Lengths of lunar months in 2019

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.

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 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

Why are lunar months different lengths?

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

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

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 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.

Dates for the 13 perigees and 13 apogees in 2019

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.

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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 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.

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.

Best New Year’s gift ever! EarthSky moon calendar for 2019

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