Placidus vs Whole sign: what changes in six real charts

Pick Placidus vs Whole sign and your planets can land in different houses, even though nothing in the sky has changed. We cast our six example charts both ways and counted. Across the six, 39 of 72 house placements moved.

Below: which ones, why Lagos behaves differently from Berlin, and a straight answer on which system to use. Every number comes from the same engine as our free birth chart calculator.

How many placements change house?

We took the ten planets, the North Node and the Midheaven for each person in our example circle (all fictional, with birth times we picked), and looked up the house in Placidus and in Whole sign. The Ascendant is left out: it is the start of the 1st house in both systems. That leaves 12 points per chart.

That is 39 of 72, or 54%. Robin has the fewest movers, 3. Lena has the most, 9, including a Moon that jumps from the 10th house to the 12th. Most moves are by a single house. 3 of the 39 jump two, as that Moon does.

The planets themselves do not move. Their signs and degrees are the same in every system. Only the fence lines between houses change. Our positions are checked against NASA’s JPL Horizons, and the house maths is separate from that.

Does the Whole sign ascendant move?

Not the degree, and not the sign. The Ascendant stays where the sky puts it. What moves is the cusp of the 1st house. In Placidus it starts on the Ascendant degree. In Whole sign it starts at 0° of the rising sign, so the whole sign becomes the 1st house.

The “moves back by” column is the key. A Whole sign chart is an Equal-house chart slid back until the first cusp sits on a sign line, so every house boundary shifts back by the Ascendant’s degree within its sign. Ade’s Ascendant is at 11°47′, so all twelve fences move about 12° and 7 points change house. Robin’s is at 21°12′, which is a bigger slide, yet only 3 points change. How many move depends on how many planets happen to sit near a boundary.

The Midheaven gets caught out too. It changed house in 3 of the six charts (Robin, Sam, Lena). In Whole sign it is not a cusp at all, just a point that happens to lie in the 9th, 10th or 11th. Astrologers who use Whole sign read the 10th house as a sign, and treat the Midheaven as a separate marker.

Why latitude changes the size of the gap

There is a second effect, and this one belongs to Placidus. At the equator the ecliptic cuts the horizon at much the same angle all day, so Placidus houses come out close to equal ones. Further from the equator the angle varies, and Placidus stretches some houses and squeezes others. That pulls the 2nd, 3rd, 5th and 6th cusps away from where Equal puts them.

We measured it. For each birth city we cast the houses every 10 minutes through 28 October 2026 (UTC) and compared ten Placidus cusps with Equal cusps, which are simply Ascendant plus 30° steps.

In Lagos, at 6.5° north, a Placidus cusp sits 2.5° from its Equal twin on average, and never more than 7.0°. In Berlin, at 52.5°, the average is 16.0° and the worst case 40.7°. London is close behind, at 15.4° on average. That is roughly 6 times the Lagos gap, from latitude alone.

What is the difference between Placidus and Whole sign houses?

Here it is without the jargon.

• Placidus is based on time. It splits the journey of each degree between the horizon and the meridian into thirds, so houses have different widths. A house can span anything from about 20° to nearly 40° of the zodiac at these latitudes. It cannot be drawn in the polar circles, and Charthood falls back to Porphyry there.
• Whole sign is based on signs. The rising sign is the 1st house, the next sign the 2nd, and so on. Every house is exactly 30° wide and houses line up with signs, which makes it very easy to read by eye.

Neither is more “true” in the sense of astronomy. Both start from the same Ascendant and the same planets. They are two ways of deciding what “the 7th house” means. The Wikipedia article on astrological houses lists the many systems, and our own houses guide shows one chart in all four.

What about Equal and Porphyry?

Charthood supports four systems, so we ran the other two as well. Equal starts at the Ascendant and adds 30° each time. Porphyry divides each quarter of the chart, between the angles, into three equal slices.

Porphyry is the near-twin of Placidus here: only 3 points in six charts land differently. Equal changes 24. Whole sign changes 39. If you are choosing a system to compare against Placidus, Porphyry is the quiet one.

Which house system should you use?

Honestly, astrologers disagree, and no experiment settles it. Here is how we would choose.

• Start with Placidus if you want the most common modern view. Most books, apps and online guides assume it, so you can compare notes.
• Try Whole sign if you like clean rules. Its houses always match signs, and many traditional astrologers prefer it. It is also forgiving when your birth time is a little off, since a house only changes when the rising sign does.
• Be wary of Placidus if you were born far north or south, or have a planet within a few degrees of a cusp. That is where the systems disagree most.
• Use Equal or Porphyry as a tie-breaker, or if Placidus will not draw at your latitude.

A sound habit: read the chart in your usual system, then switch and see which planets move. Keep what holds in both. A planet that stays in the same house either way is a safer reading than one that changes.

Take Sam, born in London with Capricorn rising. 8 of 12 points shift, among them the Moon, from the 7th house to the 8th. That is worth knowing before you build a story on Sam’s Moon house.

Common mistakes

• Comparing two readings in different systems. If a friend’s “Venus in the 10th” does not match your chart, check the system before the birth time.
• Assuming Whole sign is “less accurate”. It is a different rule, not a rougher calculation.
• Ignoring the Midheaven. In Whole sign it can sit in a different house from your 10th-house cusp.
• Forgetting the birth time. Both systems start from the Ascendant, so a time that is out by half an hour can matter more than the system.

To see a real chart in any of the four, give a saved person a different House system in their settings, or start with the seven-step birth chart guide. For the maths behind the positions, read our accuracy review against NASA data. If you only have a rough birth time, the rising sign study shows how much slack you really have.