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The Duration of the Lunar Month





The period of time between two successive New Moons is called a lunation, lunar month or synodic month. One lunation has a mean duration of 29.530588 days (29 days, 12 hours and 44 minutes). Depending on the lunation, however, it can vary from about 29.26 to 29.83 days, a difference exceeding 13 hours.


The period of time between the September and October 2013 New Moons measures 29 days, 12 hours and 58 minutes. This lunation is about 14 minutes longer than the mean, while the lunation between the February and March 2013 New Moons is approximately 13 minutes shorter than the mean lunar month.

Image credit: Wikimedia Commons


The other lunations in 2013 are either longer or shorter than the two lunar months mentioned above. Click on this timeanddate.com link to know the duration for each lunar month in 2013 or any year.


Longest Durations: New Moon Near Lunar Apogee


When the New Moon is in the vicinity of lunar apogee - the moon's farthest point in its monthly orbit - the length of the lunar month exceeds the mean period. That why the lunations as measured between the May and June 2013 New Moons, and the June and July 2013 New Moons give the longest lunar months of 2013. These lunar months are over 29 days and 15 hours in length.


Lunar Perigee and Apogee Calculator


Duration of the Lunar Month


Shortest Durations: New Moon Near Lunar Perigee


On the other hand, when the New Moon is in the vicinity of lunar perigee - the moon's closest point in its monthly orbit - the length of the lunar month is shorter than the mean period. That's why the lunation between the December 2013 and January 2014 New Moons has a lifespan of less than 29 days and 11 hours.


Extreme Durations of the Lunar Month


The above examples hardy represent extreme lunations. In fact, the variation in the year 2013 is minimal. Looking at the graph below, you can see much greater variation in the length of the lunar month several years previously or several years afterwards. See timeanddate.com for specific time periods for lunations in any year


The mathematical wizard Jean Meeus computed the longest and shortest lunations from 1760-2200. The longest happened between the New Moons of December 9, 1787, and January 8, 1788: 29 days 19 hours and 58 minutes. The shortest lunation took place between the New Moons of June 12 and July 12, 1885: 29 days 06 hours and 34 minutes.

Image credit: Wikimedia Commons


Possible to Have Only Two Full Moons in One Season?


Moon's Orbit Explained


The longest lunation happens when, at the instant of the New Moon, the Moon aligns with apogee and the Earth is at perihelion. The shortest lunation takes place when the New Moon is near perigee and the Earth is near aphelion.


copyright 2013 by Bruce McClure


August 2013 Feature * October 2013 Feature