Lengths of lunar months 2021

Quite by coincidence, the true lunar month straddling the years 2020 and 2021 matches the mean lunar month in length: 29 days 12 hours and 44 minutes. That almost never happens.

Animation of moon passing through shadow and light phases.

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.

Quite by coincidence, the lunar month straddling 2020 and 2021 – between the new moons of December 14, 2020, and January 13, 2021 – matches the mean lunar month of 29 days 12 hours and 44 minutes. The lunar month almost never equals the mean lunar month in duration. Most of the time, the true lunar month is either longer or shorter than the mean.

Keep reading to learn more about the lengths of lunar months in 2021.

Lengths of the lunar months in 2021

Successive new moons Length of lunar month
Dec 14, 2020, to Jan 13, 2021 29 days 12 hours 44 min
Jan 13 to Feb 11 29 days 14 hours 06 min
Feb 11 to Mar 13 29 days 15 hours 15 min
Mar 13 to Apr 12 29 days 16 hours 10 min
Apr 12 to May 11 29 days 16 hours 29 min
May 11 to Jun 10 29 days 15 hours 53 min
Jun 10 to Jul 9 29 days 14 hours 24 min
Jul 9 to Aug 8 29 days 12 hours 34 min
Aug 8 to Sep 6 29 days 11 hours 02 min
Sep 6 to Oct 6 29 days 10 hours 14 min
Oct 6 to Nov 4 29 days 10 hours 09 min
Nov 4 to Dec 4 29 days 10 hours 28 min
Dec 4, 2021, to Jan 2, 2022 29 days 10 hours 50 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 new 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 apogees and 13 perigees in 2021

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 labeled 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 2021, the longest lunar month happens between the April 12 and May 11 new moons, and the shortest one between the October 6 and November 4 new moons. See Astropixel.com’s complete list of the length of each lunar month in the 21st century.

Bruce McClure