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hhEb09'1
2005-Nov-10, 06:33 PM
One interesting demonstration in intro geology classes, when discussing the idea that the moon has protected the earth from meteor impacts, is to set up a scale model. If the earth is a pool ball, then the moon is a marble, so we ask, how far away is the marble from the pool ball. Inevitably, most suggestions place the marble a lot closer than the five feet, and most discern that the marble is not much protection for the pool ball at that distance.

Now, how long does it take the marble to go around the pool ball? Should we reduce the time dimension by the same factor of 240,000,000? That would mean it would be whizzing around the pool ball once every hundredth second. Maybe it could be sweeping up space debris. But wouldn't the speed of space debris as it approaches earth put it through the tunnel of the moon's orbit a lot faster than that?

And how massive would that pool cue have to be to make the marble orbit it every hundreth of a second? The mass of the moon would be just 1/81 of that, right?

NEOWatcher
2005-Nov-10, 07:14 PM
My non-professional take:
If an object were headed directly toward the Earth, I would tend to think the same way (the moon not helping much). But; being that most of the orbits are in orbit around the sun, they wouldn't head directly toward Earth, but would pass closer and closer and eventually one of the passes would impact. This means that as more passes occur, the more likely the moon will either be there, or at least have some influence on the object's trajectory.

George
2005-Nov-10, 11:06 PM
I wonder if people have heard we have protection in Jupiter, and extend this thought to a closer potential protector - the moon.

Using the pool ball analogy, Jupiter would be a 25" beach ball, but 1.75 miles away (minimum). It also does not whiz around us very fast. :) This might confuse them; maybe not.

LurchGS
2005-Nov-10, 11:12 PM
I've heard both, and goggled (not Googled, sorry, ToSeek)..

All one needs to do is look around at all the large craters on earth that erosion hasn't rubbed away to nothing (yet)

though, if one wanted to have fun, you could point out that the moon used to be a LOT closer to the Earth, and therefore offered a little more protection (orbited faster)

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ha! when it's near the horizon, the moon really IS closer to the earth !

hhEb09'1
2005-Nov-14, 05:23 PM
One of the problems is that the moon looks like it has taken a lot more hits, because of its cratered terrain--but most of those are billions of years old, and just have not weathered away like a crater on earth would have.

LurchGS
2005-Nov-14, 09:35 PM
right, though there have been significant impacts to the moon in recorded history.

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I love "on average, a large [dinosaur killer] meteoroid strikes the earth every 65 million years".

Enzp
2005-Nov-15, 02:55 AM
We contemplated that billiard ball, and then one fateful day, God was overheard to say, "Whose break?"

Hugh Jass
2005-Nov-15, 03:09 AM
I wonder if people have heard we have protection in Jupiter, and extend this thought to a closer potential protector - the moon.

Using the pool ball analogy, Jupiter would be a 25" beach ball, but 1.75 miles away (minimum). It also does not whiz around us very fast. :) This might confuse them; maybe not.

http://www.troybrophy.com/projects/solarsystem/index.html :cool:

almost off topic but cool