Placidus houses

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I hope someone out there can answer a question for me. I have read that Placidus house cusps are unique in that the diurnal semi-arc is trisected so cusps 12, 11, and 10 are 1/3, 2/3 and 3/3 along " their own diurnal semi-arc," as opposed to say, alcabatius, that trisects the ascendant semi-arc. What is the advantage of this? Is Placidus a better representation of the sky at the moment in question? Thanks.

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hi donald - welcome to skyscript.. skyscript is broken up into different forums.. you've posted this in the philosophical forum where you are unlikely to get the technical question you ask, addressed... it would be better to address this in the general astrology forum.. cheers james

Re: Placidus houses

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Donald wrote:I hope someone out there can answer a question for me. I have read that Placidus house cusps are unique in that the diurnal semi-arc is trisected so cusps 12, 11, and 10 are 1/3, 2/3 and 3/3 along " their own diurnal semi-arc," as opposed to say, alcabatius, that trisects the ascendant semi-arc. What is the advantage of this? Is Placidus a better representation of the sky at the moment in question? Thanks.
Hello Donald,
That's actually a very good question. I think many astrologers will give you a different answer, each from their own frame of reference and experience as an astrologer. So any attempt of answering this question must be done with humility, because there are no absolute answers.

My answer is: yes, Placidus houses are unique in the manner that you describe and no, that does not mean that it is better or worse than other house systems, just different.
My own take on the whole arena of house systems is that each system was formulated on a specific moment in history and each system answered a particular question, or met a specific need at that time.
Actually you could write an entire book just on this subject.

The Placidus system was developed by several astrologers/astronomers/mathematicians in the early 17th century. These people knew about the latest developments in astronomy and trigonometry and they wanted to use these insights to make real the vision of Ptolemy, at least how they interpreted that, as he described the motions of the sky in his chapter on primary directions in the Tetrabiblos.
You could say that these astrologers were fundamentalists, who wanted to take the word of God (in this case the word of Ptolemy) as litteral as possible.

As a result, a system was developed that is particularly good at describing the motions of the sky due to the rotation of the earth, the so-called Primum Mobile, or first motion of the earth (the second motion being its yearly path around the sun.)

So if you care about these astronomical elements and want to take into account the differences these astronomical elements cause in the shape of the horoscope at different latitudes, then maybe you as an astrologer will be attracted to this system. Further advantage is that quite a lot of astrologers use this system and a lot of material is published on the interpretation of the houses with the Placidus system in mind.
But does that mean that "Placidus is a better representation of the sky at the moment in question?" I'm not so sure. It depends on the things you desire to take into account when you construct your houses.

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Donald

It really depends on what you’re trying to model specifically - all the house systems model something.

If you want to model something observable for example, so that you could look up into the sky and imagine roughly where the houses are, Placidus will not help you. If you’re trying to model the rising and setting of a point, fixed for a given moment in time (like birth), Placidus won’t help. If you want to model the relative movement of planets and points as they actually rise and set etc such that you can divide it up temporally evenly then Placidus excels and outshines any other house system.

But it really depends what your focus is in terms of what you want to model. Because no house system can model everything all at once with equal weight.
"The only true wisdom is in knowing you know nothing" - Socrates

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Placidus houses

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Another question please. If Placidus cusps 12, 11, and 10 represents 1/3, 2/3 and 3/3 of the diurnal cycle for each degree on its own declination circle, does that also represent 30, 60, and 90 degrees respectively of the space from the horizon to the midheaven? That is, do Placidus cusp represent a true picture of the sky?

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No, not 30, 60 and 90 degrees and yes, it is a true picture of the sky.

The 1/3, 2/3 and 3/3 cut-off points are 30, 60 and 90 degrees ONLY when the diurnal arc is 180 degrees. Therefore, this is only true for the equinoxes and any other point on the celestial equator. But Placidus takes all diurnal arcs into account, (and all nocturnal arcs for that matter,) not just the one that happens to be 180 degrees, so in that way it is a rendering of the true sky, seen through the lens of ascension and descension in relation to the horizon.

Placidus and planetary hours

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As per Deborah Houlding's book, Placidus houses are based on the same concept as planetary hours in that each Placidus house contains 2 planetary hours. I get the idea but I have a point of confusion here since planetary hours are based on solar time (sunrise to sunset) yet Placidus houses are based on sidereal time. Am I missing something here? Anyone have any thoughts on this issue?

Re: Placidus and planetary hours

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Your understanding is correct. The principals behind the planetary hours and Placidus houses are the same, but the calculation is slightly different in the way you describe.

Both the planetary hours and the Placidus houses are calculated by dividing how much time something is above the horizon by the entire time it will spend above the horizon. Ecliptic degrees are points that do not move at all, because they are part of the tropical system itself, but the Sun is always moving and changes its declination all the time. And a change in declination results in a different diurnal arc, while ecliptic degrees have always the same declination. (The obliquity of the ecliptic changes very very slowly.)

Therefore, it is more practical to calculate the planetary hours in one way (using the rising and setting times of the Sun) and the Placidus houses in another way (using the diurnal and nocturnal arcs,) but the end result is the same.
At the start of the 5th hour of the day, for example, the Sun is exactly on the placidean cusp 11, while the rising and setting times of that same point on the ecliptic, are very slightly different from those of the Sun. Again: because the Sun is moving and the ecliptic is not.