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House systems: Placidus, Porphyry, Whole Sign and the rest

Why astrology has about a dozen house systems. Whole Sign, Placidus, Porphyry and Koch compared: strengths, weaknesses, and who actually uses each one.

8 min read · Updated 2026-06-11

The last chapter told you what the 12 houses are. What is left to explain is how you draw them. And here comes the surprise: there are about a dozen mathematically different systems for cutting the local sphere into 12 houses. None is "true" in any absolute sense. Each trisects differently and gives different results, sometimes fairly close, sometimes wildly divergent at extreme latitudes.

Here is the tour, the comparison, and where Astrolabica lands.

Why there are several systems

The local sky at the time and place of your birth is a complex 3D object. Several important circles run through it:

• The ecliptic (the line of the Sun and planets)
• The celestial equator (the projection of Earth's equator)
• The local meridian (the circle passing through the zenith and the celestial poles)
• The local horizon (the circle separating the visible sky from the invisible)

These four circles define 4 crucial points on the ecliptic:

• The Ascendant (Asc): where the ecliptic crosses the horizon in the east
• The Descendant (Desc): where the ecliptic crosses the horizon in the west (opposite the Asc)
• The Midheaven (MC): where the ecliptic crosses the meridian above the horizon
• The Imum Coeli (IC): where the ecliptic crosses the meridian below the horizon (opposite the MC)

These 4 points are the angles of the chart. Every house system computes them the same way (give or take negligible details), and every one assigns them to houses 1, 7, 10 and 4 respectively.

The argument starts with the 8 intermediate cusps (the starts of houses 2, 3, 5, 6, 8, 9, 11, 12). How do you trisect the 4 quadrants marked off by the angles? Along which dimension do you trisect: time, ecliptic space, the equator, the diurnal arc? That one geometric question is what spawns dozens of systems.

Whole Sign: the oldest (~300 BCE)

• First appeared: ~300 BCE (early Hellenistic)
• Logic: each house = one whole sign. House 1 = the entire sign of the Ascendant. House 2 = the next sign. And so on.
• H1 cusp: 0° of the Ascendant's sign (not the exact degree of the Asc)
• H10 cusp: 0° of the tenth sign counting from the Asc

If your Asc is at 17° Leo, then under Whole Sign the whole of H1 is Leo (0° to 30° Leo), the whole of H2 is Virgo, and so on. The exact degree of the Ascendant (17°) lands somewhere in the middle of H1.

Strengths:

• Mathematically the simplest (no intermediate cusps to compute)
• Works everywhere on Earth, the poles included
• The original Hellenistic system, rediscovered by the traditional revival of the 2010s

Weakness:

• The MC does not necessarily land on the H10 cusp (it often falls in H9 or H11). Moderns see this as a serious flaw; Hellenists see it as normal and meaningful.

Who uses it: reconstructed Hellenistic astrology, Vedic astrology (under the names bhava chalit or rasi chart), and a growing slice of contemporary Western astrologers since 2015.

Equal House: the modern cousin

• First appeared: ~1st century CE (Manilius), popularized again in the 20th century
• Logic: 12 sectors of 30° starting from the exact degree of the Ascendant
• H1 cusp: exact degree of the Asc (so if Asc is at 17° Leo, the H1 cusp is 17° Leo)
• H2 cusp: 17° Virgo; H3 cusp: 17° Libra; and so on

This is the "shifted" version of Whole Sign: you keep the equal 30° blocks, but anchor the origin on the exact Asc degree rather than on 0° of the sign.

Strengths: simple, works everywhere. Weakness: like Whole Sign, the MC may not land on the H10 cusp.

Who uses it: a minority but a real one, mostly in the line of Alan Leo and among certain British astrologers.

Porphyry: the simple ecliptic trisection

• First appeared: 3rd century CE (Porphyry of Tyre, Neoplatonist philosopher)
• Logic: trisection of the ecliptic arcs between Asc and MC, MC and Desc, Desc and IC, IC and Asc
• H1 cusp = Asc, H4 = IC, H7 = Desc, H10 = MC (the angles)
• The 8 intermediate cusps come from trisecting each quadrant along the ecliptic

Strengths:

• Very simple mathematically
• The MC lands exactly on H10 (the angles are respected)
• Works at extreme latitudes

Weakness:

• The houses come out unequal in size (a quadrant can be 60° or 120° depending on your latitude and the season). Some astrologers find that acceptable, others do not.

Who uses it: medieval Western astrology, Co-Star (which markets it as "modern astrology"), and some modern humanistic astrologers.

Placidus: the contemporary pro standard

• First appeared: 17th century (Placidus de Titis, an Italian Benedictine monk)
• Logic: trisection of the time each ecliptic point takes to travel its diurnal arc (the time between its rising and its passage across the meridian)
• H11 cusp: the point that has covered 1/3 of its diurnal arc. H12: 2/3. Same idea for the houses below the horizon.

This is more complex mathematically: you are not trisecting a spatial arc but a temporal arc. Why? Because planets move through the local sky at unequal speeds depending on latitude, and the Placidus system claims to better mirror the real motion of planets above and below the horizon.

Strengths:

• The most widely used system in modern Western astrology (probably more than 70% of practitioners and pro apps)
• Reflects the real diurnal motion of the planets
• The de facto standard in software (Solar Fire, Astrology King, AstroSeek)

Weaknesses:

• Mathematically unstable at extreme latitudes: beyond about 66° N/S (the polar circle), some cusps simply cannot be computed (a point's diurnal arc may not exist if the point never rises or never sets). For Reykjavík in summer, or Saint Petersburg in winter, Placidus breaks.
• The houses come out very unequal in size (up to 60° in a single house at high latitudes).

Who uses it: the modern Western standard, professional astrologers in France and the US, Astrolabica by default, AstroSeek, Astro.com (its default option), Solar Fire.

Koch: a variant of Placidus

• First appeared: 1962 (Walter Koch, Germany)
• Logic: trisection of the time the Ascendant takes to reach each cusp (not the ecliptic point itself, as in Placidus)
• A "personal" variant: everything is referenced to the motion of the Asc

Strengths: popular in German astrology and among some humanistic astrologers; conceptually elegant. Weakness: the same problems as Placidus at extreme latitudes.

Who uses it: Germanic astrology, some modern French-speaking astrologers.

Other systems (in brief)

• Regiomontanus (15th century, Johannes Müller von Königsberg): trisects the celestial equator, then projects onto the ecliptic. A forerunner of Placidus, still used in horary astrology.
• Campanus (13th century, Campanus of Novara): trisects the prime vertical (the great circle perpendicular to the meridian, running through east and west). Elegant but little used.
• Topocentric (1961, Polich-Page): a modern variant very close to Placidus, designed to fix its flaws at extreme latitudes.
• Alcabitius (10th century, an Arab astrologer): trisects the time the Asc takes to reach the MC. A forerunner of Koch.
• Morinus (17th century, Jean-Baptiste Morin): trisects the pure celestial equator and ignores the ecliptic entirely. Marginal.

Comparison table

At extreme latitudes: why Whole Sign earns its keep

Key idea. Beyond the polar circle (~66° N/S), Placidus, Koch and Regiomontanus break down mathematically because they depend on the diurnal arc, which can fail to exist (midnight sun, polar night). Whole Sign and Porphyry stay usable everywhere on Earth. If you are casting a chart for someone born in Tromsø, that matters.

For the 99% of humans who live between the tropics and the middle latitudes, the problem never comes up. But it is a substantive argument for the Hellenistic revival: Whole Sign applies universally, which the modern standard does not.

Where Astrolabica stands

Astrolabica uses Placidus by default, because it is:

• The de facto standard in modern Western practice
• What your astrologer friends probably use
• What the general user expects

But the app lets you switch to Whole Sign, Koch or Equal House in two clicks. Viewing the same chart across several overlaid systems is, in our view, the best exercise for grasping that the houses are an interpretive grid, not an objective fact. They shape how you read; they do not describe some hard reality.

The verdict

Key idea. No house system is "true." They are all tools. The debate between Placidus and Whole Sign is less an astronomical one than a philosophical question about what you want to measure. Placidus measures real diurnal motion; Whole Sign measures a pure symbolic structure. Pick the one that fits your practice, or use both and get both readings.

A few practical recommendations:

• Just starting out? Stay on Placidus. It is the standard, and you can follow any course without confusion.
• Drawn to traditional or Hellenistic astrology? Learn Whole Sign alongside it. You will notice that certain configurations come through more clearly.
• Casting a chart for someone born above the polar circle? Use Whole Sign.
• Want to understand Co-Star or run a comparison? Use Porphyry.

Next chapter: the angles, what structures the entire chart.