Regiomontanus

Regiomontanus was developed by the German astronomer-mathematician Johann Müller (1436–1476). It divides the celestial equator (not the ecliptic) into equal 30° arcs, then projects those arcs onto the ecliptic to determine house cusps. The result is an unequal-house system that differs from Placidus, particularly in the intermediate houses. Regiomontanus is strongly preferred by horary astrologers working in the William Lilly tradition, as Lilly himself used this system.

William Lilly, the most influential traditional horary astrologer (1602–1681), used Regiomontanus. When studying Lilly's methods and working horary charts in the traditional manner, using Regiomontanus is considered essential for authenticity and accuracy. The Regiomontanus system gives house cusps that often differ from Placidus by several degrees, most notably in the 2nd, 3rd, 11th, and 12th houses — which can change house rulerships significantly in horary work.

Use Regiomontanus if you practice traditional horary astrology in the Lilly tradition, or if you are studying Medieval and Renaissance-era astrological texts that specify it. For natal chart work, most modern astrologers prefer Placidus, Koch, or Whole Sign — but Regiomontanus is worth experimenting with if you find Placidus placements consistently unsatisfying for the intermediate houses. Primary directions work particularly well with Regiomontanus in the traditional framework.