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 shortest lunar month of 2017 starts with the May 25 new moon and ends at new moon on June 24, lasting 29 days 6 hours and 46 minutes. The longest lunar month of the 21st century (2001 to 2100) actually comes at the year’s end, starting on December 18, 2017 and ending on January 17, 2018, lasting for 29 days 19 hours and 47 minutes.
The longest lunar month (Dec 18, 2016 to Jan 17, 2018) is 7 hours and 3 minutes longer than the mean lunar month, and the shortest lunar month (May 25 to Jun 24) is 5 hours and 58 minutes shorter than the mean lunar month.
Added all up, the duration of the longest lunar month in this period of 13 lunar months is 13 hours and 01 minute greater than that of the shortest lunar month.
Follow the links below to learn more:
Lengths of the lunar months in 2017
|Successive new moons||Length of lunar month|
|Dec 29, 2016 to Jan 28, 2017||29 days 17 hours 14 min|
|Jan 28 to Feb 26||29 days 14 hours 51 min|
|Feb 26 to Mar 28||29 days 11 hours 59 min|
|Mar 28 to Apr 26||29 days 09 hours 19 minutes|
|Apr 26 to May 25||29 days 07 hours 28 min|
|May 25 to Jun 24||29 days 06 hours 46 min|
|Jun 24 to Jul 23||29 days 07 hours 15 min|
|Jul 23 to Aug 21||29 days 08 hours 45 min|
|Aug 21 to Sep 20||29 days 11 hours 00 min|
|Sep 20 to Oct 19||29 days 13 hours 42 min|
|Oct 19 to Nov 18||29 days 16 hours 30 min|
|Nov 18 to Dec 18||29 days 18 hours 48 min|
|Dec 18, 2017 to Jan 17, 2018||29 days 19 hours 47 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. The lunar month beginning December 18, 2017 starts at 6:30 UTC, with the instant of new moon (when the moon is most nearly between the Earth and sun for this month). Apogee is about 19 hours later, on December 19 at 1:27 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.
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 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.
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.