Planet
Neptune
Neptune is read as the part of a chart where edges dissolve — between a person and an ideal, between what a thing is and what it was hoped to be, between one life and another it quietly borrows from. It is the faculty that makes imagination, compassion and self-deception close relatives rather than opposites.
It is an outer planet, and it is the one most often recognised backwards. A Neptune passage tends to become describable only once it is over, which makes it the least rewarding placement to read forwards and one of the most useful to read in review.
Found by arithmetic, before anyone saw it
Neptune was located in 1846 by people who had never seen it. Uranus was not moving as it should; two mathematicians, working separately, calculated where a further body would have to be to account for the discrepancy, and a telescope was pointed at the answer. The planet was inferred entirely from what it was pulling on. The rulership tables astrologers were using by then were already many centuries old, so Neptune is read on modern authority from end to end.
The manner of the discovery is as good a description of the reading as anything written since. Neptune seldom shows up as an event. It shows up as a drift in something else — a standard quietly lowered, an account of a relationship that keeps needing revision, a year that felt like it was about one thing and turns out to have been about another.
Which is why the honest way to work with it is retrospective. Ask what has already gone soft rather than what is about to; the first question is answerable and the second mostly is not.
Three words that get used loosely here
Illusion. Useful only if it is allowed to mean a picture that is partly true. A Neptune placement is not usually wrong about the thing it idealises — it is wrong about the proportions, and a mistake of proportion is much harder to catch than a mistake of fact.
Spiritual. It does the least work of the three. It describes a register somebody is comfortable operating in rather than a belief they hold, and plenty of charts with a strong Neptune belong to people with no religion at all and an unusually high tolerance for ambiguity.
Generational. Neptune takes about fourteen years to cross a sign, so a reading built on the sign is a description of everyone born across a decade and a half. That is a real fact about a cohort. It is not a portrait of a person.
What it does to whatever it touches
Neptune's effect on another planet is to soften its outline rather than to argue with it. Whatever it contacts does not stop working; it stops being easy to describe. That is why these contacts are so regularly missed while they are happening and so obvious once they are not.
A body considered on its own cannot say whether this is a quiet background quality or the organising fact of a life. That is settled by where it falls and by what else in the chart is pulling at it, and neither is legible from the planet alone.
A strong Neptune is also not a character flaw. The same faculty that makes somebody a poor judge of a bad situation is the one that lets them picture a better one that does not exist yet, and there is no version of the chart in which those come apart.