In the first time-based house system that we looked at, the Alcabitius system, we carefully measured the time it took for the point of the ascendant (the point where the horizon meets the Sun’s path along the meridian), to reach the culmination point of the Sun at the midheaven, or MC (Medium Coeli).
We then took the total time of the Sun’s path along the ecliptic from the ascendant to the MC and divide the time it took into three equal parts. Adding one of these parts to the time of the ascendant gives us the time when the point of the ascendant reaches the 12th house cusp, adding the sum again gives us the 11th house cusp, and we convert those times into degree points on the ecliptic.
The beauty of this system is that it is linked to the local meridian, and thus more directly to the location of the earth-based native of the chart.
With Alcabitius, at first glance it would appear that since time is trisected between the ascendant and the MC, that the time it takes for the Sun (or any planet) to travel would be the same for each of the trisected houses, since each of the houses represent an equal amount of time.
However, this is not the case. And this is where the innovation of the Placidus system enters the picture.
The fact of the matter is that the Sun transverses some astrological signs more quickly than others. This is what William Lilly referred to as “straight signs” versus “crooked signs”. Or put another way, “signs of right ascension” as opposed to “signs of oblique ascension”, or “signs of long ascension” vs “signs of short ascension”.
Because of the way the earth is tilted with respect to the ecliptic, six of the constellations along the ecliptic seem flatter/not as tall/crooked/shorter than the other six, which appear to be standing upright, “straight”, or long.
The Sun takes less time with the crooked/oblique/short signs to “ascend” along the ecliptic than it does with the upright, straight, or long ones.
In the northern hemisphere, the signs of right ascension (straight/upright/long) are: Cancer, Leo, Virgo, Libra, Scorpio and Sagittarius.
The signs of oblique ascension (crooked/oblique/short) are: Capricorn, Aquarius, Pisces, Aries, Taurus, Gemini.
In the southern hemisphere, due to the earth’s tilt of 23.5° in relation to the ecliptic, these are reversed: i.e. the signs of long ascension become the signs of short ascension.
You can easily see for yourself how widely the ascension times vary for each sign in any astrological program capable of animating a chart wheel, such as Astro Gold or Solar Fire. I’ve prepared a demonstration video in Solar Fire to demonstrate this, you can do it yourself at home!
And here is a lovely YouTube video using Stellarium and Sumer 1.3 by Rumen Kolev which demonstrates the phenomenon of long ascension versus short ascension quite nicely, observe especially the image on the right:
So in summary, Porphyry is an ecliptic-based system which simply divides the ecliptic in three equal parts from the ascendant to the MC, and then again from the MC to the Descendant, and by extension to the rest of the 12 astrological houses.
Alcabitius is a time-based system, which measures the time it takes for the point of the ecliptic to move in a diurnal (primary, literally “to the right”) direction until it reaches the MC. We then divide the time by three (rather than dividing the ecliptic itself). Then we do this operation again from the IC to the ascendant, and by extension the rest of the twelve astrological houses.
Placidus was an innovation, in that it took into account that although the signs are a standardized 30° each on the ecliptic, the Sun moves along these signs at different rates of speed due to their positioning in relation to the local meridian. It is thus a more accurate representation of the size of an astrological house in relation to the native’s position on earth. We do not get this in any of the other previously mentioned house systems.
We will go more deeply into the Placidus system in the next post.