8. "Mars as Big as the Moon" - NOT!

Every ( Northern Hemisphere ) summer for the last six years ( at the time of writing in 2009 ), a ridiculous e-mail has done the rounds of the internet – one of those which is passed on by those naïve enough to believe it – which claims that, on a certain date in August, Mars is about to come closer to the Earth than at any time in recorded history. It exists in a number of versions, one of which even makes the absurd claim that “Mars will look as big as the Moon in the sky”! I haven’t personally received any of these, but I know people who have; last year, one friend told me that his girlfriend actually believed it, including the “as big as the Moon” part, which is obviously totally ridiculous, to anyone who gives it a moment’s logical thought.
I don’t know whether this bit of stupidity began as a deliberate hoax, or was propagated by someone stupid enough to actually believe it – but how it began is very simple. Six years ago, the first part of it was true; on 27 August 2003, Mars did indeed make an exceptionally close approach to Earth ( see Section 8.3 below, for what “exceptionally close” actually means ) – closer than at any time in the last 60000 years. To astronomers, this was merely an interesting mathematical curiosity, but the popular media made a huge deal of it, with all kinds of ridiculously exaggerated claims.
Since then, some unknown idiot has apparently laboured under the delusion that the planet makes a close approach to Earth every August, and has simply regurgitated the same e-mail every year, with the date updated. This is, of course, completely false.
So let’s take a look at the real facts.


8.1. Oppositions of Mars

The Earth and Mars, obviously, take different lengths of time to orbit the Sun; Earth takes 365 days, and Mars 687 days. So at certain times, they are both on the same side of the Sun in their orbits; at other times, they are on opposite sides of the Sun, and every other relative position in between. Therefore, the distance between the two planets varies hugely.

Fig. 7

In Fig. 7, the sizes of the two planets’ orbits are drawn to scale; Earth’s mean orbital radius is 149 million km, and that of Mars is 228 million. It’s easy to see that the planets are closest together when they are aligned as in diagram A, and furthest apart when aligned as in diagram B. In fact, their distance apart is more than five times greater at B than at A.
The configuration shown in A is called an opposition of Mars – because it’s opposite the Sun in the sky, as seen from Earth – and occurs roughly once every 25 months. The times around oppositions are of great interest to astronomers, as they are obviously the best times to observe Mars; it has its greatest apparent size, and is also visible all night, rising highest in the sky at midnight.
( On a related theme, those who follow spaceflight may know that NASA – and often also Russia – launches space probes to Mars roughly once every two years. This is done to coincide with particular relative positions of the planets, which minimise the velocity change which the probe has to make, and therefore the fuel required. )


8.2. Perihelic oppositions

In Fig. 7, I’ve drawn the orbits of Earth and Mars as circles, for simplicity. In reality, they are in fact elliptical, with the Sun at one focus of the ellipse. In the case of Earth, the ellipse is so close to a circle, that if it was drawn to scale, you would have to look closely to tell the difference; its distance from the Sun varies between 147 and 152 million km. But the orbit of Mars is distinctly more elongated – or eccentric, to use the correct term; its distance from the Sun varies between 206 and 249 million km ( Fig. 8 ).

Fig. 8

This has an interesting consequence for astronomers. Once every 15 years or so, Mars’ opposition occurs fairly close to its perihelion, the closest point in its orbit to the Sun. At this configuration, known as a perihelic opposition, the separation of Mars and Earth is several percent less than at other oppositions. This is clearly shown in Fig. 8, in which the eccentricity, or elongation, of Mars’ orbit is exaggerated to illustrate the point. They can, in fact, come as close together as 54 million km, or just under 34 million miles.
Perihelic oppositions are of special interest to astronomers – particularly the amateur kind - for reasons which should now be obvious; they present the best opportunities of all for observing Mars. The most recent perihelic opposition occurred on 27 August 2003 – and that was what led to all the aforementioned media hype, and the annual ridiculous e-mail misinformation which has circulated ever since.


8.3. The record close approach of 2003

As the perihelic opposition of 27 August 2003 approached, astronomers realised that this one would be a record close approach, with Mars coming closer to Earth than at any time in the last 60000 years. The minimum separation would be a mere 55.8 million km, or 34.9 million miles. This was because the opposition occurred exceptionally close to Mars’ perihelion, and also while the Earth was fairly close to its aphelion, or furthest point from the Sun, which occurs in early July.
( Yes, you did read that correctly. The Earth is furthest from the Sun in July, and closest in January. If you, the reader, are one of the dismaying proportion of people who find that fact surprising, then may I remind you that when it’s summer in the Northern Hemisphere, it’s winter in the Southern, and vice versa, and suggest that you revise what you – hopefully! – learned in school at about the age of 12, about the cause of the seasons! Summer and winter have nothing whatsoever to do with the Earth’s distance from the Sun, and everything to do with the tilt of its axis; the slight variation in temperature due to its distance from the Sun is trivial in comparison. )
It almost went without saying, that when the less reputable media got hold of this story, they would find ways to screw it up…
Naturally, the true meaning of such phrases as “a record close approach” was that Mars would come marginally closer than it does at most perihelic oppositions, and just a small fraction of one percent closer than at any other one in recorded history. To amateur astronomers, the fact that it was a perihelic opposition was of great importance, because it represented the best opportunity for observing Mars for 15 years. The fact that this particular perihelic opposition was just a tiny smidgin closer than any previous one was not a big deal; it was an interesting mathematical curiosity, and nothing more.
It’s equally obvious, to anyone with half a brain, that the “close approach” would take place gradually over a period of weeks, with the two planets drawing very slightly closer day by day, until their separation reached a minimum on 27 August, and then drawing gradually further apart again. If you plotted the separation against time, it would have looked like the bottom part of a sine curve, with its turning point on 27 August.
But try telling any of that to the “moron media”…
Many of the more dumbed-down media presented the facts in a sensationalised manner, which, while not actually saying anything untrue, was ridiculously misleading. Firstly, they made a massive song and dance about the “record close approach”, which led many of the public to believe that Mars was going to come vastly closer to Earth than ever before, instead of just a tiny fraction closer. Secondly, the emphasis which was placed on the actual date of closest approach inferred that it was going to make some kind of sensational appearance in the sky, for that one night only! Reading some of the stories, with headlines such as “Tonight’s the Night!” the naïve could have been led to believe that Mars was somehow going to zoom in from the depths of space, “buzz” the Earth at some incredibly close distance on that single night, and then zoom back out into space again! They seemed to miss the point, that on 27 August, it would simply be a miniscule fraction of one percent closer to us than on the previous and next days, a slightly bigger fraction closer than it was two days either side, and so on. D’ohhh!
On the day, I happened to speak on the phone to my parents. My mother, who knows nothing about astronomy, but had seen the headlines in the papers, remarked, “You can see Mars tonight, apparently” – as if to say that you couldn’t see it the night before or the night after, or every night for several weeks either side!


8.4. “Mars as big as the Moon”? I don’t think so!

As mentioned earlier, some versions of the e-mail which was circulated in 2003 about the close approach, and which have been falsely “recycled” every year since, made the absurd claim that Mars would “appear as big as the Full Moon”, and that – for one night only, naturally! – it would “look as if there were two Moons in the sky”! I hardly need say that this is utter drivel!
Phil Plait, on his Bad Astronomy blog, wearily repeats the debunking of this stupidity every year. A commenter on the blog has pointed out how this misconception probably originated. It seems that, in 2003, a badly made Powerpoint presentation on the perihelic opposition did the rounds of the internet, which included the following statement: “If Mars is viewed through a small telescope, at a modest magnification of 75x, it will appear as big as the Full Moon.”
While this was almost exactly correct ( see below ), the way it was worded was somewhat misleading. It should have said, “If Mars is viewed through a small telescope, at a modest magnification of 75x, it will appear as big as the Full Moon does to the naked eye”. It’s perhaps not even surprising, that this subsequently became distorted by the naïve, and someone repeated the information in an e-mail, but omitted the vital words about viewing through a telescope, and simply claimed that Mars would “appear as big as the Full Moon”. What is surprising, and dismaying, is the number of people who are apparently naïve or gullible enough to believe this!
Anyone with the slightest knowledge of maths or physics can see that the apparent angular size of an astronomical body is proportional to its actual size, and inversely proportional to its distance from Earth. Anyone who wished to check the facts could have found out the sizes and distances of the Moon and Mars, with a two-minute Google search. So let’s do the maths…
The diameter of the Moon is 3476 km; that of Mars is 6787 km. So Mars’ actual size is just under twice that of the Moon – 1.95 times, to be exact. The mean distance of the Moon from Earth is 384400 km; at the time of closest approach in 2003, the distance of Mars was roughly 55.8 million km – a factor of 145 times greater. So its apparent angular size in the sky, at its closest, was 1.95 / 145 = 0.0134 times, or only about 1 / 74, of that of the Moon.
So there you have it – a prime example of how a simple misinterpretation of the correct facts, combined with the “it must be right, because I read it on the internet” mentality, can lead to the widespread propagation of misinformation.

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