Regiomontanus
Regiomontanus (Johannes Müller, 1436-1476)
medieval default · celestial equator-based · for horary · historical reference
How it divides the sky
Regiomontanus uses the celestial equator as base — divides the equator into 12 equal arcs, projects each division onto the ecliptic. So natural for astrologers working with equatorial coordinates. Quadrant-based. Yields different results from Placidus, especially at mid-to-high latitudes.
When to use it
If you do horary (hourly question) astrology and want to stay faithful to medieval sources, Regiomontanus is the default. William Lilly used this system. May be found too complex for modern astrology.
History
Regiomontanus published in 1467; was the default system in Europe until the end of the Middle Ages. Placidus replaced it in the 17th century.