Half a circle, eight degrees of orb
The opposition is a hundred and eighty degrees, and this site allows eight degrees either side, the widest orb in the set. Neptune moves about two minutes of arc a day, so it is fixed for the purpose of this aspect, and the Moon does all the travelling at roughly thirteen degrees a day. The full sixteen degree window opens and closes in a little over twenty four hours.
It occurs once per lunar circuit rather than twice, which puts it near one birth in twenty two. The same arithmetic applies to the conjunction, and those two are the only Moon to Neptune contacts that happen just once each month.
Two things follow from the geometry. The bodies are in opposing signs, and if you know your birth time, in opposing houses. That axis is where the reading lives. Neptune retrograde covers around five months of every year, so it is the ordinary case and changes none of this.
Dialogue rather than fusion
The Moon is need, mood and the automatic response. Neptune is the dissolving of edges: idealisation, imagination, the picture that stands in for the thing. Where the conjunction blurs the two into a single permeable layer, the opposition keeps them separate and facing.
The usual description is a mismatch between what a person actually feels and what they have decided they feel. Feelings get interpreted through an ideal, and the ideal is often more coherent than the feeling was. Astrologers frequently report a pattern of disappointment that follows from that, not because events disappoint but because the picture was built first and reality was measured against it afterwards.
Oppositions also tend to be read through other people, since whatever sits opposite has a habit of turning up in human form. In this pairing that is the person who keeps meeting others who are hard to see clearly, and who has to work out how much of the vagueness belongs to them. That is an interpretive convention rather than a measurement, and it deserves testing against your own experience.
The axis, the signs and the orb
Because an opposition always occupies two houses, the axis is the first thing to establish and it needs a birth time. A fourth to tenth opposition reads through family against public role. A first to seventh reads through self against partner, which is the version most often described in print. A second to eighth reads through money, support and dependence, and is far less romantic than the standard write up assumes.
Signs supply the register. Virgo to Pisces is the sharpest version, because one end insists on precision and the other refuses it, and the contradiction is explicit. Gemini to Sagittarius softens it into a difference of scale rather than of kind. Cancer to Capricorn makes it a question about what a person is allowed to need.
Orb, as always, sets the volume. Under three degrees the pattern is usually recognisable to the person. Past six it is faint, and a wide opposition where the Moon sits at the very start of a sign and Neptune near the end of the opposing one has the geometry without much of the meaning.
What not to make of it
Neptune aspects invite overstatement, and oppositions invite drama, so this combination gets written up badly more often than most. It does not mean you will be deceived, or that you deceive others. It describes a gap between the felt thing and the pictured thing, which is a modest claim about how a person processes their own reactions.
It is also worth remembering the scale difference. Neptune belongs to everybody born in your decade and its sign says almost nothing about you personally. What is yours is the Moon's position on your birth date and the houses that follow from your birth time. Any reading that leans on Neptune's sign rather than the contact and its house is describing a generation, not a person.
Run the check above to get the orb and the two houses. If the orb is tight and the axis is one you recognise, the description is worth taking seriously. If it is wide and lands on houses that are quiet in the rest of your chart, it is background and this page has told you so.
Questions people ask about this
What does Moon opposite Neptune mean?
It means the Moon sits close to a hundred and eighty degrees from Neptune. The tradition reads it as a standing gap between what you actually feel and the version of it you have pictured, played out across two opposing houses.
How wide is the orb, and how long does it last?
Eight degrees either side, so a sixteen degree window. Since the Moon travels roughly thirteen degrees a day and Neptune is effectively still, the aspect opens and closes within about twenty six hours.
Does this aspect mean I will be deceived?
No. Nothing in an aspect predicts events. What is described is a tendency to interpret feeling through an ideal, which can produce disappointment when the two are compared, but that is a disposition and not a forecast.
Why does the axis matter more than the sign?
Neptune stays in one sign for about fourteen years, so its sign is shared by everyone born in that period. The houses depend on your birth time and are unique to you, which makes the axis the individualising part of the reading.
Is a retrograde Neptune worth noting?
Rarely. Neptune is retrograde for close to five months a year, so a natal retrograde Neptune is the common case. It does not alter the geometry of the opposition in any noticeable way.
Is the opposition stronger than the square?
It carries a wider orb, eight degrees against seven, so it can register at greater distance. In character the two differ rather than rank: the square is read as friction that demands action, the opposition as tension held in view.