The Astronomic Soli-Lunar Calendar or Astronomical Blue Moon Calendar was developped by Walter Ziobro, who posted a description to the CALNDR-L mailing list on 22 August 2013. Karl Palmen suggested alternative month names early / mid / blue / late per quarter.
This lunisolar calendar is based on the seasonal blue-moon rule: If four full moons occur between two seasonal points (equinoxes and solstices), the third is a seasonal blue moon. It becomes the leap month here.
The year is divided into a notos half, where the full moon will occur south of the celestial equator, and a arctos half, when it will appear north. Months within these halves are labeled by Greek letter names sequentially, but leap months use ancient letters Qoppa and Digamma respectively.
|Season||Ordinal number||Months||Approximate Hebrew||Approximate Gregorian|
|North Spring, South Fall/Autumn, Southward / Southither / Paschal / East||01||01||01||01||01||α||Notos Alpha||Early||Nisan||March–April|
|03||ϟ||Notos Koppa||Blue||blue leap|
|North Summer, South Winter, Southback / Southence / Post-Paschal / South||04||05||04||04||04||δ||Notos Delta||Early||Tammuz||June–July|
|06||ϝ||Notos Digamma||Blue||blue leap|
|North Fall/Autumn, South Spring, Northward / Northither / Anti-Paschal / West||07||08||08||07||07||Α||Arctos Alpha||Early||Tishrei||September–October|
|09||Ϟ||Arctos Koppa||Blue||blue leap|
|North Winter, South Summer, Northback / Northence / Pre-Paschal / North||10||11||11||11||10||Δ||Arctos Delta||Early||Tevet||December–January|
|12||Ϝ||Arctos Digamma||Blue||blue leap|
Months start with the new moon per UT, therefore their lengths are variable.
In the 19 year period between 2014-03-31 and 2033-03-30, there will be seven leap months:
- Notos Koppa
- Notos Digamma
- Arctos Koppa
- Arctos Digamma
This uneven distribution exemplifies that northern spring and summer are longer than the southern ones, i.e. northern fall/autumn and winter, currently.