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. Months start with the new moon per UT, therefore their lengths are variable.