{"id":47764,"date":"2023-07-27T10:00:09","date_gmt":"2023-07-27T10:00:09","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=54282"},"modified":"2023-07-27T11:32:37","modified_gmt":"2023-07-27T11:32:37","slug":"what-actually-is-a-blue-moon","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/what-actually-is-a-blue-moon\/","title":{"rendered":"What actually is a blue Moon?"},"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 Jane Williamson\n \t\t<\/p>

2023-07-27 09:58:09<\/p>

\n\n

Are you looking forward to August 2023\u2019s Super Blue Moon<\/a>? Should you be, and what does this actually mean in terms of visually observing the Moon?<\/p>\n

We all know the phrase \u2013 if something never happens, or very rarely, it happens once in a blue Moon<\/em>.<\/p>\n

But what is a blue Moon, what does the term mean and how often does a blue Moon occur?<\/p>\n

Our modern calendar is rooted in astronomy: 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>\n

For the most part, the lunar cycle and our calendar are in sync, the Moon waxing and waning each month, giving rise to the phases of the Moon<\/a>, with about 29.5 days between full Moons.<\/p>\n

For more practical lunar guidance, read our guide on how to observe the Moon<\/a>, or get lunar dates, times and phases delivered directly to your email inbox every week by signing up to the BBC Sky at Night Magazine newsletter<\/a>.<\/strong><\/em><\/p>\n

\""The\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
The phases of the Moon. Credit: Yaorusheng \/ Getty Images<\/span><\/figcaption><\/div>\n

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

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>\n

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>\n

\""A\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A \u2018blue Moon\u2019 \u2013 the second full Moon in a calendar month, appears over Bedfordshire, UK, 31 July 2015, imaged by Roger Skillin with a Nikon D610 DSLR camera and Sigma 120-400mm APO VR lens.<\/span><\/figcaption><\/div>\n

Seasonal blue Moon<\/strong><\/h3>\n

You\u2019d be forgiven for thinking the definition of a blue Moon as the second full Moon in a month dates back centuries, but it\u2019s actually startlingly modern: a 1946 edition of Sky & Telescope <\/em>magazine.<\/p>\n

Writing for the magazine, James Hugh Pruett misinterpreted blue Moon dates in old copies of the Maine Farmer\u2019s Almanac<\/em> periodical. This astronomically incorrect definition is now arguably the most widely-used understanding of a blue Moon.<\/p>\n

But the older, more traditional meaning is a \u2018seasonal blue Moon\u2019 and is specific to the astronomical season.<\/p>\n

\""Equinoxes\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
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<\/span><\/figcaption><\/div>\n

An astronomical season is between a solstice and an equinox. In the northern hemisphere, for example, the\u00a0summer solstice<\/strong>\u00a0(around 20\u201322 June) denotes the start of astronomical summer and is followed three months later by the\u00a0Autumn equinox<\/strong><\/a>\u00a0(around 21\u201323 September), the beginning of astronomical autumn.<\/p>\n

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

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.<\/span><\/p>\n

We\u2019ll get one of these seasonal blue Moons on 22 August 2021.<\/p>\n

\""A\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A monthly blue Moon on 31 January 2018 also happened to be a so-called \u2018supermoon<\/a>\u2018. Imaged here by Nick Talbot, Telford, UK, using a Canon EOS 600D DSLR camera and a Mak 150 telescope.<\/span><\/figcaption><\/div>\n

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

Like many idioms, \u201conce in a blue Moon\u201d is not especially accurate. Blue Moons occur relatively frequently, every 2\u20133 years.<\/p>\n

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>\n

Looking ahead, there will be monthly blue Moons on 31 August 2023 and 31 May 2026.<\/p>\n

When are the next blue Moons?<\/strong><\/h2>\n

Second full Moon in a month:<\/strong><\/h3>\n