A hundred and eighty degrees, and what sits at each end
An opposition is half the circle, with an orb of eight degrees on either side. The two bodies fall in opposing signs, which always share a mode and always differ in element: Aries against Libra, Taurus against Scorpio, Gemini against Sagittarius. They agree about how to operate and disagree about what they are operating on.
The Moon takes about twenty seven days to circle the zodiac and Mars about two years, so the opposition forms once in each lunar month and holds within orb for roughly a day. The aspect is therefore no rarer than any other lunar contact, and its interest lies in its structure rather than its scarcity.
Mars retrograde is worth noting. Mars is retrograde around the time it is closest to Earth and brightest in the sky, which is also when it is opposite the Sun. That does not tie it to the Moon, but it does mean an opposition to the Moon formed during a retrograde period involves a Mars that is unusually prominent physically, and the tradition reads a retrograde Mars as turning its force inward.
The confrontation the aspect describes
The Moon stands for what you need and what you do without choosing. Mars stands for what you are prepared to fight for and how you go about cutting something off. An opposition arranges them so that each is permanently in the other's field of view.
The common description is a person who is aware of their own aggression, which is not the same as controlling it. Where a conjunction fuses the two so that the person cannot see the join, an opposition separates them enough that the conflict is legible. That is why the tradition treats oppositions as workable: you cannot negotiate with something you cannot see.
The other standard reading is projection. An opposition is the classic aspect for meeting your own material in other people, so the Mars end is often described as arriving from outside, in the form of people who provoke exactly the reaction the chart already contains. Astrologers differ on how much weight to give this, and it is an interpretive claim rather than a technical one.
Axis, sign and dignity
The house axis names the arena. On the first and seventh it is the relationship version, and it is the placement where the projection reading is most often applied. On the fourth and tenth it becomes home against career, with the older sources describing a domestic life that carries the strain of a demanding public one. On the third and ninth it turns into argument as a way of thinking.
Dignity decides the balance. Mars in Aries or Scorpio opposing a Moon in Libra or Taurus gives the Mars end the upper hand. Mars in Cancer or Libra, where it is weak by the traditional scheme, opposing a strong Moon produces the reverse, and the aspect reads as irritation that never quite converts into action.
Orb, birth time and the check above
An opposition inside two degrees is a defining feature of a chart. From three to five degrees it is a working pattern. At seven or eight it is a background note, and once it crosses into adjacent signs it loses the mode agreement that makes the opposition coherent, so it should be read down accordingly.
Birth time matters more here than for slow pairs, because the Moon covers about half a degree an hour. An unrecorded time makes both the orb and the house axis uncertain, and the house axis is where most of this aspect's meaning lives.
The check above returns the exact separation, both signs, both houses and the state of Mars at your birth, which is the information needed before deciding whether the description above applies to you at all.
Questions people ask about this
What does Moon opposite Mars mean in a natal chart?
It means the two bodies were about a hundred and eighty degrees apart at birth, in opposing signs, within an eight degree orb. The tradition reads it as instinctive need and assertive drive confronting each other openly.
Is this aspect harder than the square?
Not harder, differently shaped. A square operates out of sight and tends to produce reaction, while an opposition is visible from both ends and tends to produce negotiation, or an argument the person can at least describe.
What is the projection reading?
It is the idea that one end of an opposition is met through other people rather than owned directly, so the Mars end shows up as provocation from outside. It is a widely used interpretation, not a technical feature of the geometry.
Do the two signs always oppose each other?
In a tight opposition, yes, and they will share a mode while differing in element. A wide one can cross a boundary and lose that structure, which is a good reason to read wide oppositions cautiously.
How does a retrograde Mars change it?
Mars spends roughly two months of every twenty six retrograde. In that state it is conventionally read as holding its force in rather than discharging it, so the opposition is described as more internal and slower to surface.
Why does the house axis matter so much?
Because the aspect describes a confrontation, and the houses say what it is about. The same opposition on the fourth and tenth axis and on the third and ninth produces two descriptions with almost nothing in common.