn at thirty degrees, around 19 or 20 January. On 9 January it is ten or eleven degrees short of that, which is ten or eleven days of travel. No leap year adjustment, no time zone and no birth hour closes a gap of that size.
The degree itself is very stable. The calendar slips about six hours a year against the orbit and the leap day cancels the slip, so this date has held to a band under a degree and a half wide for more than a century. Boundary uncertainty on birthdays is a real thing, but it belongs to dates around 19 to 21 January, not to this one.
The tropical zodiac is measured from the March equinox rather than from the constellations, which is another reason this stays fixed. Talk of the signs having slipped refers to the constellations behind them, and it does not move the boundaries used here.
Everything the date leaves open
The decan and the degree describe one placement. The Moon moves about thirteen degrees a day and may be in either of two signs on 9 January depending purely on the hour. The rising sign passes through all twelve in a day and cannot be guessed from the date at all.
The widget above computes your solar degree from your own year and birthplace instead of a noon default, which on this date is the difference between assuming the decan and knowing it, and reports the distance from your Sun to the nearest sign boundary.
Capricorn itself is unchanged by any of this. It is cardinal earth under Saturn, read by the tradition as structural and patient, and that reading applies at nineteen degrees exactly as it does at nine or twenty nine.
Questions people ask about this
What decan is 9 January?
The second decan of Capricorn, but near its end. The Sun sits around nineteen degrees and the boundary is at twenty, so the exact figure is worth calculating rather than assuming.
What happens if my Sun is past twenty degrees?
You would be in the third decan of Capricorn, ruled by the Sun in the Chaldean order and by Mercury through Virgo in the triplicity scheme. The sun sign itself would be unaffected.
Is 9 January ever Aquarius?
No. The Sun does not reach Aquarius until it completes thirty degrees of Capricorn, which happens around 19 or 20 January. On 9 January it is roughly ten days short.
What degree is the Sun on 9 January?
Between about 18.1 and 19.5 degrees of Capricorn at noon across 1930 to 2035. A late evening birth sits toward the top of that band.
Do decan boundaries matter as much as sign boundaries?
Less, since the sign is unchanged either way. A decan boundary changes a secondary modifier, whereas a sign boundary changes the primary placement.
Why is the degree so consistent across a century?
Because the leap year rule is designed to keep the calendar aligned with the orbit. The Sun slips about six hours later each year and the extra February day resets it, producing an oscillation rather than a drift.