TGVG Blog

Once in a Blue Moon (or Every 2 1/2 to 3 Years)

Mind Bending | September 10, 2020

Barlow SkyWatch By Alan J Peche, Director, Barlow Planetarium

Many don’t know that there are two definitions for “Blue Moon”

Seasonal Blue Moon

The oldest definition – the Seasonal Blue Moon – is when there are four full moons (one more than the normal three full moons); the third Full Moon of the season is called a Blue Moon. The next Seasonal Blue Moon will be on August 22, 2021; the other Full Moons that summer will be seen on June 24, July 23, and September 20. On average, a Seasonal Blue Moon will occur every 3 years or so.

MONTHLY Blue Moon

The newest and most used definition – the Monthly Blue Moon – was created after the original was misinterpreted and simplified in 1937 to be the second Full Moon in a month. Looking back at 1937, there was a seasonal Blue Moon on August 21, but there weren’t any months with two Full Moons. This newer definition allows the monthly naming conventions to continue. October’s Full Moon is called the “Hunter’s Moon”; October’s second Full Moon is a “Blue Moon”; finally, November’s Full Moon is called the “Beaver Moon”. The next Monthly Blue Moon will occur on October 31, 2020 (Halloween) with the first Full Moon occurring on October 1. The next will occur in August 2023. On average a Monthly Blue Moon will occur every 2.5 years. However, the next Halloween Blue Moon will occur in 2039.

The time between consecutive Full Moons is approximately 29.5 days – or roughly a month. In fact, the word “month” is derived from the word “moon” – think of “month.”

Blue Moon Trivia:

What month cannot have a Monthly Blue Moon?
The only month that cannot have a month-based Full Moon is February. Since February only has 28 days, the Moon cannot complete a 29.5 day orbit in February.

Is it possible to have two Monthly Blue Moons in a single year?
Yes, but only if the first Blue Moon happens at the very end of January. If this occurs, the next Blue Moon will occur at the very end of March. In this scenario, February does not have any Full Moons. This is called a Double Blue Moon. This happened in 2018 – January 1 (Full Moon), January 31 (Blue Moon), March 1 (Full Moon) and March 31 (Blue Moon). It will occur again in 2037.

Is it possible to have one Monthly Blue Moon and one Seasonal Blue Moon in a single year?
Yes, this happened in 1934 – Monthly Blue Moon on March 31 and Seasonal Blue Moon on November 20. It will occur again in 2048 – January 30 and August 23.

Is it possible to have two Monthly Blue Moons and one Seasonal Blue Moon in a single year?
It is very rare and happened last in 1961 – Monthly Blue Moons on January 31 and March 31 with a Seasonal Blue Moon on November 22. It will occur again in 2143 – January 30, March 30, and November 22.


Each month, the Barlow Planetarium, provides interesting science, technology, engineering and mathematics (STEM) topics for all ages.

TO GET A CURRENT SKYCHART: Visit the Skymaps link under the “Astronomy Resources” link at BarlowPlanetarium.org.

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