ASTR 101 Lecture Notes - Lecture 4: Lunisolar Calendar, Pope Gregory Xiii, Gregorian Calendar
8 Feb 2017
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12 months of 30 days, each is 354 days. A 12-month lunar cycle is short of 11 days of a solar year of 365 days. The seasons drift among the lunar months. To synchronize lunar and solar cycles, a month is added when considered necessary by observations of yearly cycles in the skies. E. g. first appearance of the star sirius in the morning sky was what early egyptians used to observe in order to choose to add an extra month or not that year. By 5th century bc, babylonians developed fixed system where a leap month was added into seven different years, in a fixed pattern, within every period of nineteen years. 235 lunar months is exactly 19 years (metonic cycle) 7 out of the 19 years are leaf years with an extra month. Hebrew: add an extra 13th month every 2/3 years, tibetan, chinese, hindu.
