Which circle gets divided?
The celestial equator is Earth’s equatorial plane extended into the sky. Regiomontanus marks equally spaced points on that circle relative to the local meridian. Each mark helps define a house plane; the chosen intersection of that plane with the ecliptic supplies a cusp.
The calculation preserves the orientation of each house semicircle. A plane intersects the ecliptic at two opposite points, so selecting the intended branch is part of the geometry, not a cosmetic adjustment to the displayed chart.
Compare with Campanus without moving the birth
Campanus divides the prime vertical instead of the celestial equator. Both methods use great-circle geometry, but their intermediate cusps can differ substantially. Hold the birth instant, place and zodiac constant to isolate this construction difference.
Polar geometry needs an additional placement check
In polar regions, cusp order can reverse and a meridian cusp need not match the separately calculated true MC. Sky Degrees checks whether consecutive cusps form one nonoverlapping 360° partition before assigning planets to houses. Ambiguous partitions are rejected, even if individual cusp intersections can be calculated.
A CONCRETE EXAMPLE
Worked comparison: the same planet, two spatial divisions
For ARMC 100°, latitude 40° N and obliquity 23.4393°, Regiomontanus gives cusp two at 212.944997° and cusp three at 242.908712°. Campanus gives cusp two at 219.343660° for the same inputs—a difference of 6.398663°.
An illustrative planet at longitude 215°, or Scorpio 5°, falls inside the Regiomontanus second house. It remains before the Campanus second cusp, in house one. Both calculations retain Ascendant 187.988849° and true MC 99.189513° in this ordinary-latitude check. The changed house number comes from the spatial projection.
Read it in your own chart.
Keep the date, recorded time, location and zodiac visible as you compare results. Guest calculations need no account.
Create your free chartCommon questions
Are Regiomontanus houses always 30° wide?
No. The equatorial divisions are 30°. Their projection onto the tilted ecliptic produces different longitude intervals, depending on the geometry.
Does a calculable cusp guarantee a usable polar chart?
No. Defined intersections alone do not establish an unambiguous house partition. The placement check must also pass; geographic poles and degenerate plane intersections are unsupported.
Calculation & interpretation
New personal calculations use Skyfield with JPL DE440 data and the site’s documented house and zodiac rules. The preserved Greenwich teaching chart has its own historical source label. Worked arithmetic and geometry examples are labelled separately. Astrological meanings are symbolic conventions, not established scientific predictions.
Birth time and place affect angles and houses. Unsupported or ambiguous inputs return an error rather than an invented chart. Personal results stay in browser state and do not create public result URLs.
Read the calculation settings guide