What Is the Placidus House System? Placidus vs Other House Systems

The Placidus house system divides a birth chart into twelve houses by time. It takes the time each degree of the zodiac spends travelling from the horizon to the Midheaven, splits that time into three equal parts and draws a house cusp at each third, so the houses come out unequal in size, anchored to the Ascendant and Midheaven.

If you have ever wondered why your houses look uneven, or why another site put a planet in a different house, this is why. Placidus is the default on most chart sites, it is the system Celestio uses for your chart, and it is one of about a dozen ways astrologers draw the same twelve houses.

What is the Placidus house system?

Placidus is a quadrant house system. "Quadrant" means the four angles of the chart, the Ascendant, IC, Descendant and Midheaven, are the cusps of the 1st, 4th, 7th and 10th houses, and each quarter between two angles holds three houses. What sets Placidus apart is how it divides each quarter: by time, not by equal slices of the zodiac.

Every degree of the zodiac rises in the east, climbs to the Midheaven, sets in the west and passes under the earth once a day, as the Sun does. The time a degree takes to climb from rising to culmination is its semi-diurnal arc; the time from setting to its lowest point is its semi-nocturnal arc. Placidus trisects those arcs:

• The 12th cusp is the degree that has completed one third of its climb from the Ascendant to the Midheaven.
• The 11th cusp is the degree that has completed two thirds of it.
• The 2nd and 3rd cusps are found the same way below the horizon, from the IC side.
• The 4th to 9th cusps sit exactly opposite the 10th to 3rd.

In plain terms: a planet on your 12th-house cusp was one third of the way, in time, from rising to culminating when you were born. A planet on your 11th cusp was two thirds of the way.

Key features of Placidus houses

• The Ascendant is always the 1st cusp and the Midheaven is always the 10th cusp.
• The in-between cusps are set by time, from the sky's daily turn over your birthplace.
• Houses are unequal: some cover less than a sign, others more than one.
• The result depends on your birth latitude, so the same moment gives near-equal houses in Singapore and very uneven ones in Oslo.
• Beyond about 66°33′ north or south, Placidus cannot be drawn at all.

Is Placidus the same as houses?

No. The houses are the twelve areas of life in a chart, from self and resources in the 1st and 2nd to friends and solitude in the 11th and 12th. Placidus is one method of drawing their boundaries. Whole Sign, Equal, Porphyry, Koch, Campanus and Regiomontanus are others.

Each house means the same thing in every system. What changes is where each house begins, and so which house a planet near a boundary falls in. Your planets, their signs and degrees, and the aspects between them are identical whichever system you pick. Our guide to the 12 houses covers what each house is for.

Why are Placidus houses different sizes?

Placidus houses are different sizes because the zodiac crosses the horizon at a slant. The ecliptic, the Sun's yearly path and the band the zodiac is measured along, is tilted about 23.4° to the equator, so some signs rise steeply and quickly while others lie low and take far longer. In the Northern Hemisphere, Capricorn through Gemini rise fast and Cancer through Sagittarius rise slowly; south of the equator the pattern reverses.

Placidus gives each house an equal share of time, and equal stretches of time cover unequal stretches of the zodiac. Near the equator the difference is small. Far from it, a house can swallow more than 40° while its neighbour holds less than 20°.

That is also where intercepted signs come from. When a house is wider than a sign, a whole sign can sit inside it without touching a cusp, and the opposite sign is intercepted in the opposite house. Two other signs then appear on two cusps each. Interceptions are a product of your birth latitude, not a hidden flaw in your chart, and they cannot happen in Whole Sign or Equal houses.

Who was Placidus, and why is his system the default?

The system is named after Placidus de Titis (about 1603–1668), an Olivetan monk who taught mathematics at the University of Pavia. He set out the method in his Physiomathematica of 1650, presenting it as the system Ptolemy intended. He did not invent it: the twelfth-century astrologer Abraham ibn Ezra had already described the same division, and it seems to have been known in the 1300s.

Placidus houses became the standard through printing rather than through any test of accuracy:

1. In the Renaissance, Regiomontanus houses dominated, because theirs were among the first tables of houses in print.
2. In 1693 the English astrologer John Partridge championed Placidus in his Opus Reformatum.
3. From 1821, Robert Cross Smith, writing as "Raphael", published almanacs with Placidus tables of houses.
4. Raphael's Ephemeris and Raphael's Tables of Houses became the standard English-language references through the nineteenth and twentieth centuries.
5. When astrology moved to computers, software kept the system everyone already had in their books.

So Placidus is the default because it was the system on everyone's desk for two centuries. It is a strong one: it ties each house to the real movement of the sky over your birthplace and keeps the angles as cusps. That is why Celestio starts every chart with it.

Where does Placidus fail?

Placidus fails beyond the polar circles, at latitudes above about 66°33′ north or south, which is 90° minus the tilt of the ecliptic. There, some degrees of the zodiac never rise and others never set during the day of birth. A degree that never rises has no climb to divide into thirds, so some Placidus cusps simply do not exist.

A birth in Tromsø, Norway, at about 69.6° north, cannot have a complete Placidus chart. A birth in Reykjavik, at about 64.1° north, can, but its houses will be strikingly uneven, with several intercepted signs. Koch houses fail at the same latitudes for the same reason.

Chart software handles this in different ways. Some programs quietly switch to Porphyry houses, some show an error and ask you to choose another system, and some draw the cusps in a strange order. Celestio uses Whole Sign houses for births where Placidus cannot be drawn, and the chart says which system it used.

How does Placidus compare with other house systems?

House systems fall into three families: those that divide the zodiac itself, those that divide time, and those that divide space around the birthplace. All quadrant systems share the same four angles; they differ only in the eight cusps in between.

Divided along the zodiac

• Whole Sign. The whole rising sign is the 1st house, the next sign is the 2nd, and so on: one sign per house. It is the oldest system, the main one in Hellenistic astrology from around the first century BCE, and it is standard in Vedic astrology. The Ascendant sits inside the 1st house rather than on its cusp, and the Midheaven usually lands in the 9th, 10th or 11th. Our comparison of Whole Sign vs Placidus houses goes deeper.
• Equal. Each house is exactly 30°, starting from your Ascendant degree. It is clear and works at every latitude; the Midheaven floats free of the 10th cusp. A rarer variant counts equal houses from the Midheaven instead.
• Porphyry. Each quadrant is split into three equal parts along the zodiac. Vettius Valens described it in the second century CE, and it is named after the philosopher Porphyry. Because it keeps the angles and works everywhere, it is the usual software fallback at polar latitudes.

Divided by time

• Placidus. Each degree's own climb to the Midheaven, divided in three.
• Koch. The Midheaven degree's climb, divided in three, with cusps at the degrees on the horizon at those moments. It was devised by Friedrich Zanzinger and Heinz Specht and popularised by the German astrologer Walter Koch. Its cusps usually land within a few degrees of Placidus, and it fails at the same latitudes.
• Alcabitius. A medieval method that divides the Ascendant degree's day and night arcs. It first appears around 500 CE and is named after the tenth-century astrologer al-Qabisi.

Divided by space

• Regiomontanus. Divides the celestial equator into twelve equal parts. Named after the fifteenth-century astronomer Johannes Müller, it was the Renaissance standard and is still used in horary astrology in the tradition of William Lilly.
• Campanus. Divides the prime vertical, the circle running from the east point through the point overhead to the west, into twelve. It is named after the thirteenth-century mathematician Campanus of Novara.
• Topocentric. A 1960s system by Wendel Polich and A. P. Nelson Page that gives cusps close to Placidus and can be drawn beyond the polar circles.

What changes when you switch house systems?

Only the house a planet is read in can change. Take an Ascendant at 24° Gemini at a mid-northern latitude, with Venus at 8° Cancer:

• Whole Sign: Gemini is the 1st house and Cancer the 2nd, so Venus is in the 2nd house, the house of resources and what you value.
• Equal: the 2nd cusp is 24° Cancer, so Venus at 8° Cancer is still in the 1st house, the house of self and appearance.
• Placidus: with a 2nd cusp at around 14° Cancer, which varies with latitude and time, Venus is again in the 1st house.

Venus's sign, degree and aspects are the same in all three. One reading puts her charm in how you look and arrive; another puts her in what you earn and treasure. Planets close to a cusp, or early in a sign, are the ones that move.

One subtle point surprises people. Switching between the tropical and sidereal zodiacs never changes a planet's house in Placidus, because the planets and cusps all shift together. In Whole Sign it can, because the sign boundaries that define each house move under the planets.

Which house system should you use?

There is no single correct house system, and good astrologers disagree. Placidus is the strongest default for modern Western astrology: it is built on the sky's real motion over your birthplace, it keeps the angles as cusps, and most of the modern literature you will read assumes it. Whole Sign is favoured by Hellenistic and Vedic practitioners and by many younger astrologers; it is simple, ancient and works at every latitude.

The practical way to choose:

1. Read your chart in Placidus first and note every planet's house.
2. Switch to Whole Sign and mark the planets whose house number changes.
3. Read both house meanings for those planets and notice which one describes your life.

Most people find the planets that move speak with a little of both houses. Start with your own birth chart: Celestio draws it in Placidus and writes a reading for that exact chart, so you can see which planets sit near a cusp.

Frequently asked questions

What is the Placidus house system in simple terms?

The Placidus house system divides the chart into twelve houses by time rather than by equal slices of the zodiac. Each house covers one third of the time the sky takes to turn a degree from one angle of the chart to the next, so houses come out in different sizes. It is the default system on most chart sites and the one Celestio uses.

Is Placidus the same as the houses in astrology?

No. The houses are the twelve areas of life in a chart, and Placidus is one method of drawing where each house begins. Whole Sign, Equal, Porphyry and Koch are other methods. The meaning of each house stays the same in every system; only the boundaries move, which can shift a planet near a cusp into the next house.

Why are my Placidus houses different sizes?

Placidus houses are different sizes because the zodiac crosses the horizon at a slant, so some signs rise much faster than others. Placidus gives each house an equal share of time, and equal time covers unequal stretches of the zodiac. The further from the equator you were born, the bigger the difference. Uneven houses are normal, not an error.

Is Placidus or Whole Sign more accurate?

Neither Placidus nor Whole Sign is more accurate; they answer the question "where does this planet show up?" in different ways. Placidus follows the sky's daily motion over your birthplace, while Whole Sign gives each house one full sign. Read your chart in Placidus, then check the planets that change house in Whole Sign and see which reading sounds more like you.

Why can't Placidus be calculated for my birthplace?

Placidus cannot be calculated above about 66°33′ north or south, beyond the Arctic and Antarctic circles. At those latitudes some degrees of the zodiac never rise or never set, so the time-based division Placidus depends on breaks down. Software then switches to another system; Celestio uses Whole Sign houses and the chart says so.

What does "Placidus, Orb 0" mean on a chart?

"Placidus, Orb 0" names two separate settings. Placidus is the house system. On the free chart generators that print this exact line, the software's documentation points to a house-cusp setting, how close a planet may be to the next cusp before it counts in that house, and 0 means that option is off. It is most likely not an aspect orb. Placidus and Orb 0 explained walks through the whole line.