Will Mars look as big as the Moon in August?

2010-08-02 00:00

Dear Urban Astronomer

I recently received this presentation (http://tinyurl.com/2fxrk64) in an email. Apparently, Mars will be closer to us on 27 August than ever before in recorded history and will look as big as the Moon in the sky. Is this true?

I was tempted to simply dismiss this as a joke. After all, it's not like anybody still falls for this, right? Surely people remember seeing the same hoax this time last year, and the year before that all the way to 2003? Nevertheless, the hoax comes back to life every year and so I must challenge it. There are two ways to test the claims in the presentation. First, I will use computer software (http://www.stellarium.org/) to simulate the night sky at 7 pm on 27 August 2010. Then, after we've had a good long look at the results, I will wait for the big day and step outside and look up so that I can compare the predictions of the presentation and the simulation and see which was right. I expect from experience that the software will give accurate results, but you shouldn't trust me on this - go outside and have a look for yourself!

Having set up Stellarium to show me the western sky on 27 August at 7pm local time, I find that Mars is currently very close to Venus, with Saturn hanging beneath them close to the horizon. On that date, Mars is about 2.13 AU away from Earth, and has an angular diameter of 4.4 arc seconds. (AU? Arc Seconds? What does all this mean? Well, AU stands for Astronomical Units - 1AU is the average distance from the Earth to the Sun, or very roughly 150 million km. An arc second is one sixtieth of an arc minute, which is a sixtieth of a degree. As you will remember from school, there are 360 degrees in a circle. The entire sky is therefore 180 degrees across - the other 180 are beneath our feet. The full moon has an angular diameter of about 30 arc minutes or half a degree, which means that you can pack 360 full moons end to end from one side of the horizon to the opposite).

Now, if we compare these figures to the presentation, which claims a distance of about 55 million kilometers (0.3AU), and an angular diameter of 25.11 arc seconds we find that they don't even come close to reality. It's tempting to shrug this off as a bit of false fluff, invented by somebody with too much time on their hands, but did you notice the other big discrepancy? According to page 4 of the presentation, Mars is supposed to rise in the East at 10pm, but in fact we see it's already close to setting by only 7pm. (They also claim it will reach it's 'azimuth' at 3 am, but azimuth only means "Horizontal position" which shows that the author doesn't really know what he's talking about. It's like saying that if you drive to another city, you will reach your "duration" by the middle of the trip. It's nonsense.)

As it happens, Mars did pass particularly close to Earth in August 2003 (the year this hoax first started circulating). As an experiment, I set the date in Stellarium to 27 August 2003, and centered the view on Mars. Bingo! The distance to Mars shows as a little over 0.3AU, and the angular diameter is a little over 25 arc seconds, which is exactly what the hoax email described. The presentation, incidentally, states that the view of Mars "At a modest 75x magnification" will look the same size as the moon to the naked eye, even if the layout makes it seem as if Mars and the Moon will be the same size to the naked eye.

So what's going on here? Two things. Back in 2003, the close approach of Mars was an exciting thing for planetary observers. Mars would be exceptionally close (although not significantly closer than any of the other approaches, which happen regularly a little more than once every two years) and thus nicely positioned for viewing and photographing through a telescope. The mainstream media picked up on this, and became confused by one tiny detail The angular size of the moon (30 arc minutes) looks close to the size of Mars (25 arc seconds), and didn't notice the difference between seconds and minutes. With that, the story spread worldwide via both news reports and email chains. The only mystery now is who keeps dusting off this old mistake, crossing out the old year and replacing it with the current one and restarting the chain!

Comments? Questions? Why not mail me at uastronomer@gmail.com

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