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Seeing a green moon on 420? You must be high...

Puff, puff, science. A glaring-green-moon rumor has reappeared in time for 420 "weed day" celebrations of all things pot.

Greenified moon

A photo manipulation program turned this NASA moon photo green.

NASA/Green tint by Amanda Kooser/CNET

After laying low for a year, a particularly entertaining celestial rumor has reappeared for 2018. Back in 2016, word spread around the internet that the moon was going to glow green on April 20. People who missed the significance of the "420" marijuana-celebration day joke were sorely disappointed with their moon watching that night.

But you can't keep a good, marginally believable joke down. Science news provider EarthSky says the green moon has been the most-searched-for post on its site for weeks. Some Twitter users are tattling on gullible family members.

The original green-moon joke traces back to a now-deleted Facebook post from 2016 and didn't have any cannabis connotations at first. But the rumor spread like a weed once it became a humorous reference to the unofficial pot day of April 20. The most commonly shared image of the green moon says the phenomenon "only happens once every 420 years."

Don't be too disappointed by this debunking. Green moons have actually happened after a fashion. NASA says this rare event occurred in 1883 when the Indonesian volcano Krakatoa erupted with the force of a 100-megaton bomb. It mainly made the moon look blue, but some moonbeams shone through looking very green.

You can also check out an astronomy image NASA shared in 2005 showing the moon sporting a green rim as seen through a telescope.

If the green-moon hoax gets a few extra people outside to gaze at the moon tonight, then at least it will have had a positive side effect.