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 actual length varies throughout the year. The lunar month beginning on October 30, 2016 is the longest lunar month of 2016. It lasts for 29 days 18 hours and 40 minutes, until November 30.
That’s 5 hours and 56 minutes longer than the mean.
And it’s 11 hours and 20 minutes longer than 2016’s shortest lunar month, which happened between the new moons of May 6 and June 5.
Follow the links below to learn more:
Lengths of the lunar months in 2016
|Successive new moons||Length of lunar month|
|January 10 to February 8||29 days 13 hours 08 min|
|February 8 to March 9||29 days 11 hours 16 min|
|March 9 to April 7||29 days 09 hours 29 min|
|April 7 to May 6||29 days 08 hours 06 minutes|
|May 6 to June 5||29 days 07 hours 30 min|
|June 5 to July 4||29 days 08 hours 01 min|
|July 4 to August 2||29 days 9 hours 44 min|
|August 2 to September 1||29 days 12 hours 19 min|
|September 1 to October 1||29 days 15 hours 08 min|
|October 1 to October 30||29 days 17 hours 27 minutes|
|October 30 to November 29||29 days 18 hours 40 min|
|November 29 to December 29||29 days 18 hours 35 min|
Why are the lunar months different lengths? In a nutshell, the longest lunar month of the year occurs when the successive new moons coincide closely with lunar apogee – the moon’s farthest point from Earth in its orbit. The lunar month beginning October 30, 2016 starts at 1738 UTC, with the instant of new moon (when the moon is most nearly between the Earth and sun for this month). Apogee is just over a day later, on October 31 at about 19 UTC. Translate UTC to your time zone here.
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.
Believe it or not, the year’s longest and shortest lunar months don’t showcase ultimate extremes. In fact, the few years ahead (2017 and 2018) will stage shorter and longer lunar months that vary even more greatly from each other and the mean than those of 2016.
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.
Each year, the shortest and longest lunar months come later in the year. For instance, in 2017, the shortest lunar month happens in between the May 25 and June 24 new moons; and the longest one in between the December 18, 2017 and January 17, 2018 new moons. Click here for a complete listing for the length of each lunar month in the 21st century.
Bottom line: October 30, 2016 marks the start of the longest lunar month of 2016. It lasts for 29 days 18 hours and 40 minutes. That’s 5 hours and 56 minutes longer than the mean. And it’s 11 hours and 20 minutes longer than 2016’s shortest lunar month, which spanned a period between early May and early June.
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.