The position both schemes agree on
At noon in every year from 1930 to 2035, the Sun on 10 July stood between about seventeen and a half and just under nineteen degrees of Cancer. That places it in the second face, the band from ten to twenty degrees, without exception across the sample.
The narrowness of the range is a by-product of the leap year cycle. A common year runs a quarter of a day short and leaves the Sun about six hours behind, and February's extra day pulls the calendar back by roughly a day every fourth year, so the two effects largely cancel.
Eighteen degrees also puts this birthday a long way from any edge. It is eighteen degrees past the Gemini boundary and twelve short of the Leo one, which at the Sun's early July rate is about nineteen days behind and thirteen ahead. Anyone who has been told they sit on a cusp can measure the claim against that, since the ingress moment itself only shifts by around a day and a half from year to year.
The Chaldean order
The older of the two schemes ignores sign boundaries and runs the seven traditional planets in their classical speed sequence, Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, continuously around the thirty six faces of the zodiac starting from the first face of Aries.
Counting through that cycle lands Mercury on the second face of Cancer. Because the sequence takes no notice of signs, the rulers of consecutive faces within one sign have no particular relationship to each other or to the sign ruler, which is part of what the scheme was for.
What the sequence is taken to add here is Mercury's particular business: naming, counting, carrying messages between one thing and another. Applied to a water sign the older writing reads it as impressions that get articulated rather than only felt. That is the concrete content of the assignment, and it is inherited description rather than anything the position measures.
The triplicity scheme
The other common arrangement keeps everything inside the element. The three faces of a sign are given, in order, to the three signs sharing its element, beginning with the sign itself. For Cancer that is Cancer, then Scorpio, then Pisces, so the second face belongs to Scorpio and takes Mars as its ruler.
This scheme is tidier and much more common in modern writing, which is why Mars turns up so often for this birthday. It is not more correct, only more recent in popular use. A page that quotes a face ruler without naming the scheme has made a choice and hidden it.
What follows, and what does not
Tradition reads Mercury on this face as sharpening a Moon ruled sign towards observation and speech, and reads the Scorpio assignment as deepening it towards persistence. Those are inherited descriptions and should be treated as such. The measurement underneath them is only a longitude.
Nothing else about the chart follows. The Moon changes sign every two or three days and may cross during this date, and the ascendant depends on the minute and the latitude of birth. The widget above uses your year, hour and birthplace to give a single degree and the distance from it to the nearest sign boundary.
The Moon is the placement that most separates two charts on this date, and it is also, unusually, the ruler of the sign. Cancer belongs to the Moon, so the body the tradition treats as the chart's first reference is the fastest one in the sky. It can be in either of two signs across a single 10 July and in any of the twelve across different years, while the ascendant covers all twelve in a day and changes degree every four minutes.
Questions people ask about this
What sign is 10 July?
Cancer, in every year checked. The date sits well past the June ingress and well short of the move into Leo around 22 July.
Is the decan ruler Mercury or Mars?
Both, in different schemes. The Chaldean order gives the second face of Cancer to Mercury, while the triplicity scheme assigns it to Scorpio and so to Mars.
Which scheme should I use?
Either, as long as you say which. They answer slightly different questions and have both been in use for a long time, so mixing them without labelling is the only real error.
What degree is the Sun on this date?
Roughly seventeen and a half to nineteen degrees of Cancer at noon, depending on the year, with the hour of birth shifting it slightly within that.
Does the face boundary at twenty degrees affect this date?
Not in the sampled century. The highest noon figure is a little under nineteen degrees, so the position stays inside the second face all day in every year.
Are decans the same as faces?
In practice the words are used interchangeably for the ten degree divisions. Some writers reserve decan for the triplicity scheme and face for the Chaldean order, which is worth knowing when sources disagree.