How Placidus divides the sky
Placidus divides time rather than space. Every point on the ecliptic takes a certain number of hours to travel from the horizon to the meridian, and Placidus splits that journey into equal thirds.
That is why the houses come out uneven. A degree that rises quickly reaches the meridian quickly, so its thirds are short; one that rises slowly produces long houses. The result is a chart where house sizes vary considerably, sometimes by a factor of two.
Why it became the default
Largely through printing. Placidus tables were the ones widely published in the nineteenth and twentieth centuries, so they were what most astrologers had on the shelf, and the convention stuck.
That is a historical accident rather than an argument for the system. It does mean, though, that most published interpretations assume Placidus, which is a practical reason to use it unless you have a reason not to.
Where it fails
Above roughly sixty-six degrees of latitude, some degrees of the ecliptic never rise at all. A house circle that never crosses the horizon has no daily arc to divide, so there is no Placidus cusp to compute.
This is not a rounding problem or a software limitation. The cusp genuinely does not exist. Tools that print one anyway have substituted something without saying so.
This calculator falls back to Porphyry in that case and tells you it has done so. If your birthplace is in northern Scandinavia, Alaska, or anywhere above the Arctic Circle, you will see that notice.
| Latitude | Placidus status |
|---|---|
| Up to about 60° | Works normally |
| 60° to 66° | Increasingly distorted; very uneven houses |
| Above about 66° | No solution; falls back to Porphyry |
The intercepted sign problem
Because Placidus houses are uneven, a house can be wide enough to swallow an entire sign without that sign appearing on any cusp. That sign is called intercepted, and the opposite sign is intercepted too.
Interceptions become common at higher latitudes and are almost universal above fifty degrees. The traditional reading is that the intercepted sign's qualities are harder to access, which is a claim worth holding loosely: an interception is a consequence of the division method rather than a feature of the sky.
Switching to Whole Sign houses makes interceptions impossible, which some practitioners consider an argument for that system.
Placidus against the alternatives
The planets are identical in every system. What moves is the boundaries, and in a typical chart two or three bodies change house between methods.
The comparison table in your result shows the same chart in every system this site offers and marks which bodies actually move. That turns an abstract argument into a specific question about your own chart, which is the only version of it worth having.
If a body sits within a degree or two of a cusp, treat its house as uncertain whichever system you choose. That is more honest than picking a method and forgetting the ambiguity.
Who Placidus was
Placidus de Titis was a seventeenth-century Italian monk and mathematician who published the tables the system is named after. He did not invent the method, which appears in earlier sources, but his tables are what carried it.
The name attached to the system is worth knowing mainly because it is not descriptive: nothing about the word tells you it divides time, which is the one thing that distinguishes it.
Why it needs a birth time
All twelve cusps derive from the ascendant and Midheaven, which sweep the whole zodiac each day. Without an hour there is nothing to anchor them to, and this calculator says so rather than defaulting to noon.
Common questions
Why is Placidus the default everywhere?
Largely because its tables were the ones widely printed in the nineteenth and twentieth centuries. That is a historical accident, but it does mean most published interpretations assume it.
What does Placidus actually divide?
Time, not space. It splits the hours a degree takes to travel from horizon to meridian into equal thirds, which is why the houses come out uneven.
Why does it fail at high latitudes?
Above about sixty-six degrees some degrees never rise, so there is no daily arc to divide and the cusp genuinely does not exist. This calculator falls back to Porphyry and says so.
What is an intercepted sign?
A sign swallowed entirely inside one house, appearing on no cusp. It happens because Placidus houses are uneven, and it becomes common above fifty degrees of latitude.
Should I use Placidus or Whole Sign?
Placidus for modern Western interpretations, Whole Sign for traditional or Vedic material. The comparison table shows which of your bodies actually change between them.
Who was Placidus?
A seventeenth-century Italian monk and mathematician who published the tables. He did not invent the method, which appears in earlier sources.