House systems, and why they disagree

Twelve houses, several incompatible ways of drawing them, and no consensus after two thousand years.

All house systems agree on the horizon and the meridian: the Ascendant and Midheaven are astronomical facts. They disagree on how to divide the space between them.

Whole Sign, the oldest, gives each house one entire sign, starting with the sign of the Ascendant. It is defined everywhere on Earth and never produces a distorted chart.

Placidus, the default here and almost everywhere since the eighteenth century, divides the time it takes a degree to rise from the horizon to the meridian. It produces houses of markedly unequal size, and above the polar circles it fails outright — some degrees never rise at all. The instrument refuses rather than fudges it, and suggests Whole Sign instead.

Koch, Regiomontanus, Campanus and Porphyry each slice the sphere differently. For a chart near the equator they broadly agree. At high latitudes they diverge sharply, and a planet can land in different houses depending only on which system you chose.