Begin with the observer’s local sky
Zenith means directly overhead; nadir is its opposite. Together with east and west, they define the prime vertical. Sky Degrees constructs equally spaced rays in this plane, then combines each ray with the north–south horizon axis to define a house plane.
Intersecting that plane with the ecliptic produces two opposite candidates. The calculation selects the intersection aligned with the intended ray. The local orientation matters: dividing a flat zodiac wheel into equal-looking wedges does not reproduce the construction.
Campanus versus Regiomontanus
Regiomontanus starts with equal divisions of the celestial equator. Campanus starts with the prime vertical. The distinction concerns which circle is divided before projection. Use actual cusp degrees when comparing the systems; the common description ‘spatial houses’ is not enough to identify a boundary.
An explicit limit at unusual latitudes
Some polar configurations reverse the sequence of ecliptic cusps. Sky Degrees rejects house placement when those cusps do not partition a single zodiac turn without overlap. It also rejects undefined intersections and geographic poles. The true MC is returned independently and must not be inferred merely from a polar tenth-cusp label.
A CONCRETE EXAMPLE
Worked projection: equal sky arcs, unequal zodiac intervals
At ARMC 100°, latitude 40° N and obliquity 23.4393°, Campanus returns Ascendant 187.988849°, second cusp 219.343660° and third cusp 249.927440°. The first zodiac interval is 31.354811°; the next is 30.583780°, despite equal 30° divisions of the prime vertical.
The eleventh cusp is 127.887847°, compared with 134.645885° in Regiomontanus for the same input geometry. The shared true MC is 99.189513°. These numbers show the effect of choosing a different circle for projection; they do not establish that one system predicts events better.
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
Is the prime vertical the same as the zodiac?
No. It is a local great circle through east, west, zenith and nadir. The zodiac follows ecliptic longitude. Campanus projects from the former onto the latter.
Can I switch to Campanus without recalculating?
Select Campanus in calculation settings and apply the change. Keep the same birth inputs and zodiac, then inspect the newly calculated cusps and house assignments rather than reusing the previous labels.
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