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.

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.