Every couple of years, the Earth zips past Mars in their orbits around the sun. And - without fail - someone tries to trick people into believing it'll be as big in the night sky as the moon.
The hoax, which dates back to 2003, used to spread via email. Now it's evolved into its 2018 form - appearing on a fake news site.
"Two moons in the sky on July 27!" claims science.info. "This is the first time that humanity will be able to observe this exceptional phenomenon... The next time this cosmic event will happen again will be 36,996."
That's not true. But the reality is almost as impressive, at least from an astronomical perspective. This year's close encounter coincides with a lunar eclipse and an even rarer event called a 'selenelion'. That's when a lunar eclipse - or 'blood moon', as the media has taken to calling them in recent years - happens so early in the morning it's visible at the same time as the sun.
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Mars orbits the sun slightly further out than Earth. That, combined with Earth's greater speed, results in the two planets passing each other every 26 months. Mars and the Earth will be closest this year on July 31/August 1.
But even at its closest, Mars will still be 57.6 million kilometres away - slightly more than a third of the distance from here to the sun. NASA says to the naked eye, it won't look any bigger - but it will be about 10 times brighter.
There are other clues in the science.info article that give away the fact it's rubbish. It claims Mars will be "32 million light-years" away from Earth, which would place it not only well out of the solar system, but in another galaxy entirely.
In 2013, astronomer and writer Phil Plait said the initial fake story in 2003 was likely the result of a genuine mistake.
"Through a telescope that magnifies an image 75x, Mars would look as big as the Moon does to the naked eye," he wrote. "But this being the internet, that got all mished and mashed up."
He said the rumour seems to crop up every year, even when Mars is on the complete other side of the sun, seven times further away than it is this weekend.
If Mars was close enough to appear as large as the moon, it would probably result in massive tsunamis, climate changes and potentially a collision between the two planets, as their gravity pulled them closer together. They could potentially end up a binary planet system.
"We'd be in big trouble given the gravitational pulls on Earth, Mars, and our moon!" the NASA website states. Figuring out how three bodies - such as the Earth, Moon and Mars - interact with each other according to the laws of motion continues to frustrate even the best physicists.
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Many scientists now believe the Earth was hit by a planet about the size of Mars, dubbed Theia, around 4.5 billion years ago.
The next flyby in October 2020 won't be quite as close as this one. Earth and Mars' orbits are elliptical, slightly tilted and affected by larger planets such as Jupiter, so they don't always pass at the exact same distance.
Newshub.
