A full moon, measured
An opposition is a hundred and eighty degrees between two bodies, with an orb of about eight for the Sun and Moon. The Moon covers around thirteen degrees a day against the Sun's one, so the pair drifts through that sixteen degree window in a little over a day either side of the exact full moon.
That gives roughly the same slice of the lunar month as the conjunction, so this is a common aspect rather than a special one. The distinguishing feature is geometric. Your two most important bodies are as far apart as the zodiac allows, which means they sit in opposite signs and, in most house systems, in opposite houses.
The astronomy is unusually literal here. At a full moon the whole lit face of the Moon is turned towards us because the Earth is between it and the Sun. Nothing is hidden and nothing is merged.
What the opposition does to the pair
The tradition reads the Sun as deliberate purpose and the Moon as the reflex under it, the thing you do before you have decided anything. An opposition puts those two in permanent dialogue. Each one can see the other, and neither can absorb it.
The usual description is awareness at the cost of ease. You tend to know what you are doing and why, because the two positions are constantly commenting on each other. What is harder is acting without a counter-argument arriving first. People with this aspect often describe a lifelong habit of arguing both sides of their own decisions, sometimes usefully and sometimes to a standstill.
Opposite signs share a polarity and a mode, which softens the picture. Aries and Libra are both cardinal, Taurus and Scorpio both fixed. The pair is not random opposition, it is two ends of one axis, and the reading improves when you treat them as one subject argued from two positions rather than two subjects.
Other people as the third party
Because an opposition is the aspect of the other person, this configuration is often read through relationships. What you cannot reconcile internally tends to get handed to whoever is nearest. One half of the axis gets lived out at home and the other at work, or one half in yourself and the other in a partner.
That is not a prediction, it is a pattern astrologers have found convenient. Its practical use is diagnostic. If you find yourself repeatedly attracted to, or irritated by, people who embody one end of your Sun and Moon axis, the tradition would say you have parked that end outside yourself for the time being.
Orb, houses and how much of this applies
Orb does most of the work. Inside two degrees the split is obvious to the person living it. Past six it is a structural note rather than a felt one, and a Sun at twenty eight degrees Leo with a Moon at three degrees Pisces has broken the clean sign opposition altogether, which changes the reading substantially.
The house axis then aims it. The same aspect across the fourth and tenth reads as family against career. Across the first and seventh it reads as self against partner. Across the sixth and twelfth it is far less visible from outside and often shows up as a private argument about work and withdrawal.
The check above gives you the exact separation, both signs and both houses. That is the part a general description cannot guess, and it is where the useful reading actually starts.
Questions people ask about this
Does Sun opposite Moon mean I was born at a full moon?
Yes, within about a day either side. The exact full moon is the moment the two bodies are a hundred and eighty degrees apart, and the aspect holds while they are within roughly eight degrees of that.
Is this a difficult aspect?
The tradition treats it as demanding rather than damaging. An opposition supplies awareness and withholds ease, so the same configuration reads as balance in one chart and stalemate in another depending on sign, house and what else aspects the axis.
Are my Sun and Moon signs always opposite?
Usually, since the two are close to a hundred and eighty degrees apart. A wide opposition can slip a sign at either end, giving something like Sun in Leo and Moon in Pisces, and that version loses the neat polarity the classic reading depends on.
How is it different from Sun conjunct Moon?
A conjunction merges the two so there is no second opinion available internally. An opposition keeps them apart so there is always one. The new moon birth is single-minded and the full moon birth is self-aware, and each lacks what the other has.
Does the Moon's speed matter here?
It matters for how quickly the aspect forms and separates, not for its meaning. The Moon moves about thirteen degrees a day, so an exact opposition becomes a wide one within half a day, which is why birth time precision changes the orb noticeably.
Which houses should I look at?
The two the Sun and Moon occupy, and the axis they make. The house axis is what turns an abstract polarity into a specific and recurring subject, and it is usually more informative than the signs on their own.