No double moon in 2018……


This image sometimes circulates on Facebook, with the claim that Mars will appear as big and bright as a full moon. It’s a hoax. Don’t believe it.

Will Mars and the moon will appear the same size in 2018? Gosh, no. What’s really amazing is the staying power of this hoax, which has its roots in a real 15-year cycle of Mars, that’s peaking – giving us an excellent year to observe Mars – in 2018. You’re likely to see it as an email – or on social media – in the form of a claim that Mars will appear as large as a full moon in Earth’s sky on a particular date. Sometimes there’s a suggestion that Mars and Earth’s moon will appear as a double moon. I’ve also seen the photo above, circulating on Facebook. And that is just not true. It’s not true in 2018. It’s never been true. It never will be true. Here’s a sample of what the hoax typically says:

SEE MARS AS LARGE AS THE FULL MOON. Should be spectacular! Truly a once in a lifetime experience!

It sounds amazing! Can it possibly be true?

No. It can’t.

Mars can never appear as large as a full moon as seen from Earth. As seen from Earth, in months when Mars does appear side by side with a full moon, Mars’ diameter appears, on average, about 1/140th the diameter of the full moon.

In other words, you’d have had to line up 140 planet Mars – side by side – to equal the moon’s diameter.

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The red planet Mars, as seen by the Hubble Space Telescope. Can you imagine seeing this in our night sky? It’d be terrifying! Luckily, it can’t happen. Image via NASA.

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 reddish starlike point.

View larger. | This photo is closer to what Mars actually looks like in our sky. At best, it’s a bright reddish pinpoint, like a red star. Mars was brilliant while near the star cluster M44 in mid-April 2010. It’s the bright object in the upper left of this beautiful image by Peter Wienerroither. Used with permission.

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. Center-to-center, Earth and Mars 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! Mars 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 …

The 2003 event was part of that 15-year cycle for Mars, mentioned above. Think of Earth and Mars in orbit around the sun again. Neither Earth nor Mars has a circular orbit. Both worlds have elliptical orbits … like squashed circles. So both Earth and Mars have a closest point to the sun. Maybe you can see that – when Earth passes between the sun and Mars (opposition) around the time Mars is closest to the sun (perihelion) – Earth and Mars come closest. The diagram below, used with the kind permission of the Royal Astronomical Society of Canada, helps show why.

Diagram by Roy L. Bishop. Copyright Royal Astronomical Society of Canada. Used with permission. Visit the RASC estore to purchase the Observers Handbook, a necessary tool for all skywatchers. This diagram explains why, in 2016, Mars was closer than it had been in 10 years. In 2018, it’ll be even closer … but never moon-sized in Earth’s sky. Read more about why Mars is sometimes bright and sometimes faint.

If you look closely at this diagram, you can see that Earth and Mars will have a particularly close opposition in mid-2018. Mars will be closer than it’s been since 2003! It’ll be bright and very reddish! Again, like a dot of flame.

No doubt the Mars-as-big-as-the-moon and double moon rumors will be flying!

Bottom line: Mars can never appear as large as a full moon in Earth’s sky. The email – or social media – claims to the contrary are a hoax. However, 2018 will be an excellent year for Mars. Watch for this hoax to rev up as Mars brightens in our sky, culminating in an extra bright Mars (but still, a starlike Mars) around late July.

Read more: Mars brighter in 2018 than since 2003

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