Why can a planet change houses depending on the house system we use?

If you have ever changed the house system in an astrology program, you have probably noticed that the chart changes too. A planet may appear in the 2nd house with Placidus, in the 3rd house with Koch, and again in the 2nd house with Campanus.

So which version is correct? How is it possible to have two different versions of the natal chart describing who I am or how I function?

The short answer is: they are all correct. House systems are calculated using different mathematical methods; that is why the division of the houses changes. And we can look at the philosophical correspondence of those calculations to determine what we are reading according to the house system we use.

The chart you see on the screen is two-dimensional, but it represents a three-dimensional, spherical geometry. It is the sky seen from a specific point on Earth, at a specific moment. The different house systems are different ways of translating that geometry into the circle of the chart, and that is why the cusps can change position.

The key is not to think that each system is dividing the same circle in an arbitrarily different way. In reality, each one first decides which dimension of celestial space it wants to divide into twelve sectors. It then projects that division onto the ecliptic, which is what we ultimately see drawn in the chart. Placidus, Koch, and Campanus are quadrant systems and preserve the Ascendant and Midheaven as the cusps of houses I and X; Equal Houses follows a different logic.

That is why we can translate the mathematics into an interpretive question.

I would separate them like this:

System	What it takes as reference	Interpretive analogy	Question
Placidus	Diurnal/nocturnal movement; temporal division of arcs	process / becoming	Through what experience does this develop?
Koch	Also movement and time, but calculated differently from local latitude and the MC	rhythm of actualization / situated temporality	When, or through what rhythm, does this become effective in my experience?
Campanus	Three-dimensional division of local space around the observer	existential position / orientation in space	From what place in my existence do I enter into relationship with this?
Equal Houses	30° of the ecliptic from the Ascendant	zodiacal order of experience	In what structural territory of my life does this express itself?

And I think each one can be refined much further.

Placidus: the chart as process

Astrodienst describes it precisely as a division of the phases of movement of celestial points above and below the horizon. In other words, beneath the geometry of Placidus there is a temporal idea: something is rising, culminating, descending, or moving toward the bottom of the sky.

So, interpretively, I would not see a Placidus house simply as “an area of life.”

I would see it as a phase of actualization. It is as if the planet is something that is happening, and the house describes where it has to pass through in order to manifest.

Mercury in the 4th house in Placidus, for example, would not necessarily have to mean only “Mercury = family/home.”

It could mean:

Mercury becomes actualized through a process of interiorization, roots, memory, belonging, and the construction of a subjective foundation.

The house would almost be a verb.

In that sense:

Placidus asks less “where is this?” and more “through what process does this come into being?”

And that explains why it can feel so psychological: it turns the chart into a kind of movement between birth, culmination, descent, and gestation.

Campanus: the chart as lived space

Campanus does something very different. It divides the prime vertical, a great circle passing through the zenith, nadir, and east-west points, into twelve equal sectors, and then projects those divisions onto the ecliptic. It is literally a division of the local sphere around the observer.

That is why a very powerful analogy appears here.

Campanus could describe:

Where is this located within my sphere of existence?

Not so much the process through which something happens, but the orientation of the individual in relation to the world. It is like standing inside a sphere. You are not looking at a sheet of paper.

There is sky above.
Earth below.
A horizon around you.
An east, a west.
A meridian.

Campanus divides precisely that space.

So a Campanus house could be understood as a kind of existential direction.

For example, Mercury in Sagittarius in the 5th house in Campanus:

It does not have to mean simply “creative communication.”

It could be something much more elemental:

thought and language occupy, within the individual’s sphere, the place from which something of one’s own moves out into existence.

Creating, formulating, playing, naming, producing something that did not exist before.

Here, the 5th house would not be so much “an experience that produces Mercury,” but the direction toward which Mercury opens.

That is why the formula we have been exploring works very well:

Campanus: “How does this part of the individual open toward existence?”

And perhaps even:

“Where does it point?”

Koch: the chart as situated time

Koch is interesting because conceptually it is related to Placidus. Both belong to the so-called temporal house systems. Swiss Ephemeris documentation explicitly notes that Koch is also based on diurnal and nocturnal arcs, although it constructs the cusps differently.

That is why I would not try to turn Koch into something radically different from Placidus.

The difference could be more subtle.

If Placidus says:

“How does this develop?”

Koch could say:

“How does this enter my time?”

Or:

“What is the rhythm through which this becomes present?”

There is something almost biographical there.

Placidus seems more processual to me.

Koch, more temporal-dynamic.

An analogy:

Let us look at a flower.

Campanus would ask:

Where is it growing toward?

Placidus:

What process does it go through from seed to flower?

Koch:

What is its season, its rhythm of growth, the moment when each phase becomes effective?

We cannot yet claim that this is “the correct interpretation” of Koch. This is already my own philosophical construction based on the geometry. But it seems coherent to me with what it is doing mathematically.

Equal Houses: the chart as architecture

Here something completely different happens.

The exact degree of the Ascendant is taken, and from there twelve consecutive sectors of exactly 30° are constructed. The MC continues to exist as an astronomical point, but it does not necessarily coincide with the cusp of the 10th house.

And this radically changes the question.

We are no longer dividing time.
Nor are we dividing space three-dimensionally.

We are imposing a regular structure on the ecliptic:

30°.
30°.
30°.
30°.

It is almost a matrix.

That is why we could relate Equal Houses to something like:

the architecture of individual experience.

If the Ascendant says:

“my experience begins from here,”

then every 30° a new field appears.

1 → me.
2 → what I possess.
3 → what I relate.
4 → what I come from.
etc.

But not because the local sky itself produced those divisions, but because we have constructed a regular sequence taking the Ascendant as the point of origin.

It could be formulated as:

Equal Houses: “How is experience organized from my point of entry into existence?”

That gives it a much more structural quality than Placidus.

So perhaps we should not ask:

“Which house is Mercury really in?”

but rather:

“What dimension of being am I exploring when I ask which house Mercury is in?”

Because each system could be answering something different.

Let us suppose:

Mercury in Sagittarius.

Campanus → 5th house
Placidus → 4th house
Koch → 4th house
Equal Houses → 5th house.

We have already seen that this does not necessarily create a contradiction.

We could read it like this:

What Mercury in Sagittarius is: a mind that seeks meaning, conceptual relationships, horizon, interpretation.

Campanus — H5: where it opens toward → toward creation, formulation, and the externalization of something of one’s own.

Placidus — H4: through what process it develops → through interiorization, memory, roots, and the construction of a subjective perspective.

Koch — H4: where it finds its rhythm of actualization → in cycles of interiorization, belonging, and the establishment of a foundation.

Equal Houses — H5: what position it occupies within the individual’s overall architecture → the Mercurial function belongs structurally to the field of creation and individual expression.

So we can arrive at something quite sophisticated:

Campanus = orientation.
Placidus = process.
Koch = rhythm.
Equal Houses = structure.

And they would not be four competing interpretations. They would be four projections of the same multidimensional object.

I think this last image is probably the most precise: we are not changing the planet; we are changing the plane onto which we project its position. The same three-dimensional figure can produce completely different two-dimensional profiles depending on the angle from which it is projected.