The straddle, in figures
Sampled at noon from 1930 to 2035, the Sun on 12 July stood between about nineteen and a quarter and a little over twenty and a half degrees of Cancer. The twenty degree mark sits inside that range rather than outside it.
So the honest answer for the face is year dependent. In the earlier part of a leap cycle the noon figure tends to fall below twenty and the position lies in the second face, while later in the cycle it clears the line and lands in the third. A page that names one face flatly for this date is right for some readers and wrong for others.
What the two faces carry
Below twenty degrees the second face of Cancer applies. The Chaldean order rules it with Mercury and the triplicity scheme assigns it to Scorpio and therefore to Mars. Above twenty degrees the third face begins, which the Chaldean order gives to the Moon and the triplicity scheme gives to Pisces and so to Jupiter.
The traditional readings differ accordingly, and the third face is the one where the face ruler and the sign ruler coincide in the Chaldean order, since Cancer itself is ruled by the Moon. Whether that doubling means anything is interpretation. That the two faces are described differently is simply a fact about the tradition.
Why the hour and the place matter more here
The Sun advances close to a degree from midnight to midnight, so on a date sitting on a face boundary the hour of birth can decide the answer on its own. A birth in the early hours and a birth late in the evening on the same 12 July can fall on opposite sides of the twenty degree line in a borderline year.
Time zone plays into it too. A position quoted for noon in one place corresponds to a different clock time elsewhere, so the birthplace shifts the figure as well as the hour. This is exactly the situation where a default noon calculation can hand you the wrong face without any warning that it was close.
This is where the widget above earns its keep against a printed range. A range cannot report a margin, because it has no year and no hour to work from, so it will hand a 12 July reader a face label with no indication that the label was decided by less than a degree. The widget returns a longitude and the distance to the nearest boundary, which on this date is the only way to know which side of the twenty degree line you were actually born on.
The sign, by contrast, is safe
None of this touches the sun sign. Twenty degrees of Cancer is ten degrees from the Leo boundary, which is around eleven days of solar motion, so no hour, year or birthplace in the sample makes this date anything but Cancer.
The Moon and the ascendant remain undetermined as on any date. Enter your year, hour and birthplace in the widget above and it will give your exact solar degree, which is what settles which side of the twenty degree line you were born on.
Nor is the borderline case unique to the Sun. The Moon changes sign every two or three days and may cross during this date, giving either of two answers, and across the sampled years it appears in all twelve. The ascendant runs the full circle daily, shifting about a degree every four minutes and depending on latitude as well as clock time. Those two, rather than the sun sign, are what actually distinguish two people who share this birthday.
Questions people ask about this
What sign is 12 July?
Cancer, without any doubt. The date sits about ten degrees short of Leo, which is far more than any variation between years or hours could cover.
Which decan is 12 July?
It depends on the year. The noon position ranges from a little over nineteen degrees to a little over twenty, so some years fall in the second decan of Cancer and others in the third.
How do I find out which one applies to me?
Use the widget above with your year, birth time and birthplace. It returns the exact solar degree, and anything below twenty is the second decan while anything above is the third.
Why do sources disagree about the decan for this date?
Because they pick one and print it. The position genuinely lies on either side of the boundary depending on the year, so a single flat answer cannot be right for everyone born on this date.
What is the difference between the two decans?
The rulers change. The second is Mercury in the Chaldean order and Mars in the triplicity scheme; the third is the Moon and Jupiter in those same schemes.
Does the hour of birth matter here?
More than usual. The Sun moves close to a degree in a day, which in a borderline year is enough to carry the position across the twenty degree line between morning and night.