Placidus and Whole Sign are different house systems, not different planetary calculations. Whole Sign assigns each entire zodiac sign to one house beginning with the rising sign; Placidus is a quadrant system whose house cusps depend on time and geographic latitude.
Whole Sign Houses
In Whole Sign, the zodiac sign containing the Ascendant becomes the entire first house, the next sign becomes the second house, and so on. Each house therefore spans exactly one sign, although the Ascendant degree itself can fall anywhere inside the first sign.
Placidus Houses
Placidus is a time-based quadrant house system. The Ascendant and Midheaven anchor the chart, and intermediate cusps are calculated through a specific astronomical method. The houses are usually unequal in zodiacal size.
Why a planet can “move” houses
The planet has not moved. Its zodiac longitude stays the same. The boundary of the house changed. A planet at 28° of one sign may sit in the first house in one system and the second house in another.
Which system is correct?
Astrology has no scientific test that has established one house system as objectively true. Different astrological schools prefer different systems and interpret them through different traditions.
Why Ava Lux checks both
Ava Lux uses Placidus as a detailed quadrant view and can compare Whole Sign where it clarifies the broader sign-house structure. The reading does not hide a major house-system discrepancy. If a placement changes houses, that becomes interpretive information rather than an inconvenience to erase.
High-latitude complication
Quadrant systems such as Placidus can create extremely unequal houses at high latitudes and may encounter calculation limitations in extreme cases. Whole Sign avoids that specific geometry because each sign is assigned as a house.
The practical rule
Do not choose a house system merely because it gives the interpretation you prefer. Choose a coherent method, state it and keep the astronomical positions separate from the interpretive house framework.
Sources and textual boundary
Ava Lux editorial rule: source texts, later Kabbalistic interpretation, modern astrology and the Ava Lux reading method are not treated as if they were the same layer. Where traditions disagree or textual versions vary, the page says so.