How often is there a blue moon?

Once in a blue moon...

...

...

...

...

...

...

...

...

...

...

...

...

OK, but seriously...

We all know the old saying that refers to a rare event... but what does a blue moon really mean and how do I know when to see one?

Luckily for you, a Blue Moon will appear in the night’s sky on August 22, here is everything you need to know ahead of the event.

What is a blue moon?

There are two different descriptions used to describe a Blue Moon.

The first being a definition by the Maine Farmers’ Almanac which blue moon dates for farmers say an original Blue Moon happens whenever a season has four full Moons, instead of three traditionally. It means that the third one is known as the Blue Moon - which is expected to happen this weekend.

As a result, the event occurs around every 2.7 years - the last one was back in February 2019 and the next one will be in August 2024.

It is at the year’s two solstices and two equinoxes is where the astronomical seasons begin and end – with typically three moons in each astronomical season, adding up to 12 moons for the lunar year altogether.

Though thanks to a mistake made back in 1946, the modern - and simpler - definition was made and it looks a little different.

With the aim to educate readers on what the Maine Farmers’ Almanac definition of Blue Moon was, Sky & Telescope published an article where they mistakenly wrote: “This second in a month, so I interpret it, was called Blue Moon.”

Some they managed to misread the original description where it referred to the twice-in-a-month kind of Blue Moon.

And thus a new definition was born.

But this is not how most people use the phrase today. The more modern definition refers to a time when one particular month has two full Moons within it.

The next chance to see a Blue Moon by the more modern definition will come in August 2023.

More often than not, the phrase Blue Moon is not used to refer to either kind of Blue Moon, or a literal Blue Moon. It is often used in songs with the aim of recalling the sense of the word used in the blues, of loneliness or sadness.

The Moon only turns the actual colour of blue very rarely indeed. Occasionally, the eruption of volcanoes or large fires can put smoke in the sky that changes its colour.

When do they occur?

If we’re going by the original definition, a Blue Moon occurs every 2.7 years. The last one is in February 2019, and after this one there will not be another until August 2024.

Meanwhile, if you’re using the more modern definition, August 2023 will be the next time one occurs.

When’s the latest?

A Blue moon by the original definition will happen this Sunday on August 22 - see keep your eyes peeled.

So which definition of a Blue Moon moon will occur this weekend?

It is the traditional Maine Farmers’ Almanac definition of a Blue Moon that is set to occur on Sunday.

Is the moon really blue?

Short answer - not really.

Blue Moons are just like any other moon on a normal night - however, there are rare occasions where Blue Moons can actually turn slightly blue.

But this feat requires a volcanic eruption or a large fire for smoke in the air to change the colour of the moon - so an uncommon occurrence.