Choose a simple house convention

1. Begin twelve equal intervals at the Ascendant

Imagine marking a circular dial every 30°. Twelve marks return you to the first because 12 × 30° = 360°. In the Equal convention implemented here, the first mark is the Ascendant. A house cusp is a boundary longitude; the twelve boundaries describe twelve equal intervals around the ecliptic. This division is an astrological convention applied after the astronomical directions have been calculated.

Use an invented Ascendant of 350°. The first three boundaries are 350°, 380° and 410° before wrapping. Subtracting a full turn from the last two gives 350°, 20° and 50°. The circle has not reversed direction at zero: the second boundary is still 30° beyond the first.

cusp h = wrap(Asc + (h − 1) × 30°), for house h = 1, …, 12

Here Asc is a longitude in degrees and wrap removes complete turns. House numbers begin at one, whereas the array index n in equal_cusps begins at zero. For house 10, n = 9, so the offset is 270°, not 300°. The complete list for Asc = 350° is [350, 20, 50, 80, 110, 140, 170, 200, 230, 260, 290, 320] degrees.

The supplied midheaven does not enter this formula. If our invented MC is 80°, the tenth Equal cusp is still 260°. house_set retains both the separate MC and the cusp array; it does not move a boundary to force MC onto house 10. The next lesson will introduce a convention that anchors the tenth cusp at MC.

Once Equal is selected, house_set returns Some containing twelve values. Latitude and time have already helped produce the Ascendant, but equal_cusps needs only that longitude. Changing the house choice therefore changes the subdivision without resampling the planetary positions.

const wrap = (x: number): number => { const r = x % 360; return r < 0 ? r + 360 : r; };
const equalCusps = (ascendant: number): number[] =>
  Array.from({ length: 12 }, (_, n) => wrap(ascendant + n * 30));

2. Keep a convention separate from its representation

Equal houses follow the exact Ascendant degree. Whole Sign convention instead associates the first house with the entire zodiac sign containing that direction. A sign here is one of twelve 30° zodiac sectors; it is not a constellation with an irregular sky boundary. This conceptual distinction helps us read the interface, but does not tell us that the runtime constructs sector boundaries.

In fact, house_set’s WholeSign branch returns cusps: None. Take the same invented Asc = 350° and MC = 80°. The HouseSet retains those angles and the WholeSign selection, but contains no twelve-number array. None means an absent optional value, rather than an empty list, twelve zeros or a failed Equal calculation.

WholeSign → HouseSet { system: WholeSign, ascendant: 350°, midheaven: 80°, cusps: None }

The public calculate_chart result copies the optional cusp field and returns its body records separately. It does not compute a Whole Sign array or assign planets to houses. A caller that wants sign-based house assignments needs an additional convention outside this runtime path. Filling in twelve longitudes here would teach behavior the engine does not implement.