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When it does - at 8.19pm precisely, according to NASA - it could be shining down on a club which has just completed the fabled “treble” of League Championship, League Cup and FA Cup at the world-famous football venue - and one whose fans, appropriately, long ago adopted the song Blue Moon as their club anthem. Former welterweight world boxing champion Ricky Hatton, a City fanatic who will be in the crowd, told Express.co.uk of the blue moon: "It does seem strange. I guess you could call it destiny - but only if we win!

"It would be amazing if we finish up the season winning the treble with a blue moon shining down. But we have to beat Watford first - which won't be easy. I'll be there at Wembley to cheer City on. We'll be singing Blue Moon at the final whistle, win or lose." Author Gary James, a fanatical supporter as well as the author of several books chronicling the history of the club, including Manchester City Folklore: What Every Blue Needs to Know, will also be cheering on his club - and admitted he had not made the connection. He told Express.co.uk: “It would be nice if we could sing Blue Moon just as a blue moon was rising.”

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Like many a superstitious football fan, Mr James was reluctant to talk of victory, and to lift the cup City will need to beat Watford, who booked their passage to the final with a remarkable 3-2 comeback victory over Wolverhampton Wanderers. He said: “I’m sure we’ll sing it anyway, whether or not we win, but if we did win the Cup - I don’t really want to call it fate or destiny or whatever, but that’s what it would feel like! “It’s also appropriate because, despite our recent success, we only seem to get to the FA Cup final once in a blue moon as well!” He explained the club’s fans adopted Blue Moon - written by songwriters Richard Rodgers and Lorenz Hart in 1934 - in the 1989/90 season.

Former world boxing champ Ricky Hatton is a huge City fan who will be at Wembley tomorrow

Mr James said: “We were at Anfield at in those days they used to keep the away fans behind after the final whistle. “A couple of the lads who went home and away just started singing it in a really mournful sort of way and everyone joined in. “Football fans are like that - they’ll just latch on to a song for some reason and make it their own. “Stoke City did that with Delilah for example.”

Since then the song has become an important part of Manchester City’s club culture - not least, a mascot known as Moonchester. Mike Devlin, author of Manchester City: The Secret History of a Club That Has No History, was likewise taken aback by the news. He said: "I wasn't aware of that and it's a bit uncanny to be sure. "I think we sing Blue Moon to take the micky out of ourselves really, and stop us getting too big for our boots."

In terms of the spectacle itself, the Royal Observatory’s Dr Emily Drabek-Maunder - while no football authority - urged City and Watford fans alike to do their best to enjoy the spectacle. She said: “The view will be a bright, full moon rising in the East. Despite the name, the moon doesn’t actually turn blue (unlike a blood moon when the Moon turns red), it is just the extra full moon that we have in the year. “The blue moon on May 18 will look like every monthly full moon, as a beautiful silvery-grey in our night sky. “Even using just your eyes, you’ll still be able to see the darker regions, which are called ‘maria’ or ‘seas,’ on the surface of the Moon. READ MORE: Vincent Company reveals all ahead of FA Cup final

I don’t really want to call it fate or destiny or whatever, but that’s what it would feel like! Manchester City fan and author author Gary James on the strange FA Cup final blue moon

“These maria are large regions of basalt on the Moon, which is dried lava from ancient lunar volcanoes. “Blue moons happen every two to three years. Normally, we have one full moon every month and twelve full moons in a year. “However, it only takes 29.5 days to cycle through all of the phases of the Moon, which is slightly under a month. “Eventually, this adds up over time and means that there are thirteen full moons in a year, which is when a blue moon will occur.”

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Dr Drabek-Maunder added: “There are a couple of definitions of a ‘blue moon,’ a traditional definition and a more modern definition. In astronomy, seasons begin and end at equinoxes and solstices. “For example, we are currently in the spring season which began with the spring equinox (March 20) and will end with the summer solstice (June 21). “Traditionally, a blue moon is the third full moon in an astronomical season that has four full moons. “There are four full moons in the astronomical spring this year – March 21, April 19, May 18, June 17 – so this means the May full moon is the blue moon. READ MORE: What is a blue moon? Will the moon really turn blue?

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“The next traditional blue moon after this year will be August 22, 2021 and this will be the third full moon in the astronomical summer. “There is another definition of a full moon that is more common in modern day, which is when two full moons occur in the same month. The second full moon of that month is often referred to as a ‘blue moon.’ “The next blue moon by this definition is October 31, 2020 on All Hallows’ Eve.” While blue moons will be uppermost in the minds of many a football fan inside Wembley on Saturday evening, our celestial companion has been in the news for other reasons in recent months, not least the Super Blood Wolf Moon phenomenon earlier this year, during which the moon appeared to glow red while seeming brighter and closer to Earth than normal.