House systems: Placidus and Whole Sign

A house system is a rule for cutting the sky into twelve houses. The two systems used here agree on the four angles and disagree on the lines between them, because there are several defensible ways to divide a curved sky. Both systems place the Ascendant and the Midheaven at the same points. Only Placidus makes the Midheaven the start of the 10th house.

The two used here differ in kind. Placidus divides time: it splits the span each point of the ecliptic takes to travel between the horizon and the meridian. Its houses have unequal widths, and it cannot be computed beyond the polar circles, where the engine falls back to Whole Sign.

Whole Sign divides the zodiac: the sign of the Ascendant is the whole 1st house, the next sign the 2nd, and so on. Every house is exactly 30°, and the Ascendant can fall anywhere inside the 1st house, not only at its start.

Switch systems and the same wheel redraws. The planets stay where they are. Only the lines between the houses move, and with them the house a body lands in. A change of house is a change of label, not of position.

Neither is the correct one: they are different rules, not different measurements. Hellenistic astrologers used Whole Sign houses, and modern psychological astrology usually uses Placidus. The choice changes which area of life a body is read in, so Genastro lets you switch and always says which system a chart uses.