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The formal lunar day is therefore the time of a full lunar day-night cycle. Due to tidal locking, this equals the time that the Moon takes to complete one synodic orbit around Earth, a synodic lunar month, returning to the same lunar phase. The synodic period is about 29.53 Earth days, which is about 2.2 days longer than its sidereal period.
The average duration in modern times is 29.53059 days with up to seven hours variation about the mean in any given year. [7] (which gives a mean synodic month as 29.53059 days or 29 d 12 h 44 min 3 s) [a] A more precise figure of the average duration may be derived for a specific date using the lunar theory of Chapront-Touzé and Chapront (1988):
Since each lunation is approximately 29 + 1 ⁄ 2 days, [1] it is common for the months of a lunar calendar to alternate between 29 and 30 days. Since the period of 12 such lunations, a lunar year , is 354 days, 8 hours, 48 minutes, 34 seconds (354.36707 days), [ 1 ] purely lunar calendars are 11 to 12 days shorter than the solar year .
Each complete cycle of phases is called a "lunation". [7] The approximate age of the Moon, and hence the approximate phase, can be calculated for any date by calculating the number of days since a known new moon (such as 1 January 1900 or 11 August 1999) and reducing this modulo 29.53059 days (the mean length of a synodic month).
Years began on a jiǎzǐ (甲子) day and a 72-day wood phase, followed by a bǐngzǐ (丙子) day and a 72-day fire phase; a wùzǐ (戊子) day and a 72-day earth phase; a gēngzǐ (庚子) day and a 72-day metal phase, and a rénzǐ (壬子) day followed by a water phase. [8] Other days were tracked using the Yellow River Map (He Tu).
The time between each gēng is 1 ⁄ 10 of a day, making a gēng 2.4 hours—or 2 hours 24 minutes—long. The 5 gēngs in the night are numbered from one to five: yì gēng ( 一 更 ) (alternately chū gēng ( 初更 ) for "initial watch"); èr gēng ( 二更 ); sān gēng ( 三更 ); sì gēng ( 四更 ); and wǔ gēng ( 五更 ).
TIL in 2000 a Mexican woman performed an hour-long C-section on herself with a kitchen knife after 12 hours of constant pain. After 3 attempts to cut open her abdomen, she made a 17cm vertical ...
TJD was a 4-digit day count from MJD 40000, which was May 24, 1968, represented as a 14-bit binary number. Since this code was limited to four digits, TJD recycled to zero on MJD 50000, or October 10, 1995, "which gives a long ambiguity period of 27.4 years". (NASA codes PB-1–PB-4 used a 3-digit day-of-year count.) Only whole days are ...