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If you feel something good happens to you so rarely that it’s “once in a blue moon,” then this month you’re in luck. On 31 May, we are treated to a blue moon.
Let me say at once that you’re not likely to perceive the moon as any kind of azure shade. That does sometimes happen, the most famous case being back in September 1950, when people around the northern hemisphere were amazed that the Moon appeared a brilliant blue. The culprit was a vast forest fire in Canada. It wafted specks of ash high into the air that were just the right size to obstruct the long wavelengths of red and orange light. As a result, the moonlight filtering down to the world below was predominantly blue.
The eruption of Krakatoa in 1883 caused literal blue moons; and – to a lesser extent – the ash from Mount St Helens (1980) and Pinatubo (1991). But they are very rare. According to this definition, you’d get lucky only two or three times in a lifetime.
As far as we know, the phrase “once in a blue moon” wasn’t prompted by anyone actually observing the Moon this colour. The saying surfaced in 1823, when the British journalist Pierce Egan added an entry to the latest edition of the Classical Dictionary of the Vulgar Tongue: “Blue moon. In allusion to a long time before such a circumstance happens.”
Presumably it had been circulating on the streets of London for a while before Egan picked up on the phrase. This was around the time that Cockney rhyming slang started to develop, so who knows if the phrase might have meant something totally different, only connected by rhyme with ‘blue moon’?
As the phrase spread around the English-speaking world, some people looked for an astronomical meaning. In 1937, the editor of the Maine Farmers’ Almanac, H. P. Trefethen, devised a theory that came – as far as we can tell – out of the blue.
Usually there are 12 full moons in a year, and they have traditional names in North America, such as the Wolf Moon in January and the Flower Moon in May. In most years, then, there are three full moons each season of the year. Occasionally, though, there are 13 Full Moons in a year, and Trefethen felt he had to devise a name for the interloper.
He picked on the season with four full moons, and gave the traditional names to the first, second and fourth full moons. The third full moon in this season he called the “blue moon.” Some writers refer this definition as the “traditional blue moon,” even though the tradition dates back – as far as we know – only to the 1930s.
In 1946, astronomer James Hugh Pruett, writing in the prestigious astronomy magazine Sky & Telescope, misinterpreted Trefethen’s definition. When two full moons fall in the same month, Pruett declared, the second one is called a blue moon. Forty years later, the Trivial Pursuit board game included this definition as the answer to their question ‘What is a Blue Moon?’
And this version – the “calendrical blue moon” – became the norm, used by almost everyone today. It means that a blue moon falls once every two to three years: the last was in August 2023, and the next is coming at the end of this month
What’s Up
Venus is at its most spectacular this month, blazing as the Evening Star in the western sky and visible after the sky has turned completely black. If you are well away from light pollution, check for one of astronomy’s rarer sights: shadows cast by Venus’s iridescence.
The night sky at around 11pm this month (Nigel Henbest)
Just a bit higher in the sky lies Jupiter, second only to Venus in brilliance, and lying near the twin stars Castor and Pollux.
We’re in for a gorgeous sight just past mid-month, when the three great luminaries of the night sky – Venus, Jupiter and the Moon – play tag in the west. On 18 May, the crescent Moon lies right next to Venus. The following evening, the Moon is right between Venus and Jupiter, and on 20 May, Jupiter is just below the Moon.
From 20 May, you have a chance to spot the innermost planet, Mercury, hugging the horizon to the lower right of Venus.
On the starry front, look up to spot the familiar pattern of the Plough (or Big Dipper in North America) soaring overhead. To the ancient Greeks, these seven stars were part of a larger pattern, the Great Bear (Ursa Major), outlining the creature’s body and bent tail. Follow the curve of the tail (the handle of the plough or dipper) and it will guide you to bright orange Arcturus in Boötes (the herdsman), and then on to Spica, the leading light of Virgo (the virgin).
To the right of Virgo you’ll find Leo (the lion), its outline resembling a crouching feline. Below Virgo and Leo, a long line of faint stars traces out the sinuous body of Hydra, the water snake, with a little stellar group near Procyon marking its head. Hydra is the largest constellation and the longest, stretching more than a quarter of the way around the sky. It’s also one of most ancient star patterns, dating back 5000 years to a time when its elongated shape marked out the equator of the sky.
Diary
9 May, 9.10pm: Last Quarter Moon
16 May, 8.01pm: New Moon
18 May: Moon near Venus
19 May: Moon between Venus and Jupiter
20 May: Moon near Jupiter
22 May: Moon near Regulus
23 May, 11.11am: First Quarter Moon
26 May: Moon near Spica
27 May: Moon near Spica
30 May: Moon near Antares
31 May, 8.45am: Full Moon; ‘blue moon’