With the coming summer, the rumour is circulating again. I've already received 3 copies of it in my email from various sources.
So let me spell it out for everyone, once and for all:
1. Mars has not, and will not ever within the lifetime of all humans, appear to be as large as the full moon.
2. Mars is not making its closest approach in <large number> of years. That event took place.
3. The whole thing is a hoax, based on a factual, if poorly worded, internet circular from many years ago.
Explanation:
Point 1 - Mars has not, and will not ever within the lifetime of all humans, appear to be as large as the full moon.
The full moon is LARGE in the sky. Its apparent size varies with its orbital anomaly (the relative position along its orbit). As you might imagine, it appears largest when it is closest, and smallest when it is far away. The change in size can be as much as 12%. However, give or take that little bit, the full moon appears to be about 31 arc-minutes in diameter (a little more than half a degree). An arc-minute is 1/60 of a degree, and 1/21600 of a circle. For most people "31 arc-minutes" is a relatively useless measure, so let me put it in simpler terms... it's about 1/720 of a circle.
How big is 1/720 of a circle? To find this out, take a quarter from your change jar. A Canadian 25-cent piece is 24mm in diameter. If that 24mm was 1/720 of a circle, that would mean the circle is 17.28 metres around. That would mean the circle had a radius of 2.75m
So, a quarter held up 2.75m away is about the size of the full moon, give or take.
How big is Mars? It turns out that the next closest approach of Mars occurs on 27 Jan, 2010, when the Red Planet will be 99 million km from Earth. It will have an apparent diameter of just over 14 arc-seconds in the sky.
An arc-second is 1/60 of an arc-minute, or 1/3600 of a degree or 1/1296000 of a circle. It's a pretty tiny measure. Mars, at its biggest that we will see between 2008 and 2011, will be only 14 / 1296000 of a circle.
How big is that? Since you've currently got a quarter sitting 2.75 m away, we can use the same quarter. The 24mm of the quarter is now 14/1296000 of the circle. That means our circle has a circumference of 2.21 km! Such a circle has a radius of just over 350 metres. Mars, at its biggest that we will see between 2008 and 2011 will be the same apparent size as a quarter held 350 metres away.
In fact, for Mars to appear as large as the full moon, it would have to approach to something like 1 million km give or take. That is never going to happen, and if, as a result of some horrible cosmic cataclysm it did happen, it would be bad.
Point 2 - Mars is not making its closest approach in <large number> of years. That event took place.
In August of 2003, Mars and the Earth came into their closest approach that will occur for a bit less than 300 years. That was six years ago, and Mars was, at that time 55.76 million km from Earth - about half what it will be at the Jan 2010 approach. The August 2003 approach was the smallest distance between the two planets for the last 57500 years. On 28 August, 2287, Mars will approach 70000 km closer than in 2003, although none of us will see it, AND IT STILL WILL NOT BE AS BIG AS THE FULL MOON... although the rumour will still be circulating, I'm sure. In August 2050, Mars will approach to only 0.2 million km farther than in 2003, so many of us WILL get to see that again, looking about the same as it did in 2003.
How big did it look in 2003? It was just over 25 arc-seconds. Yell down to your man holding the quarter at 350 metres to move 152 metres toward you. When he's holding the quarter at 198 metres away from you, and you'll see that quarter to be the same size Mars appeared in the sky at its closest approach in 2003.
Point 3 - The whole thing is a hoax, based on a factual, if poorly worded, internet circular from many years ago.
Yes, it is.
The original text of the message that people were circulating in 2003 said this:
At a modest 75-power magnification Mars will look as large as the full moon to the naked eye .
And that is correct for the time. Tiny, little, red Mars when magnified 75x in a telescope will appear, IN THE TELESCOPE, to be as big as the full moon does TO THE NAKED EYE. After that, a telephone game took over and the telescope requirement fell by the wayside.
The notification, once true back in 2003, now circulates... wandering the internet like some kind of electronic zombie, spreading disinformation wherever it goes. If you get this the "Mars is going to be huge" chain letter, please do not forward it on, but feel free to reply to the sender with a link to this post.
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