The term 'EQUINOX MO' is composed of three abbreviations: SEPT (4 letters), MAR (3 letters) and SEP (3 letters). SEPT stands for September, MAR stands for March, and SEP stands for the astronomical term equinox, which refers to the two times a year when the sun is directly above the equator and the length of day and night are nearly equal. The two equinoxes occur in March and September, hence the abbreviations MAR and SEP.