Yes, I’m tardy to the party – in more than one way. First, I’m writing about this after August 27, and also, this particular bit of foolishness has already been debunked about half a billion times. So why bother writing about it? I’ll explain, but first let’s address the history and details of this hoax (and it is a hoax).
Earth orbits the Sun in about 365.24 days, at an average distance of about 150 million kilometers. Earth’s orbit isn’t perfectly circular, so during the course of its orbit it moves closer to and farther away from the Sun. At its closest approach to the Sun (called perihelion) Earth is approximately 147 million kilometers out. At its most distant point from the Sun (called aphelion), Earth is roughly 152 million kilometers away.
The orbit of Mars is outside of Earth’s orbit, and also more eccentric – meaning Mars’s orbit is more oval-shaped than Earth’s orbit is, and there’s a bigger difference between Mars’s perihelion and aphelion distances. The average Sun-Mars distance is about 228 million kilometers. Its perihelion is about 206.6 million kilometers, and its aphelion is more than 249 million kilometers.
Earth, being closer to the Sun, completes its orbit faster than Mars does. Every 26 months, Earth overtakes and passes Mars on their respective trips around the Sun. When this happens, Mars is at a point in the sky directly opposite the Sun. We call this opposition. During an opposition of Mars, the two planets are as close as they’ll get until the next opposition occurs.
The Earth-Mars distance at opposition varies from one occurrence to the next. If Mars is near aphelion or Earth is near perihelion, the distance will be great. If Mars is near perihelion or Earth is near aphelion, the distance will be smaller. The closest possible approach of Earth to Mars would be during a super-rare concurrence of three astronomical events: opposition, Mars perihelion, and Earth aphelion. During such a passage, Earth and Mars would be a little less than 55 million kilometers apart. Unfortunately, Earth’s aphelion is not currently lined up with Mars’s perihelion, nor will it be at any time soon; ergo, this remains a theoretical minimum for the extended future.
Still, there are isolated local minima – points in the orbits of both planets where the distance between them shrinks to a relatively small number, if not the smallest possible. On August 27, 2003, the Earth-Mars distance was about 55.8 million kilometers, the closest the two planets had been in nearly 60,000 years. There will not be a closer approach between Earth and Mars until 2287. The 2003 opposition is where this hoax began.
At some point prior to the 2003 close opposition, somebody shot out an email alerting people to this upcoming astronomical rarity. The email contained a line to the effect that through a modest 75-power magnification telescope, Mars would look as large as the full moon looks to the unaided eye. This distinction was quickly lost, however, and it was soon being reported that Mars would look as large as the Moon…period.
Okay, an honest mistake, right? Maybe it was a mistake in 2003, but the message appeared again in 2005. It has circulated every year since then, either as an email or as a meme. Each time the message claims that on August 27 of the current year, Mars will look as big as the Moon in the night sky, despite the facts that (1) the opposition of Mars has not occurred on August 27 since 2003, (2) the oppositions that have occurred were not particularly close, and (3) during none of them would Mars have appeared as large as the full moon.
Just how large does Mars appear during an opposition? Let’s pause for a minute to use a small-angle approximation formula: θ = 206265*(d / D). In this formula, θ (theta) represents the apparent angular diameter of Mars; in other words, how wide it seems to be when pasted against the sky. The lowercase d represents the diameter of Mars (6779 kilometers), and the uppercase D is the distance between our planet and Mars.
θ = 206265*(6779 km / 54,600,000 km) = 25.6 arcseconds
An arcsecond is 1/3600 of a degree, and a degree is 1/180 of the distance across the full sky, so 25.5 arcseconds represents a pretty small piece of sky real estate. By comparison, the angular diameter of a full moon is about 31 arcminutes, or 1860 arcseconds. Even at its closest possible approach, Mars would appear nearly 73 times smaller than the full moon as seen from Earth. It would be a dot – a bright red dot, but a dot nonetheless.
Just for a moment, let’s abandon our knowledge of orbital mechanics and assume that Mars could get close enough to Earth to look as big as the Moon. How close would that be? Well, the diameter of Mars is roughly twice the diameter of the Moon, so Mars would have to be about twice as far away as the Moon. That would put Mars at a distance of about 750,000 kilometers. At that distance, the Martian gravity would have significant effects on Earth’s tides – more than the Moon, in fact. If the Moon, Earth, and Mars were roughly aligned at the time of Mars’s closest approach, there would be a fearsome spring tide. Coastal areas would be flooded by the inrushing water. Mars would also disturb the Moon’s orbit, possibly sending the Moon careening into outer space (or worse, but less likely, crashing down to Earth). There would be a significant uptick in the number of earthquakes as the tug of Mars’s gravity released strain in fault lines deep beneath Earth’s surface.
But the worst of it would pass as soon as Mars left the vicinity of Earth, right? Wrong. A close passage by a body as massive as a planet would leave a lasting impact on Earth’s orbit. Depending on the particulars of the encounter, Earth’s orbit could be enlarged (leading to a dramatic and devastating drop in global temperatures) or shrunken (leading to a dramatic and devastating rise in global temperatures). Whole climate systems would be rewritten. Humanity, if it survived, would face a radically altered world. It would take decades, maybe centuries, for us to adapt to our new home.
Needless to say, this did not happen in 2003. It did not happen in 2005, or in 2006, or in any other year since then. It will not happen in any year going forward. The “Mars as big as the Moon” email and meme are utter hoax garbage. Every year the meme flies, and every year a host of scientifically literate people rush to debunk it. Why add my voice to the din of other debunkers?
I figure there has to be some critical mass beyond which the debunking will take hold and the hoax will stop finding traction each year. I don’t know what that mass is – we obviously haven’t reached it yet – but each time a person says “Not so fast!”, I like to think that one more would-be propagator of the Mars-Moon hoax decides to trash the message rather than spreading it. Who knows: maybe by 2287, when the next relatively close Mars opposition occurs, people will look at the sky and know the truth.
In the mean time, I humbly submit that we move April Fools’ Day to August 27. We’ll probably have to rename it.