A sign is a month, not a point
The Sun spends around thirty one days crossing Cancer, moving a bit less than a degree a day. Someone born in the opening days is at one or two degrees; someone born in the closing days is at twenty eight or twenty nine. Calling both of them the same thing is accurate but very coarse.
The face system exists to make that distinction. Three bands of ten degrees split the month into thirds, and older technical writing treats those thirds as meaningfully different. It is the layer that answers the question of which Cancer birthday you actually have.
Sixteen degrees, measured
At noon in each year from 1930 to 2035, the Sun on 8 July stood between about fifteen and a half and just under seventeen degrees of Cancer. That is past the fifteen degree midpoint in every year of the sample, and comfortably inside the ten to twenty degree band.
The tightness of the range comes from the leap year cycle. A common year leaves the Sun about six hours late at a given longitude and a leap February pulls it back about a day, so the drift and the correction largely cancel and the figure stays put over a century.
What the second face carries
Chaldean order runs the seven traditional planets in sequence through the thirty six faces of the zodiac, and the second face of Cancer comes out to Mercury. The triplicity scheme instead assigns the faces of a sign to the three signs of its element in order, which gives this one to Scorpio and therefore to Mars.
In the inherited reading, a Moon ruled sign with Mercury on the face is described as watchful and articulate rather than simply receptive, while the Scorpio assignment is read as adding depth and persistence. Both are tradition. Neither follows from the measurement, and this site keeps the two labelled separately.
The disagreement between the schemes is not a corruption of one by the other. They were built for different purposes. The Chaldean order is a continuous cycle through the whole zodiac and takes no interest in where signs begin and end, so a face ruler under it carries no relation to the sign ruler at all. The triplicity arrangement starts over inside each sign and keeps everything within the element, which makes it tidier and easier to teach. Sources differ on this face because they inherited different halves of the material, not because one of them made an arithmetical slip.
The parts still missing
Position in the sign is one piece of a chart and the date supplies only that piece. The Moon crosses sign boundaries every two or three days and may change during this very date. The ascendant depends on the minute of birth and on the latitude of the place, and it works through the entire zodiac each day.
That is why the widget above asks for more than a date. Given a year, an hour and a birthplace it reports a single solar degree instead of a range, and it states how far that degree sits from the nearest boundary between signs.
Distance from the edges is worth quoting too, since it settles the cusp question outright. Sixteen degrees of Cancer leaves sixteen degrees behind to Gemini and fourteen ahead to Leo, which at roughly a degree a day is about seventeen days and fifteen days respectively. The ingress itself only moves by a day and a half or so between years, so nothing about the year, the hour or the time zone brings this date within reach of either boundary.
Questions people ask about this
Is 8 July Cancer?
Yes, in every year checked at noon from 1930 to 2035. The date is roughly halfway through the sign and nowhere near either ingress.
How far into Cancer is this date?
About fifteen and a half to seventeen degrees, which is a little over halfway through the sign's thirty degrees.
Which decan is that?
The second decan of Cancer, ten to twenty degrees. It is ruled by Mercury in the Chaldean order and by Mars through Scorpio in the triplicity scheme.
Do people born in different parts of Cancer differ?
In the traditional material, yes, because the face changes and with it the second ruler. Whether that difference means anything is a question of interpretation rather than measurement.
Will I be Leo in some years?
No. Leo begins around 22 July and the Sun on this date is still roughly a fortnight of motion short of that boundary in every year sampled.
What decides my exact figure?
The year, the hour and the birthplace. Those three together give a single degree instead of a range, which is what the widget above returns.