The Moon is what is really being talked about
Cancer is ruled by the Moon, which stands for habit, memory and what a person needs in order to feel unthreatened. So when a list says Cancer and Aries clash, the underlying claim is that the Moon is uncomfortable under pressure from Mars, and that is a claim about two planets that can be checked in any pair of charts.
It also explains why the warnings are phrased the way they are. The complaints attributed to Cancer in these combinations are complaints about tone rather than about substance: not that the wrong thing was decided, but that it was decided too loudly, or too publicly, or without anyone noticing the mood first.
Four cardinal signs sharing one crossing
Cancer, Capricorn, Aries and Libra are the cardinal signs, each opening a season, and together they form the cross that hard angles to Cancer are drawn from. Capricorn sits 180 degrees away; Aries and Libra sit 90 degrees away on either side.
Because all four initiate, none of them waits. The friction the tradition describes is not resistance but collision: two people both moving first, in different directions, on the same subject.
Cancer and Capricorn is a division of labour
The axis runs between private life and public standing, home and career, the thing kept safe and the thing built in view. Traditional texts read this as a split of responsibilities before they read it as a quarrel, which is why so many long partnerships sit on it.
Either party can find the arrangement a relief or a grievance, and the angle has nothing to say about which. That is the general point about oppositions, and it applies here more visibly than almost anywhere on the wheel.
What actually shows strain
A square is 90 degrees between signs of the same mode and unlike elements, and its force depends entirely on how exact it is. Two suns twenty degrees off a true square are not in aspect, and a list working from birth months cannot tell.
The contacts that do predict difficulty run between personal points. Saturn from one chart on the other's Moon is the heavy one for a Cancer, read as affection rationed. Neptune on a personal planet blurs. Hard Mars contacts describe how an argument accelerates. A tight Moon aspect between two charts tells a Cancer more than any sun-sign square ever will. The synastry calculator reads those angles with their orbs, and synastry sets out the method.
Read the combination itself
The Cancer and Capricorn page treats the opposition as a pairing rather than a warning, and says what the two build well as well as where they grind. That is more use than a place in a ranking.
With two birth times, compare the charts and find out whether the difficulty exists in this particular case.
Questions people ask about this
Which signs are the hard angles for Cancer?
Capricorn opposite, Aries and Libra square. The tradition calls these hard rather than worst.
Is Capricorn a bad match for Cancer?
Not by the geometry. The axis describes a division between private and public life that either party can use or resent.
Why is the Moon central here?
The Moon rules Cancer, so the alleged clashes are really claims about how the Moon fares under contact from Mars or Saturn.
What predicts a genuinely hard pairing?
Saturn and Neptune contacts to personal planets, hard Mars aspects, and tight orbs rather than sun signs.