Can the Moon ever turn green? In a nutshell, no. But that hasn’t stopped the idea of a green Moon becoming a rather persistent meme.
Note that we’re using ‘meme’ in the pre-social media sense of the word here. That's to say, “an idea that seemingly self-propagates through a population” rather than “a funny picture on Facebook”.
But the social media sense applies too, because it was a social media post that popularised the idea in the first place.
Exploring the myth
According to myth-busting website Snopes.com, the idea can be traced back to a meme that first appeared in 2016.
That meme said simply:
"Save the date: May 29th, 2016. Green moon visible for first time since 1847."
While that original post got some traction, the rumour of the green Moon really took off when someone reposted a version of it in 2018 – now with an added pseudoscientific explanation involving the full Moon’s proximity to blue-hued Uranus in the night sky, and with the date of this supposed upcoming celestial wonder changed to 20 April.
That date should have been a clue – but more on that in a moment.
Can the Moon ever appear green?
There are only two reasons the Moon might appear green: either it actually is green, or something is happening to the light that reflects off its surface, before it reaches our eyes, that makes it appear green.
We’ll assume you know the Moon isn’t actually green. If it was, it would look green every night, not once every 420 years, as that April 2018 post claimed!
But could something make it appear green? That’s a much more sensible question.
After all, we sometimes see a ‘Blood Moon’ that appears orange or red in hue.
You’ll see these when there’s a lunar eclipse, because the only light reaching the Moon from the Sun, during such an event, is red light that has been refracted towards the Moon by Earth’s atmosphere (with blue wavelengths, conversely, refracted outwards into space).
For similar reasons, you'll often see a Moon close to the horizon appear orange, because sunlight reflected from its surface towards the observer has had to pass through a thicker part of Earth's atmosphere.
The Moon can also sometimes appear red or orange if seen through skies that are dense with pollutants such as ash from volcanos or smoke from forest fires.
In rare, extreme cases, such particles can also have the reverse effect, scattering red light and making the moon appear blue.
Though the phrase ‘blue moon’ usually refers to either the second full Moon in a month, or the third full Moon in a season of four, and has nothing to do with colour at all.
How to make the Moon green
There are no known natural or atmospheric phenomena that would make light appear greener than it is.
If you wanted to make the Moon appear green you could:
- Observe or photograph the Moon through a filter (or the bottom of a beer bottle!)
- Photograph the Moon then edit it to produce a verdant hue
Those things can make the Moon appear green, but no known natural phenomenon will.
And if the Moon isn’t green, and if nothing is happening to its light to make it appear so… then you’re not going to see a green Moon, are you?
So why do people think you might?
Green Moon and 420
It helps if you understand that ‘420’ has long been a US slang term for marijuana/cannabis, and that ‘4/20’ (April 20) is now celebrated by some as so-called 'International Weed Day'.
Once you’re equipped with that information, claims that “the Moon is going to turn the colour of marijuana for the first time in 420 years on 20 April” don’t sound quite so credible!
Basically, the whole idea is a joke and nothing more.
Except, there’s a kicker here… because in the course of researching this article, we found a letter published in scientific journal The Observatory way back in 1884, which mentions "the most interesting phenomenon" of a green-hued Moon having been observed during "the recent remarkable sunsets as seen in this climate."
That raises the intriguing possibility that someone, somewhere actually HAS seen a green Moon at some point – but if they did, then the explanation is beyond us (and apparently beyond NASA, too).
Someone who posted the 1884 letter on Quora suggested it may be "an optical illusion due to how the visual processes in your brain produce the experience of colour."
Green flash of the Moon
There is a phenomenon known as a 'green lunar flash', where a small part of the Moon may appear green very briefly when it's low on the horizon.
This, again, is to do with how light from the Moon is reflected and scattered through Earth's atmosphere before it reaches our retinas and camera sensors.
The image above was captured by Gerhard Hüdepohl and you can read about the image and what causes the greenn flash via the ESO website.
There's a similar effect that occurs with the Sun and also with the planet Venus, the latter known as the green flash of Venus.
Are there any green moons?
Ok, so our Moon never appears green, except through some sort of optical manipulation, but what about other moons in the Solar System?
You may have seen images of green moons around other planets in our Solar System, but these are likely 'false-colour' images.
Often, planetary scientists process data captured by spacecraft and rovers and give the targets false colour in order to emphasise certain features they want to study or highlight.
So as far as we know, there is no green moon in our Solar System.
But beyond our Solar System? Who knows...
Have you ever seen the Moon appear green? What do you make of that 1884 letter? Email us via contactus@skyatnightmagazine.com with any ideas you might have.
