Why we will never see a full moon-sized Mars in our skies by Pecier Decierdo

Viral stories of Mars appearing as big as the full moon in our skies crop up regularly. The most recent time such stories became widely shared online was a couple of months ago, when the Red Planet was at its closest approach to Earth in 11 years. However, even during the time of closest approach (which was close to midnight of May 30 in the Philippines), Mars was still more than 75 million kilometers from Earth.

The biggest Mars will ever get versus the full moon. 
Photo credits: Mars NASA/STSci.
Image credit: David Le Conte.

75 million kilometers - how far is that? And how big, relatively speaking, is Mars anyway? Given the distances involved, will we ever see Mars as a red disk in our skies?

To get rid of the misinformation, it is important that we answer these questions. After all, while stories like a full moon-sized Mars get people excited about space science, the disappointment brought about by such unrealistic expectations might only turn people away from sky watching. It might even make them less excited about science news in general.

The size of Mars for selected dates throughout this year, compared to the size of the full moon.
Image credit: NASA/JPL-Caltech.

Furthermore, the expectation of seeing the red disk of Mars in our skies is a symptom of a failure to understand the scales involved. To be fair, though, the scales involved are mind boggling. So let's help our boggled minds deal with the cosmic scales involved by playing with some balls.

Mars as a basketball

Not all closest approaches of Mars result in the same distance. That's because the orbits of planets are actually elliptical or squished circles (although the squishing in the case of Earth and Mars aren't that big). The closest Mars and Earth can ever get is 54.6 million kilometers. Last May 30, it came to within 75 million kilometers from Earth.

Not all closest approaches of Mars are made equal.
Image credit: lightstrider.com

Now imagine that Mars were the size of a basketball. The diameter of Mars is 6,780 kilometers. Meanwhile, that of a standard NBA basketball is around 9.5 inches or close to 25 cm. In this scale, how far was the Red Planet last May 30? Using simple ratio and proportion, we find that it is - wait for it - 2.75 kilometers away. Even at the minimum distance, basketball Mars will still be 2 km away.

Imagine standing along an empty EDSA (what a dream, I know) near the Magallanes MRT station. Then imagine staring into the direction of Ayala. Our basketball Mars would be near the flyover that leads to Rockwell. (For those not familiar with the distances in Manila, you can use Google Maps to check how these distances compare to landmarks in your place).

In comparison, the Moon would be a tennis ball just 14 meters away. (A basketball court is more than 15 meters wide).

A size comparison of the biggest rocky bodies in the inner Solar System.
Image obtained via Wikimedia Commons.

You can also watch this amazing video by Veritasium showing just how astounding the scales in the Solar System are. (Note that in the video, the Earth is the basketball. More accurately, when the Moon is a tennis ball, the Earth is a beach ball almost twice as big as a basketball.)

The right angles 

Astronomers measure sizes and distances in the sky by degrees of arc. For example, if you were in a vast plane or looking into the far distance at sea, the distance between the horizon and the point directly overhead (which astronomers call zenith) is 90 degrees of arc.

Now try holding one hand at arm's length against the sky. A hand span would measure 20 to 25 degrees, your clenched fist 10 degrees, and your pinky 1 degree. The full moon, on the other hand (heh) would measure around 0.5 degrees.

A handy way to estimate angles in the sky.
Image credit: timeanddate.com

In comparison, Mars was a mere 0.005 degrees last May 30. This means that the disk of an average full moon is a million times bigger than Mars then.

All of this is not to say Mars is not a remarkable sight with the naked eyes. The only things that can rival the Red Planet in brightness in this month's sky are the Moon (being very near - remember the tennis ball?) and Jupiter (which, despite being nearly 10 times farther than Mars, is so huge it still ends up appearing 2 1/4 times bigger).

The position of Mars in the early evening sky for the next few weeks. This image is a screen capture of the open source software Stellarium. The location for the software was set to Manila. Those living elsewhere can download Stellarium and look for Mars in their local sky.

If you missed Mars' closest approach this year, you can catch it in July 2018. Then, Mars would be closer to us, which means it would be slightly brighter and bigger. How much closer? Remember the basketball? It would be 2.1 km then. That's the distance between MRT Magallanes station and Buendia station.

The allure of the Red Planet

The red glow of Mars has intrigued sky watchers for thousands of years. For example, Mars' movement and reversals across the background of stars led Nicolaus Copernicus to propose the heliocentric model of the Solar System.

Observations made during close approaches of Mars have also placed the Red Planet squarely in the middle of the public's imagination. In 1877, Italian astronomer Giovanni Schiaparelli performed observations of Mars that included features he called 'canali', which in Italian means channels. Many later thought this meant canals were discovered on Mars. This, of course, was a misinterpretation, but this has not stopped generations of storytellers from coming up with stories about the alien races that made these canals.

In 1894, American astronomer Percival Lowell also performed observations of Mars during a close approach and decided that the canals were real. He made a detailed map of Mars showing hundreds of these canals. This fired up the public's imagination anew, and has even led to great works of science fiction from the tales of H.G. Wells, to those of Edgar Rice Burroughs.

A 1906 New York Times piece citing Percival Lowell as the greatest authority on Mars, then claiming that Lowell's
observations have confirmed life on the Red Planet.
Image credit: The New York Times.

Because of its special place in the collective imagination, hyped-up, exaggerated, or misinterpreted stories about Mars are therefore nothing new. But I think that while speculative tales about Mars and its fictional inhabitants can make great fiction, the reality of Mars and the adventures we have undergone to study it are something even greater. 

The monumental challenges of going to Mars, for example, is something many fanciful tales fail to describe. However, the red glow of Mars is also what inspired one of the pioneers of the science that would take us to its rusty surface.

Robert H. Goddard described in writing a moment in childhood when, atop the branches of a cherry tree he was pruning, he was inspired to build rockets. 

"It was one of the quiet, colorful afternoons of sheer beauty which we have in October in New England, and as I looked towards the fields at the east, I imagined how wonderful it would be to make some device which had even the possibility of ascending to Mars, and how it would look on a small scale, if sent up from the meadow at my feet," Goddard wrote. "I was a different boy when I descended the tree from when I ascended, for existence at last seemed very purposive."

He then went on to pioneer the science that has allowed us to send our many space probes and robots around and on the Red Planet.

Robert H. Goddard with an early liquid-fuelled rocket. Goddard himself was 
fueled by dreams of Mars.
Photo accessed through Wikimedia Commons.

So when you get a clear sky one of these nights, look for Mars and the other planets. They may never look as big as the full moon, but they will always be a delight to behold. Encourage young people you know to go sky watching, too. Who knows, Mars' red glow might just call out to them, and like Goddard, they might be inspired to pursue a career in science and engineering. One day, they might even end up walking on its distant but ever alluring surface.

Go sky watching whenever you can and whenever the sky is clear!
Photo credit: David Reneke's World of Space and Astronomy


1. National Aeronautics and Space Administration. (n.d.). Mars in Our Night Sky. Retrieved from: http://mars.nasa.gov/allaboutmars/nightsky/mars-close-approach
2. Gaherty, G. (2016, May 25). Mars Makes Closest Approach to Earth in 11 Years on May 30. Space.com. Retrieved from: http://www.space.com/32795-mars-closest-to-earth-11-years-may-30.html
3. Le Conte, D. (2013, October 24). Mars Hoax. Retrieved from http://www.astronomy.org.gg/hoax.htm
4. Choi, C.Q. (2014, November 4). Mars Facts: Life, Water and Robots on the Red Planet. Space.com. Retrieved from http://www.space.com/47-mars-the-red-planet-fourth-planet-from-the-sun.html
5. Chayka, K. (2015, September 28). A Short History of Martian Canals and Mars Fever. Popular Mechanics. Retrieved from: http://www.popularmechanics.com/space/moon-mars/a17529/a-short-history-of-martian-canals-and-mars-fever/
6. Atkinson, N. (2016, March 16). Inspiration And An Old Picture Full of Awesome: Robert Goddard And His Rocket. Universe Today. Retrieved from: http://www.universetoday.com/89507/inspiration-and-an-old-picture-full-of-awesome-robert-goddard-and-his-rocket/

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