Shortest lunar month of 2015. The shortest lunar month this year happens in between the new moons of April 18 and May 18. This lunar month is only 29 days 9 hours and 16 minutes long, or 3 hours and 28 minutes shorter than the mean lunar month of 29 days 12 hours and 44 minutes.
What is a lunar month? It’s just the duration between successive new moons. It’s also sometimes called a lunation or synodic month. Although the lunar month has a mean period of 29.53059 days (29 days 12 hours and 44 minutes), the actual length varies throughout the year.
That’s 4 hours and 57 minutes longer than the mean lunar month of 29 days 12 hours and 44 minutes.
All added up, that means the year’s longest lunar month in 2015 exceeds the year’s shortest by 8 hours and 25 minutes.
Lengths of the lunar months in 2015
|Successive new moons||Length of lunar month|
|January 20 to February 18||29 days 10 hours 34 min|
|February 18 to March 20||29 days 09 hours 49 min|
|March 20 to April 18||29 days 09 hours 21 min|
|April 18 to May 18||29 days 09 hours 16 minutes|
|May 18 to June 16||29 days 09 hours 52 min|
|June 16 to July 16||29 days 11 hours 19 min|
|July 16 to August 14||29 days 13 hours 29 min|
|August 14 to September 13||29 days 15 hours 48 min|
|September 13 to October 13||29 days 17 hours 24 min|
|October 13 to November 11||29 days 17 hours 41 minutes|
|November 11 to December 11||29 days 16 hours 42 min|
Why the difference in the lengths of lunar months? 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 to 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.
In 2015, the year’s longest and shortest lunar months do not actually showcase great extremes. In fact, the longest and shortest lunar months in the years ahead will vary more greatly from the mean.
The longest lunar months happen when successive new moons occur near lunar apogee – and in addition, the Earth is near perihelion (Earth’s closest point to the sun in its orbit). 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 2016, the shortest lunar month happens in between the May 6 and June 5 new moons; and the longest one in between the October 30 and November 29 new moons. Click here for a complete listing for the length of each lunar month in the 21st century.
Bottom line: A lunar month is the duration between successive new moons. It’s also sometimes called a lunation or synodic month. Although the lunar month has a mean period of 29.53059 days (29 days 12 hours and 44 minutes), the actual length varies throughout the year. The shortest lunar month of the year begins with the new moon of April 18, 2015 and ends with the new moon of May 18, 2015.
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.