Construct houses.

We have already divided the zodiac into twelve equal signs and placed Mira’s planets within them. Adding houses gives the chart a second set of divisions, related to the local sky we explored in the previous lesson. Although signs and houses both number twelve, they serve different purposes: the zodiac provides our reference for celestial directions, while a house system supplies a rule for arranging boundaries around the local chart.

Before we can say which house contains a planet, we need to locate the boundary at which each house begins. This beginning is called a house cusp. Different house systems construct these cusps differently, so choosing a system is a choice of method. We’ll follow the geometry of those methods here; symbolic interpretation is outside this course.

Start with Equal houses.

The simplest construction to follow begins exactly at the Ascendant and gives every house the same width. Since twelve houses must fill a 360° circle, dividing 360° by twelve gives 30° per house. This is the rule for Equal houses: the first cusp is the Ascendant, and each following cusp is found by adding another 30°. Whenever an angle reaches 360°, we wrap it back around the circle.

To reach the fifth cusp, we take four 30° steps from the first: 47 + 4 × 30 = 167°, or 17° Virgo. The same pattern works for any house number n: take n − 1 steps from the first cusp, then wrap the result if needed. After twelve steps we have added a full 360° and returned to the Ascendant.

Whole Sign makes a different first cut.

Another construction begins with the sign containing the Ascendant, rather than the Ascendant’s exact degree. Keep our practice Ascendant at 17° Taurus. In Whole Sign houses, Taurus as a whole becomes the first house, so the first boundary is 0° Taurus, or 30° on the full circle. Gemini becomes the second house, Cancer the third, and each remaining sign supplies one complete house.

Changing the construction has moved the first boundary, while the Ascendant itself remains at 17° Taurus, inside the first house. The horizon has stayed in the same direction. For Mira’s sidereal Whole Sign chart, we must choose the sign containing her sidereal Ascendant. Simply shifting an already constructed tropical house grid would miss this choice of a whole sidereal sign.

Let the four angles set the corners.

Equal and Whole Sign give us boundaries by taking equal steps around the zodiac. Other systems begin with all four angles we found in the previous lesson: the Ascendant, Imum Coeli (IC), Descendant and Midheaven (MC). They place these at the beginnings of houses 1, 4, 7 and 10. The four directions divide the zodiac into sections called quadrants, whose widths in zodiac degrees need not be equal.

One way to fill in the intermediate boundaries is to divide each quadrant into three equal arcs along the ecliptic. This is the Porphyry construction. For example, if the arc from Ascendant to IC spans 84°, dividing by three gives three houses of 28° each. A neighbouring quadrant can have a different width, so its three equal divisions can produce houses of a different size.

We can also base the divisions on the time involved in rising and setting motion, rather than on equal lengths along the zodiac. Placidus uses this kind of construction. To locate its intermediate boundaries, the calculation repeatedly refines a trial value until it reaches a solution. This method therefore differs from both adding 30° in Equal houses and dividing an ecliptic quadrant into three arcs in Porphyry.

The relationship between the MC and the tenth house depends on the chosen rule. In these quadrant systems the MC sets the tenth cusp; Equal and Whole Sign do not require that alignment. Wherever the MC falls, it still marks the same calculated direction in the local sky. Astrodienst’s overview of house systems provides a comparison of the constructions.

Try the rules on the same birth.

In the practice wheel above, you can compare the constructions using controlled teaching angles. The calculator below then applies the selected rule to Mira’s record, or the birth details you entered. Begin with Equal and compare the first cusp with the Ascendant. Switch to Whole Sign and calculate again: the planetary positions and Ascendant should agree, while the house boundaries can change.

Then try Placidus and read the method reported with the result. At some locations and times, the calculation cannot produce the intermediate boundaries required by that system. This engine then uses Porphyry and identifies the method actually used, so you can tell which construction produced the displayed cusps. If the polar geometry also prevents a usable Porphyry division, choose Equal or Whole Sign to continue.

This distinction also explains a difference between the two activities: the practice wheel lets you explore Porphyry directly, while the chart calculator uses it as the fallback for Placidus rather than offering it as a separate input choice.

Make it yours

Try the idea.

A few questions to put the pieces together. Take your time; you can try again.

1. The Ascendant is 352°. Where does the third Equal house begin?

Cusp longitude in degrees · a whole number is enough (within 0.01)

2. For that same sidereal Ascendant of 352°, where does the first Whole Sign house begin?

Cusp longitude in degrees · a whole number is enough (within 0.01)

3. A Porphyry quadrant spans 96°. How wide is each of its three houses?

Width in degrees · a whole number is enough (within 0.01)

Check my understanding

A rule you can explain.

We can now follow a house system from its starting boundary to the remaining cusps, and explain why changing that rule can place the same planet in a different house. Mira’s chart has its planetary positions, its local orientation and its house boundaries. In the final lesson, we will bring these layers together on one wheel.