What is a lunar month? It’s the duration between successive new moons. Also called a lunation or synodic month, it has a mean period of 29.53059 days (29 days 12 hours and 44 minutes). That’s the mean, but the the true length varies throughout the year.
The new moon on January 16-17, 2018, brings to an end the longest lunar month of the 21st century (2001 to 2100). This very long lunar month began with the new moon of December 18, 2017 and lasted for 29 days, 19 hours and 47 minutes.
The new moon on January 16-17, 2018, also begins the longest complete lunar month of 2018, which will end with the new moon on February 15. Its duration will be 29 days 18 hours and 48 minutes.
The shortest lunar month of 2018 starts with the June 13 new moon and ends at new moon on July 13, lasting 29 days 07 hours and 05 minutes.
This year’s longest lunar month (January 16-17 to February 15) is 6 hours and 4 minutes longer than the mean lunar month, and the shortest lunar month (June 13 to July 13) is 5 hours and 39 minutes shorter than the mean lunar month.
Added all up, the duration of the year’s longest lunar month is 12 hours and 43 minutes greater than that of the shortest lunar month.
Follow the links below to learn more:
Lengths of the lunar months in 2018
|Successive new moons||Length of lunar month|
|Dec 18, 2017 to Jan 17, 2018||29 days 19 hours 47 min|
|Jan 17 to Feb 15||29 days 18 hours 48 min|
|Feb 15 to Mar 17||29 days 16 hours 06 min|
|Mar 17 to Apr 16||29 days 12 hours 46 min|
|Apr 16 to May 15||29 days 09 hours 51 min|
|May 15 to Jun 13||29 days 07 hours 55 min|
|Jun 13 to Jul 13||29 days 07 hours 05 min|
|Jul 13 to Aug 11||29 days 07 hours 10 min|
|Aug 11 to Sep 09||29 days 08 hours 04 min|
|Sep 09 to Oct 09||29 days 09 hours 45 min|
|Oct 09 to Nov 07||29 days 12 hours 15 min|
|Nov 07 to Dec 07||29 days 15 hours 18 min|
|Dec 07, 2018 to Jan 06, 2019||29 days 18 hours 08 min|
Why are the lunar months different lengths? In a nutshell, the longest lunar month occurs when the successive new moons coincide closely with lunar apogee – the moon’s farthest point from Earth in its orbit.
In contrast, the year’s shortest lunar month takes place when the successive new moons fall appreciably close to lunar perigee – the moon’s closest point to Earth in its orbit.
On the average, the lunar month (new moon to new moon) is about 2.22 days longer than the sidereal month (one complete revolution of the moon relative to the background stars). However, if the moon is near apogee at the end of one sidereal month, the moon travels more slowly than average in its orbit. Therefore, the period of time between the end of the sidereal month and the end of the lunar month is longer than average.
The opposite is the case when the moon is near perigee. The moon travels more swiftly in its orbit at perigee, in which case the time period between the end of the sidereal month and the end of the lunar month is less than average.
The most extreme longest lunar months happen when successive new moons occur near lunar apogee – and in addition, when Earth is near perihelion (its closest point to the sun). Because Earth is always closest to the sun in early January, the very longest lunar months take place in between December and January new moons.
On the other hand, extremely short lunar months happen when successive new moons fall near lunar perigee – and in addition, the Earth is near aphelion (Earth’s farthest point from the sun in its orbit). Because Earth is always at aphelion in early July, the very shortest lunar months take place in between June and July new moons.
When are the longest and shortest lunar months of 21st century? The longest lunar month of the 21st century (2001 to 2100) occurs in between the December 2017 and January 2018 new moons. With a length of 29 days 19 hours and 47 minutes, this particular lunar month exceeds the mean by a whopping 7 hours and 3 minutes.
The century’s shortest lunar month takes place in between the new moons of June and July 2053, a period of 29 days 6 hours and 35 minutes. That’s 6 hours and 9 minutes shorter than the mean.
Incidentally, exceptionally long or short lunar months repeat in cycles of 9 years.
Bottom line: In 2018, the shortest lunar month happens in between the June 13 and July 13 new moons; and the longest one in between the January 17 and February 15 new moons. Click here for a complete listing for the length of each lunar month in the 21st century.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.