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2016’s longest lunar month starts October 30

Lunar months vary in part because the moon’s orbit around Earth isn’t a perfect circle. Longest (and shortest) lunar months in 2016, here.

Simulated view of the cycle of the moon's phases from new moon to new moon. This cycle is known as the lunar month. From the years 1760 to 2200, the longest lunar month was 29 days 19 hours and 58 minutes and the shortest 29 days 6 hours and 34 minutes.

Simulated view of the moon’s phases.

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

Why are lunar months different lengths?

When are the longest and shortest lunar months of 21st century?

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

Sources: Astropixels.com and TimeandDate.com

One lunar month is the period of time from new moon to new moon. As viewed from the north side of The Earth's and moon's orbital planes, the Earth goes counterclockwise around the sun and the moon goes counterclockwise around Earth. Image credit: Wikipedia

One lunar month is the period of time from new moon to new moon. As viewed from the north side of The Earth’s and moon’s orbital planes, the Earth goes counterclockwise around the sun and the moon goes counterclockwise around Earth. Image via Wikipedia

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.

Dates for the 14 apogees and 13 perigees in 2016

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.

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The moon's orbit around Earth is not a perfect circle.  But it is very nearly circular, as the above diagram shows.  Diagram by Brian Koberlein.

The variation in the length of lunar months happens because the moon’s orbit around Earth is not a perfect circle. However, it is very nearly circular, as the above diagram shows. Diagram by Brian Koberlein.

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.

2017 EarthSky Lunar Calendar pre-sale…is happening NOW!

Phases of the moon, posted to EarthSky Facebook by our friend Jacob Baker.

Phases of the moon, posted to EarthSky Facebook by our friend Jacob Baker.

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

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