▁States ▁due ▁to ▁the ▁Farm ers ' ▁Al man ac ▁( pub lished ▁since ▁ 1 8 1 8 ). ▁ ▁According ▁to ▁the ▁Farm ers ' ▁Al man ac , ▁a ▁" blue ▁moon " ▁is ▁the ▁third ▁full ▁moon ▁in ▁any ▁period ▁between ▁either ▁sol st ice ▁and ▁equ ino x , ▁or ▁between ▁equ ino x ▁and ▁sol st ice , ▁( calcul ated ▁using ▁the ▁mean ▁tropical ▁year ), ▁which ▁contains ▁four ▁full ▁mo ons . ▁These ▁seasons ▁are ▁equal ▁in ▁length , ▁unlike ▁the ▁astronom ical ▁ones , ▁which ▁vary ▁in ▁length ▁depending ▁on ▁the ▁Earth ' s ▁speed ▁in ▁its ▁elli pt ical ▁orbit ▁round ▁the ▁Sun . ▁To ▁compare , ▁in ▁ 1 9 8 3 ▁the ▁equal ▁length ▁seasons ▁began ▁at ▁ 1 : 4 8 ▁am ▁on ▁ 2 3 ▁March , ▁ 9 : 1 5 ▁am ▁on ▁ 2 2 ▁June , ▁ 4 : 4 2 ▁pm ▁on ▁ 2 1 ▁September ▁and ▁ 1 2 : 1 0 ▁am ▁on ▁ 2 2 ▁December , ▁while ▁the ▁astronom ical ▁seasons ▁began ▁at ▁ 4 : 3 9 ▁am ▁on ▁ 2 1 ▁March , ▁ 1 1 : 0 9 ▁pm ▁on ▁ 2 1 ▁June , ▁ 2 : 4 2 ▁pm ▁on ▁ 2 3 ▁September ▁and ▁ 1 0 : 3 0 ▁am ▁on ▁ 2 2 ▁December ▁( all ▁times ▁G MT ). ▁Due ▁to ▁a ▁mis inter pret ation ▁of ▁this ▁definition ▁in ▁the ▁March