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Regiomontanus houses
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What it is
Regiomontanus is a quadrant house system that divides space rather than time. It takes the celestial equator — the Earth's equator projected out onto the sky — and cuts it into twelve equal arcs of 30 degrees, then carries those divisions onto the zodiac.
As in every quadrant system, the four angles are the cusps of the angular houses: the Ascendant opens the 1st, the Imum Coeli the 4th, the Descendant the 7th and the Midheaven the 10th.
How it is worked out
Picture the celestial equator as a great ring around the sky. Starting from the point where it crosses the eastern horizon, it is marked off every 30 degrees. Through each mark, and through the north and south points of the horizon, a great circle is drawn — the astrologer's position circles. Where each of those circles crosses the ecliptic, the circle of the zodiac, is a house cusp.
The reasoning behind using the equator is that it is the sky's clock: every degree of the equator rises at the same steady rate as the Earth turns. Dividing it equally gives equal portions of the daily rotation, translated into places along the zodiac. The houses that result are unequal in zodiacal size, and signs can be intercepted — wholly contained within a house.
Where it comes from
The system carries the name of Johannes Müller of Königsberg (1436–1476), the German astronomer known as Regiomontanus, whose tables of directions — written in the 1460s and printed after his death — made it easy to calculate. He did not claim to have invented it; he called it the rational method (modus rationalis), and earlier astronomers in the Islamic world and in medieval Spain, Abraham ibn Ezra among them, may have known it. Printing is why his name stuck.
It became the standard of Renaissance and early modern Europe. The Jesuit astronomer Clavius described it as the method most used in his day, the French astrologer Morin worked with it, and in England it is the system of William Lilly's Christian Astrology (1647): the house cusps, and the house rulers that answer a horary question, were all drawn in Regiomontanus.
When astrologers reach for it
- Horary astrology, above all. Most traditional horary practitioners use Regiomontanus because the techniques and worked examples they follow were written in it.
- Electional and other traditional work in the Lilly line.
- Primary directions in the traditional manner, since Regiomontanus based his directions on the same position circles.
It is less used in modern natal astrology, where Placidus took over in the nineteenth century. At high latitudes it keeps producing cusps where Placidus and Koch fail, but the houses become badly distorted.
In a reading here
Regiomontanus is one of the seven house systems Viastaro offers for charts, which use Placidus unless another is chosen. Horary readings here are always cast in Regiomontanus, whatever system a person's other charts use, because that is the grid the horary rules were written for.