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 longest lunar month in 2019 starts with the new moon on January 6, and concludes with the new moon on February 4. Its duration will be 29 days 19 hours and 35 minutes.
The shortest lunar month of 2019 starts with the August 1 new moon and ends with the new moon on August 30, lasting 29 days 07 hours and 25 minutes.
This year’s longest lunar month (January 6 to February 4) is 6 hours and 51 minutes longer than the mean lunar month, and the shortest lunar month (August 1 to August 30) is 5 hours and 19 minutes shorter than the mean lunar month.
Added all up, the duration of the year’s longest lunar month is 12 hours and 10 minutes greater than that of the shortest lunar month.
Follow the links below to learn more:
Lengths of the lunar months in 2019
|Successive new moons||Length of lunar month|
|Dec 07, 2018 to Jan 06, 2019||29 days 18 hours 08 min|
|Jan 06 to Feb 04||29 days 19 hours 35 min|
|Feb 04 to Mar 06||29 days 19 hours 00 min|
|Mar 06 to Apr 05||29 days 16 hours 47 min|
|Apr 05 to May 04||29 days 13 hours 55 min|
|May 04 to Jun 03||29 days 11 hours 16 min|
|Jun 03 to Jul 02||29 days 09 hours 14 min|
|Jul 02 to Aug 01||29 days 07 hours 56 min|
|Aug 01 to Aug 30||29 days 07 hours 25 min|
|Aug 30 to Sep 28||29 days 07 hours 49 min|
|Sep 28 to Oct 28||29 days 09 hours 12 min|
|Oct 28 to Nov 26||29 days 11 hours 27 min|
|Nov 26 to Dec 26||29 days 14 hours 08 min|
|Dec 26, 2019 to Jan 24, 2020||29 days 16 hours 29 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 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 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 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 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 2019, the shortest lunar month happens between the August 1 and August 30 new moons; and the longest one between the January 6 and February 4 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.