By Deborah Byrd, EarthSky
The famous double moon hoax appears to be gearing up again. We hear that the word spread on Facebook in July, 2015 – and is still spreading – that there would be a double moon. Will this hoax ever die? It’s now 12 years old. Still, clearly, not everyone knows it’s a hoax. Google searches have made this post the most popular on our site for the past week. An email must be circulating – somewhere, social media must be buzzing – with the suggestion that Mars will appear as large as a full moon in Earth’s sky. And that is just not true. Here’s a sample of what it typically says:
SEE MARS AS LARGE AS THE FULL MOON. Should be spectacular! Truly a once in a lifetime experience!
I saw a version of this a few years back that included a powerpoint presentation, suggesting that Mars and Earth’s moon will appear as a “double moon” in late August. I’ve also seen the photo below, circulating on Facebook.
It sounds amazing! Can it possibly be true?
No. It can’t. The email and photo are perpetuating a hoax that rears its crazy head every summer. 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.
Mars can never appear as large as a full moon as seen from Earth. 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.
As seen from Earth, in months when Mars does appear side by side with a full moon (and, again, that’s not happening in July or August of 2015), Mars’ diameter is about 1/140th the diameter of the full moon.
You would have had to line up 140 planet Mars – side by side – to equal the moon’s diameter.
Ah, Mars. World of dreams and visions. Mars is the world orbiting one step outward from Earth’s orbit. This world is slightly smaller than Earth – but slightly larger than Earth’s moon. Mars is also much much farther away than Earth’s moon. It’s hard to comprehend what little specks the planets and moons are in contrast to the vastness of space, but let me put it this way. Earth’s moon is about a light-second away. Traveling at 186,000 miles per second, light bouncing from the moon’s surface takes about a second to reach us here on Earth. Meanwhile, light from Mars takes much much longer to reach Earth – from several minutes to about 20 minutes – with the difference being the result of Earth’s and Mars’ motions around the sun. In other words, when Mars is on the same side of the sun as Earth, its distance from us is less than when it’s on the far side of the sun from us.
The moon is much closer than Mars, and that’s why we see the moon as a bright disk in our sky. Meanwhile – to the eye – Mars never appears as anything but a starlike point.
So how did this rumor of Mars-as-big-and-bright-as-the-moon get started? It started with an actual (though much more subtle) event in 2003. On August 27 of that year, Earth and Mars came very slightly closer than they’d been in nearly 60,000 years. Our two worlds, center-to-center, were less than 35 million miles apart – just over three light-minutes apart. The last people to come so close to Mars were Neanderthals. Astronomy writers like me had a field day that year, talking about Mars at its closest. Was it a spectacular sight? Yes! It looked like a dot of flame in the night sky.
Was Mars as big and bright as the moon, even at its closest in 2003? Never.
But the legend continues …
Bottom line: Mars will not appear as large as the full moon at anytime in July, 2015 or on August 27, 2015. Mars will not ever appear as large as a full moon in Earth’s sky. The email – or social media – claims to the contrary are a hoax.