One hundred and eighty degrees, eight degrees of orb
An opposition is a half circle between two bodies, allowed an orb of eight degrees on either side. Because the Moon travels about thirteen degrees a day and Jupiter barely moves in comparison, the pair sweeps through that sixteen degree band in something over a day.
The Moon opposes Jupiter once every lunar circuit, so roughly monthly, about a fortnight after it last conjoined it. A tight opposition inside a degree occupies about two hours of that month, which is why the recorded birth time is the difference between a headline placement and a footnote.
One structural consequence is worth noticing: the two bodies sit in opposite signs and, if the time is known, in opposite houses. That axis is the subject matter of the aspect.
A dialogue rather than a fusion
The Moon stands for the automatic response, the thing you do before deciding to. Jupiter stands for enlargement, for the impulse to add rather than trim.
Set opposite each other they cannot simply merge. The traditional reading is a person whose needs and whose sense of what is enough keep having to be reconciled, usually in public rather than privately, because oppositions tend to be lived through other people. It commonly shows as generosity that outruns the actual resources, followed by a correction, followed by the same again.
The other standard reading is that oppositions give perspective. You get to see the appetite from the far side, which the conjunction never allows. Whether that turns into judgement or into balance is not decided by the aspect.
There is a further point that separates this from the conjunction. An opposition is visible from both ends, so it tends to be described back to the person by other people before they notice it themselves. That is the ordinary way oppositions announce themselves in a life, and it is why the seventh house so often comes into the reading.
What shifts the reading
Signs matter first. A Moon in Taurus opposite Jupiter in Scorpio is a slow argument about security and risk. A Moon in Gemini opposite Jupiter in Sagittarius is the same aspect running through information and belief instead, and reads almost nothing like the first.
Then the houses. The second and eighth axis makes it about shared money. The fourth and tenth makes it about home against career, and that version is often the one people recognise immediately.
Orb decides volume. Inside two degrees it is loud. Past six it is background, and a tighter aspect elsewhere in the chart will usually be doing more of the work.
Look for the ruler of each sign as well. If the sign holding the Moon is ruled by a planet that is itself under pressure, the opposition inherits some of that, and the reading shifts away from the stock description.
Time, accuracy and Jupiter's pace
Jupiter takes about twelve years to circle the zodiac and is retrograde for roughly four months a year, so a natal retrograde Jupiter here is unremarkable and not worth building a reading on. It does not meaningfully change how quickly the Moon closes the angle.
What does change things is time accuracy. Half a degree an hour of lunar motion means a birth time rounded to the nearest hour carries about half a degree of uncertainty, and a guessed time carries several degrees. The widget above reports the orb it calculates from the time you enter, so an approximate time gives an approximate orb rather than a wrong one.
A last note on proportion. This aspect appears in a great many charts, so on its own it is not a distinguishing mark. What distinguishes a chart is a tight orb, an angular house, or a third body making a T square to the pair.
Questions people ask about this
What does Moon opposite Jupiter mean?
It means the Moon and Jupiter are close to a hundred and eighty degrees apart at birth, in opposite signs. The tradition reads it as instinct and the urge to expand held in permanent dialogue rather than merged.
How wide an orb is allowed?
Eight degrees for an opposition, the same as for a conjunction. Inside two degrees the aspect is prominent, and beyond six most readers treat it as a minor note in the chart.
Is it a difficult aspect?
Oppositions are usually described as tension rather than damage. The pair has to be negotiated instead of ignored, and the tradition treats that negotiation as the source of perspective as often as of strain.
How often does it happen?
Once a lunar month, roughly a fortnight after each Moon and Jupiter conjunction. The full eight degree orb covers a little more than a day of each month.
Does an unknown birth time ruin the reading?
It weakens it. Without a time you can still often tell whether the aspect exists, since the Moon moves about thirteen degrees across a whole day, but you lose the houses and the precise orb.
Which houses make it most obvious?
The angular pairs, especially the fourth and tenth axis, where the aspect reads as home against public role. Cadent houses tend to soften it into something less visible from outside.