Divide an equatorial arc, then return to the zodiac
Right ascension measures around the celestial equator; zodiac longitude measures around the ecliptic. Their degree values generally differ because the two circles are tilted relative to one another. Equal divisions in right ascension therefore need not become equal longitude intervals.
Sky Degrees finds the rising point’s right ascension relative to the meridian, divides this semiarc into thirds and constructs the intermediate meridians. The remaining portion of the half-circle supplies the below-horizon divisions.
The important distinction from Placidus
Alcabitius uses the Ascendant’s semiarc as its starting measure. Placidus solves conditions involving each candidate cusp’s own semiarc. Calling both time-based does not make their intermediate boundaries interchangeable. Compare actual cusp degrees before comparing house readings.
When the construction needs to stop
The implementation requires the relevant semiarc to lie strictly between 0° and 180°. It rejects a degenerate semiarc, an undefined angle or a layout that cannot provide unambiguous house placement. This is not a claim that every high-latitude birth has a supported Alcabitius chart.
A CONCRETE EXAMPLE
Worked projection: thirds in right ascension are not zodiac thirds
Use ARMC 100°, latitude 40° N and obliquity 23.4393° as a geometry check. The Ascendant’s right ascension lies 87.337126° beyond the meridian. One third is approximately 29.112375°; two thirds are 58.224750°.
Projecting those meridians back onto the ecliptic gives cusp eleven at 126.720969° and cusp twelve at 156.471754°. The MC is 99.189513°. By comparison, Placidus gives cusp eleven at 132.507284° for identical inputs—a 5.786315° difference in the boundary, without changing the planetary sky.
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
Does Alcabitius divide the zodiac into twelve equal parts?
No. The division happens in the Ascendant’s semiarcs, followed by projection. The resulting house widths in ecliptic longitude are generally unequal.
What should I record when comparing an Alcabitius chart?
Record the birth instant, coordinates, zodiac, ayanamsa if applicable, and the selected system. Keep the exact cusp and axis values; a matching chart label alone is insufficient.
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