House Systems Compared: Placidus, Whole Sign, and Porphyry

What Does a House System Control?

A birth chart runs two calculations that have nothing to do with each other. One finds where each planet sits in the zodiac — a sign and a degree that come only from your birth date, time, and location. The other draws twelve lines across that same sky. Those lines are the house cusps, and they mark off twelve areas of life: career, relationships, home, and so on. A house system is just the rule for drawing those twelve lines.

Four systems come up again and again. Placidus divides the sky by how long it takes points on the ecliptic to rise, and it's the default in most astrology software. Whole Sign is the oldest, going back to Hellenistic astrology: it treats a whole zodiac sign as one house, starting with whichever sign the Ascendant falls in. Koch runs a similar time-based formula and lands close to Placidus at most latitudes. Porphyry, from the third century, splits each quarter of the chart into three equal sections. None of these four can move a planet's sign. They just argue over where the boundaries go.

One Chart, Four House Systems

The table below runs one chart, born March 2, 1994 at 6:14 a.m. in Chicago, through all four systems. The Ascendant and Midheaven come out identical in every column, since both points come from birth time and location alone. The twelve house cusps are where the systems split. Whole Sign starts every house at 0° of a sign, so its 10th-house cusp sits at exactly 240°. The Midheaven itself is still Sagittarius 17°03′ under all four systems.

Why Your Planets Move Between Houses

Whichever system draws those twelve lines decides which house a planet ends up in. Move a boundary by a few degrees and a planet can slide into the next one. Below, I sorted six planets from that same chart by house, under all four systems.

Three placements shift. The Moon and Jupiter sit close to the 8th/9th house boundary. Whole Sign draws that line at a fixed 210°, well before the Placidus cusp at Scorpio 24°14′, which pushes both planets one house later. Porphyry agrees with Placidus on the Moon but puts Jupiter in the 9th too, since its cusp falls at a different degree. Saturn moves the most. It sits at Pisces 03°54′, just before the Ascendant, landing in the 12th house under every quadrant system. Whole Sign puts it in the 1st instead, since its 1st house covers the entire Ascendant sign, not one exact degree. The Sun, Venus, and Mars stay put, since none of them sits near a boundary.

Which System Is Right?

None of the four is more correct than the others, mathematically. Whole Sign is the oldest, with a written record going back to Hellenistic astrology, centuries before Placidus existed. Porphyry dates to the third century. Placidus came along in the 1600s and became the default in most 20th-century Western software. It caught on and stuck; nobody has proven it more accurate than the rest. Koch showed up in the 20th century, using a time-based formula close to Placidus's.

Venusward defaults to Placidus. Near the poles, during the stretches of the year with continuous daylight or continuous darkness, Placidus has no valid solution. The calculator switches to Whole Sign automatically there, and says so on your results page. The full details are on the Accuracy & Methodology page. Which zodiac a chart uses is a separate question; see Tropical vs. Sidereal Zodiac, Explained for that one.

A Common Mix-Up

People often blame the house system when two sites give them different rising signs. That's not what's happening. Your Ascendant, the exact degree behind a rising sign, comes from birth time and location alone, before any house system runs. The table above shows all four systems landing on the same one. If your rising sign itself differs between sites, look elsewhere: birth time, time zone, or a different zodiac are the more likely causes. Why Is My Rising Sign Different on Every Site? and Why Is My Moon Sign Different on Every Site? cover those causes.

How Different Sites Handle It

House system choice varies from site to site, and most people don't check which one they're looking at. I looked up each site's own help pages to find its default. Know your site's default before you compare two charts side by side.

How to Check What Changed

1. Check the sign and degree first.If two calculators agree on a planet's sign and degree, and only disagree on which house it's in, a house system difference explains it. Nothing is wrong.
2. See whether the placement sits near a house cusp. A planet just a few degrees from a boundary, like the Moon or Jupiter above, is the kind that moves between systems.
3. Look up which house system the site defaults to. The table above lists several; most sites say so somewhere in a settings menu or FAQ page.
4. Rule out the other causes first. If the sign itself differs between two sites, not just the house, the likely cause is birth time, time zone, or zodiac system. Time Zone & DST Errors in a Birth Chart covers the birth-time and time-zone side of it.
5. Run your own chart and compare. Run your placements through the free chart calculator, then check the Accuracy & Methodology page for how Venusward computes houses.

If two sites put a planet in different houses, start with the checklist above. Most of the time it comes down to which house system each one defaults to.