Astronomic Soli-Lunar Calendar

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
	02	02	02	02	02	β	Notos Beta	Mid	Iyar	April–May
		03				ϟ	Notos Koppa	Blue	blue leap
	03	04	03	03	03	γ	Notos Gamma	Late	Sivan	May–June
North Summer, South Winter, Southback / Southence / Post-Paschal / South	04	05	04	04	04	δ	Notos Delta	Early	Tammuz	June–July
	05	06	05	05	05	ε	Notos Epsilon	Mid	Av	July–August
			06			ϝ	Notos Digamma	Blue	blue leap
	06	07	07	06	06	ζ	Notos Zeta	Late	Elul	August—September
North Fall/Autumn, South Spring, Northward / Northither / Anti-Paschal / West	07	08	08	07	07	Α	Arctos Alpha	Early	Tishrei	September–October
	08	09	09	08	08	Β	Arctos Beta	Mid	Cheshvan	October–November
				09		Ϟ	Arctos Koppa	Blue	blue leap
	09	10	10	10	09	Γ	Arctos Gamma	Late	Kislev	November–December
North Winter, South Summer, Northback / Northence / Pre-Paschal / North	10	11	11	11	10	Δ	Arctos Delta	Early	Tevet	December–January
	11	12	12	12	11	Ε	Arctos Epsilon	Mid	Shevat	January–February
					12	Ϝ	Arctos Digamma	Blue	blue leap
	12	13	13	13	13	Ζ	Arctos Zeta	Late	Adar	February–March

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

2016-05-07

2019-05-04

2027-05-06

Notos Digamma

2021-08-08

2024-08-05

2032-08-06

Arctos Koppa

2029-11-06

Arctos Digamma

none

This uneven distribution exemplifies that northern spring and summer are longer than the southern ones, i.e. northern fall/autumn and winter, currently.