{"id":62751,"date":"2024-08-19T06:47:45","date_gmt":"2024-08-19T06:47:45","guid":{"rendered":"http:\/\/d7b33536-9609-4a57-91f1-4b8c6271689f"},"modified":"2024-08-29T06:41:03","modified_gmt":"2024-08-29T06:41:03","slug":"blue-moon-explained","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/blue-moon-explained\/","title":{"rendered":"Blue Moon explained"},"content":{"rendered":"

What is a blue Moon and when is the next one? Find out what the term means and the astronomical definition in our blue Moon guide. <\/p>

By Jess Wilder\n <\/p>

Published: Monday, 19 August 2024 at 06:47 AM<\/p>

\n\n

We all know the phrase \u2013 if something never happens, or very rarely, it happens once in a blue Moon<\/em>. But what is a blue Moon, what does the term mean and how often do they occur?<\/p>

And what’s the difference between a monthly blue Moon and a seasonal one?<\/p>

In this guide we’ll reveal everything you need to know.<\/p>

Get monthly lunar dates, times and phases delivered directly to your email inbox by signing up to receive the BBC Sky at Night Magazine e-newsletter<\/a>.<\/em><\/strong><\/p>

And for a quick reference, download our lunar phases poster (PDF)<\/a><\/em><\/strong><\/p>

A blue Moon isn’t really blue. The term has nothing to do with a change in the Moon’s appearance or colour. Credit: Steve Brown<\/figcaption><\/figure>

Why blue Moons happen<\/strong><\/h2>

Our modern calendar is rooted in astronomy.<\/p>

A day is based on the rotation of the Earth on its axis, a year comes from the time it takes Earth to orbit the Sun<\/a>, and a month is based on the revolution of the Moon around Earth.<\/p>

And this is where the term arises.<\/p>

Typically, we see 12 full Moons a year, one on average falling neatly into each calendar month.<\/p>

But the two calendars don\u2019t precisely<\/em> match up. A calendar year contains around 11 days more than the number of days in 12 lunar cycles, so eventually the difference makes itself known.<\/p>

It\u2019s then that we get two full Moons within a calendar month, and this has led to the informal meaning: a blue Moon is the second full Moon in a month.<\/p>

\"A
A ‘blue Moon’ – the second full Moon in a calendar month, over Bedfordshire, UK, 31 July 2015, Credit: Roger Skillin<\/figcaption><\/figure>

Monthly blue Moon vs seasonal blue Moon<\/strong><\/h2>

The definition of a blue Moon as the second full Moon in a month is what’s known as a monthly blue Moon.<\/p>

You might think this definition dates back centuries, but it\u2019s actually startlingly modern: a 1946 edition of Sky & Telescope <\/em>magazine.<\/p>

Writing for the magazine, James Hugh Pruett misinterpreted dates in old copies of the Maine Farmer\u2019s Almanac<\/em> periodical.<\/p>

This astronomically incorrect definition is now arguably the most widely-used understanding of a the term.<\/p>

But the older, more traditional meaning is a seasonal blue Moon and is specific to the astronomical season.<\/p>

And what is an astronomical season? It means the seasons are marked by the equinoxes and solstices, going from spring equinox to summer solstice to autumn equinox and winter solstice.<\/p>

An astronomical season is between a solstice and an equinox.<\/p>

\"Equinoxes
Equinoxes occur when the Sun crosses the celestial equator. For Earth, this is the point that the planet\u2019s axis points neither towards nor away from the Sun. Credit: BBC Sky at Night Magazine<\/figcaption><\/figure>

In the northern hemisphere, summer solstice<\/strong> (around 20\u201322 June) denotes the start of astronomical summer.<\/p>

It’s followed three months later by the autumn equinox<\/a><\/strong> (around 21\u201323 September), the beginning of astronomical autumn.<\/p>

Within that time, commonly we would see three full Moons.<\/p>

The original Maine Farmers\u2019 Almanac<\/i> definition says a blue Moon is the third full Moon in a quarterly season of four full Moons.<\/p>

So the difference between a monthly and a seasonal one is this.<\/p>

A monthly blue Moon is the second full Moon in a month with two full Moons, but a seasonal blue Moon is the third full Moon in an astronomical season containing four full Moons.<\/p>

\"Soumyadeep
Soumyadeep Mukherjee captured this image of the Blue Supermoon from Kolkata, India, 30 August 2023.<\/figcaption><\/figure>

How often does a blue Moon occur?<\/strong><\/h2>

Like many idioms, ‘once in a blue Moon’ is not especially accurate. Blue Moons occur relatively frequently, every 2\u20133 years.<\/p>

Going by the term meaning the second full Moon in a month, there was one blue Moon in 2015, there were two in 2018 (in January and March; February had no full Moon) and one in 2020 which, as it fell on 31 October, was predictably named a Halloween blue Moon.<\/p>

The next seasonal blue Moon will occur on 19 August 2024 and the next monthy one occurs on 31 May 2026.<\/p>

A blue Moon isn’t blue<\/strong><\/h2>
\"The
The bright cloudy daytime sky makes the Moon appear blue in this image captured by Jason Meadows from Kent, UK, 11 April 2011, using a Bresser Messier N-150, 25mm eyepiece, Panasonic Lumix TZ5.<\/figcaption><\/figure>

Many people may think this barely needs pointing out, a blue Moon is not usually blue and the term does not refer to the colour of the Moon.<\/p>

The appearance of the Moon will be no more or less affected by atmospheric conditions than it is in any other month.<\/p>

So a Moon could be white, yellow, grey, or possibly orange and reddish (such as during a lunar eclipse<\/a>).<\/p>

You may have heard about the recent study that found the Moon is rusting<\/a>, but this isn’t why it appears orange or red.<\/p>

This comes down to the light that the Moon reflects passing through our atmosphere as it travels towards us on Earth.<\/p>

Depending on the size and density of those dust particles, the light we see will appear to vary in colour.<\/p>

But it is highly unlikely to look blue as the necessary atmospheric conditions are rare.<\/p>

The day the Moon turned blue<\/strong><\/h2>
\"An
An illustration depicting the eruption of Perbuatan volcano on Krakatoa Island, August 1883. Photo by DeAgostini\/Getty Images<\/figcaption><\/figure>

The Moon did turn blue at least once, that we know of.<\/p>

The eruption of Krakatoa, Indonesia in 1883, which killed 60,000 people, was one such occasion.<\/p>

The devastating explosion threw up plumes of smoke and dust, including particles 1 micron wide, a size that efficiently blocks red light but allowed blue light through.<\/p>

As a result, the Moon at night appeared bluish for several years after the eruption.<\/p>

Apart from such rare occurrences, the Moon is only likely to look blue in a photo where the photographer has used a colour filter or added the tinge in processing.<\/p>

\"A
A lunar eclipse imaged rising over Turkey in 2015. A lunar eclipse happens when the Sun, Earth and the Moon are aligned, projecting a red-brown tint onto the Moon. Credit: iStock Credit: iStock<\/figcaption><\/figure>

Where does the term come from?<\/strong><\/h2>

The English language is bulging with phrases and sayings based in our ancient connections to the seasons, the land and the sky.<\/p>

But, despite its common usage, the roots of ‘once in a blue Moon’ \u2013 linking the unusual extra Moon and blueness \u2013 are not clear.<\/p>

One early mention is in a 1528 pamphlet.<\/p>

It describes untrustworthy Roman clergy as \u201cwiley foxes\u201d who will say \u201cthe Moon is blue\u201d (\u201cthe mone is blewe\u201d): that is, will expect people to believe any absurd lie.<\/p>

\"Jeff
Jeff Bezos pictured with a model of Blue Origin’s Blue Moon lunar lander. Credit: Blue Origin<\/figcaption><\/figure>

Scroll forward to journalist Pierce Egan\u2019s 1821 bawdy bestseller Life in London<\/em>, and ‘blue Moon’ had come to denote \u2018unusual\u2019 and \u2018rare\u2019:<\/p>

\u201cTheir attention was at this moment attracted by the appearance of two persons dressed in the extreme of fashion, who, upon meeting just by them, caught eagerly hold of each other\u2019s hand, and they overheard the following\u2014\u2019Why, Bill, how am you, my hearty?\u2014where have you beentrotting your galloper<\/em>?\u2014what is you arter?\u2014how\u2019s Harry and Ben?\u2014haven\u2019t seen you this blue moon.\u2019\u201d<\/p>

Which term do you use? Do you look forward to the appearance of a large full Moon? If you have any thoughts on the terminology or even want to send us your observations and images, get in touch via contactus@skyatnightmagazine.com<\/a><\/em><\/strong><\/p> <\/body><\/html>\n

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