Longest lunar month starts February 23

The longest lunar month in 2020 starts with the new moon on February 23, and concludes with the new moon on March 24. Details about 2020’s lunar months, here.

Animation of moon passing through shadow and light phases.

Simulated view of the moon’s phases.

The longest lunar month in 2020 starts with the new moon on February 23, and concludes with the new moon on March 24. Its duration will be 29 days 17 hours and 56 minutes. What is a lunar month? It’s simply 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 2020 starts with the August 19 new moon and ends with the new moon on September 17, lasting 29 days 8 hours and 19 minutes.

Keep reading to learn more about the lengths of lunar months in 2020.

Lengths of the lunar months in 2020

Successive new moons Length of lunar month
Dec 26, 2019, to Jan 24, 2020 29 days 16 hours 29 min

Jan 24 to Feb 23 29 days 17 hours 50 min
Feb 23 to Mar 24 29 days 17 hours 56 min

Mar 24 to Apr 23 29 days 16 hours 58 min

Apr 23 to May 22 29 days 15 hours 13 min

May 22 to Jun 21 29 days 13 hours 03 min

Jun 21 to Jul 20 29 days 10 hours 52 min

Jul 20 to Aug 19 29 days 09 hours 09 min

Aug 19 to Sep 17 29 days 08 hours 19 min

Sep 17 to Oct 16 29 days 08 hours 31 min

Oct 16 to Nov 15 29 days 09 hours 36 min

Nov 15 to Dec 14 29 days 11 hours 09 min

Dec 14, 2020, to Jan 13, 2021 29 days 12 hours 44 min

Sources: Astropixels.com and TimeandDate.com

Diagram of positions of moon in orbit with pictures of what it looks like from Earth for each phase.

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 14 apogees and 13 perigees in 2020

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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Dotted circle around Earth and line representing moon's orbit slightly offset from it.

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 the 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.

Nine photos of the moon going through its different phases.

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

Bottom line: In 2020, the shortest lunar month happens between the August 19 and September 17 new moons; and the longest one between the February 23 and March 24 new moons. See Astropixel.com’s complete list of the length of each lunar month in the 21st century.

Bruce McClure