Mars as seen by the Hubble Space Telescope, which is about the best view we can get from Earth.... [+]
Let's get right to it: Mars will never appear in the sky as large as the full Moon. The Red Planet is a long way off (much to the chagrin of scientists who want to study it better), and even at the closest approach of Mars and Earth, the two planets are still nearly half the distance Earth is from the Sun, and will still look like a bright reddish spot in the night sky.
Yet every year about this time, the rumor goes around that Mars will appear as large as the full Moon in the sky. Fifteen years ago it was passed around as email forwards; today you see it all over Facebook. The version someone sent me last night reads:
12:30 August 27th you will see two moons in the sky but only one will be the moon. The other will be Mars. It won't happen again until 2287. No one alive today has ever witnessed this happening.
At least the last sentence of that is correct: no one, living or dead, has ever or will ever witness this happening, because it can't.
The size an object appears in the sky depends on how big it is and how far away it is. The Moon is pretty close to Earth: 240,000 miles (384,000 km), which is practically on top of us in astronomical terms. By contrast, Mars and Earth will never get closer to each other than about 34 million miles (55 million kilometers). And that's the closest: I ran the numbers, and Mars will be more like 230 million miles (380 km) away on August 27, nearly as far away from Earth as it ever gets.
Simulation of how Mars will appear just before dawn on August 27, 2015. Note that it is still... [+]
In other words, on August 27, Mars will appear about as small as it ever does. It's on the opposite side of the Sun from us right now. If you want to see it, you need to get up really early in the morning (not 12:30 AM like the Facebook post claims). At least the Moon will be very nearly full that night: the Facebook post got that one right.
Thanks to a convenient bit of geometry, the size something appears on the sky is directly related to how far it is away. When Mars is twice as far away from Earth, it will appear half the size, so it changes size quite a bit during its orbit (and ours). The Moon and Sun are the most famous example of the correlation of apparent size with distance. The Sun is about 412 times the diameter of the Moon, but it's also about 395 times farther away on average — so they appear to be the same size in the sky, even though the Sun is much larger. (I explained how this works on my own blog.)
Mars is roughly twice the diameter as the Moon, so for Mars to appear to be the same size as the Moon, it would need to get to about 500,000 miles away from us — something like 700 times closer than it can ever get.
The motion of the planets is about as predictable as it can be. That's how NASA engineers like Yanping Guo can plot the trajectory of the New Horizons spacecraft and have it arrive at Pluto precisely where she intended. And that's how we can be sure Mars will never leave its orbit and drift close enough to Earth to appear the size of the full Moon.
August can be kind of a boring month. In the United States, the weather is generally pretty hot, and there are no holidays, festivals, or major sporting events to distract us from the heat. It's not even a national election year (not that you would know from the number of candidates running around). I choose to blame that for the eternal recurrence of the "Mars is big as the full Moon OMG!!!1!" trope, but let's watch baseball or stay inside and play video games or something else instead, OK?
