House Systems
There are many different ways of calculating houses. Porphyry is our favorite way of doing it (and the way we showed you).
Porphyry
Porphyry is the default house system used at Co–Star. At the time of birth, the precise degree of the sign corresponding with the MC, or Midheaven — aka, where the highest point of the Sun’s path through the sky (the ecliptic) intersects with the meridian — is determined, as is the precise degree of the sign rising on the Eastern Horizon (the Ascendant).
The degree of the sign corresponding with the MC marks the cusp of the 10th house. The degree of the Ascendant marks the cusp of the 1st house. The rest of the space between the MC and the Ascendant is divided into three equal parts.
The Descendant and the IC (Imum Coeli, or “bottom of the sky”) are taken to be 180° from (i.e., directly opposite) the Ascendant and the MC, respectively, and the space between them is also divided into three equal parts.
Porphyry prioritizes the angles of the chart (the Ascendant, the Descendant, the IC, and the MC) as the 1st, 7th, 4th, and 10th house cusps, respectively, and then determines the other houses from these. Because many people see these four angles as the most powerful parts of the natal chart, they prefer to use the Porphyry system rather than one like Placidus, in which the angles may fall between houses.
WHO USES IT?
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Whole Sign
The Whole Sign house system is the oldest form of house division, and remained the preferred method of determining houses for about a thousand years after its inception. Whole Sign takes the entire span of the zodiac sign that appears on the Ascendant at the time of a person’s birth as the 1st house. The following sign makes up the 2nd house, the next the 3rd house, and so on. All houses are of equal size (30°) in the Whole Sign system.
Whole Sign houses are very easy to calculate and use. More complicated phenomena such as intercepted signs cannot occur using the Whole Sign system.
WHO USES IT?
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Placidus
The most popular house system in use today is Placidus. Placidus is a time-based method of calculating the houses that was devised during the Renaissance. It works by marking the cusps of the houses in two-hour increments from your time of birth. The degree of the zodiac sign rising on the Eastern Horizon at your time of birth marks the cusp of the 1st house. After roughly two hours, another measurement is made to see where that degree is now: that marks the cusp of the 12th house. This continues for twelve hours, until the 7th house cusp has been marked. The angles of these six houses are then extended to make up the angles of the six remaining houses. Because each house is proportioned according to the amount of time the signs spend on the horizon from the perspective of earth, this generally results in houses of unequal size. The further your birth place is from the equator, the greater the distortion of the houses.
The Placidus system does not work for extremely northern or southern latitudes (greater than 66°N or 66°S) because certain degrees of the zodiac never touch the horizon in those regions. If you were born at a latitude exceeding 66°N or 66°S, you will not be able to calculate your chart using the Placidus house system.
Originally, it was popular in England, possibly because the Catholic Church banned it. It later became the default because while the underlying math is complex, it’s relatively simple to represent & reproduce the Placidus system in table form.
WHO USES IT?
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aspects
Aspects are special angles (like 0°, 180°, etc) between any two planets.