Double moon 27 August: hoax returns to fool Facebook yet again

People around the world will marvel at a spectacular "once in a lifetime experience" of a "double moon" in the night sky on 27 August, according to a message doing the rounds on Facebook.

The post, which has been debunked by astronomers everywhere, claims that Mars will be so bright and large that it will look as though there are two moons next to one another.

The astronomy site EarthSky.org, affiliated with the award-winning science radio show of the same name, said that the rumour has become so popular that the site's feature discrediting the double moon theory has been its most-read feature this week.

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The hoax often includes a Photoshopped picture of what looks like two bright moons hovering over a domed building in the early evening.

But "the email and photo are perpetuating a hoax that rears its crazy head every summer," EarthSky explains. "The hoax has circulated every summer since 2003. Twelve years running! That's a long time for a hoax to run, in our world of information."

The site explains that viewed from the Earth, it is impossible for Mars ever to appear as large as a full moon. Editor-in-chief Deborah Byrd adds "Mars isn't even visible in July, 2015, and, although it might come into view in the east before dawn by August 27, 2015, it won't be anywhere near the July or August full moon.

"What's more, Mars is nowhere near its brightest or closest in July or August of 2015, or at any time in 2015. In 2015 so far, Mars has been relatively inconspicuous in our sky. It's on the far side of the sun from Earth. That'll continue to be the case throughout the rest of this year."

So how did the rumour begin?

In 2003, the Earth and Mars came as close as they have ever been to one another in 60,000 years, the BBC reports. At 56 million kilometres (35 million miles) apart, the two planets were "about the closest they can get." As the celestial event approached, some fantastical claims about how big Mars would appear began to circulate. Although the planet appeared very large and bright, it still looked significantly smaller than the moon.