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MHS
2002-Apr-06, 07:18 AM
I was reading the chapter in Phil's book about the names of a meteor in it's different stages (meteoroid, meteor and meteorite) and I decided to look them up in my 'Concise Oxford Dictionary'. The explanations for meteoroid and meteorite seemed pretty accurate, but for meteor the dictionary gives the following definition:

"meteor a small body of matter from outer space that becomes incandescent <u>as a result of friction with the earth's atmosphere</u> and is visible as a streak of light."

This is exactly what Phil says is wrong. I quote:

"So that's why we get meteors. But why are they so bright? Almost everone thinks it's friction - our atmosphere heating them up, causing them to glow. Surprise! That answer is wrong. When the meteoroid enters the upper reaches of the Earth's atmosphere, it compresses the air in front of it. When a gas is compressed it heats up, and the high speed - perhaps as high as 100 kilometers per second - of the meteoroid violently shocks the air in its path. The air is compressed so much that it gets really hot, hot enough to melt the meteoroid. The front side of the meteoroid - the side facing this blast of heated air - begins to melt. It releases different chemicals, and it's been found that some of these emit very bright light when heated. The meteoroid glows as its surface melts, and we see it on the ground as a luminous object flashing across the sky. The meteoroid is now glowing as a meteor."

Later on Phil does mention that "this is the usual explanation given in books and on TV", but a dictionary... come on!

<font size=-1>[ This Message was edited by: MHS on 2002-04-06 02:21 ]</font>

GrapesOfWrath
2002-Apr-06, 05:51 PM
On 2002-04-06 02:18, MHS wrote:
Later on Phil does mention that "this is the usual explanation given in books and on TV", but a dictionary... come on!

No dictionary is perfect. For instance, 9 out of 10 don't even have the word "gullible." Does yours?

In this case, I think the most common general science explanation of meteors has been friction--that has certainly been my experience. In the case of the atmosphere, or any fluid, how do you define "friction"? Clearly, in order for that hot high-pressure air to cause the meteor to melt, some sort of tranfer of heat has to take place, between the air and meteor. Is that a sort of friction? Again, what would we mean by "air friction"?

The confusion over exactly what is friction is not restricted to fluids.

The Bad Astronomer
2002-Apr-06, 06:33 PM
Heating from friction occurs when two particles (fluid, solid, whatever) rub against each other in some way, and transfer heat through conduction (transferring kinetic energy). Meteoroids heat up because the rammed air in front of them heats up (Boyle's law, I think... any chemists here?) and transfers that heat through radiation.

So it isn't friction that heats up meteoroids. There is only a little friction, comparitively, with the rock and the air around it. That's what causes ablation, but not heating.

MHS
2002-Apr-06, 08:18 PM
On 2002-04-06 12:51, GrapesOfWrath wrote:

No dictionary is perfect. For instance, 9 out of 10 don't even have the word "gullible." Does yours?


Yes it does. The dictionary has over 1600 pages so it really should contain the word gullible. Although I have to admit I'd never heard of it, but hey I'm a foreigner right? /phpBB/images/smiles/icon_wink.gif

(edited for spelling)

<font size=-1>[ This Message was edited by: MHS on 2002-04-06 15:19 ]</font>

Donnie B.
2002-Apr-07, 12:03 AM
On 2002-04-06 12:51, GrapesOfWrath wrote:
No dictionary is perfect. For instance, 9 out of 10 don't even have the word "gullible." Does yours?

Hee hee... I'm not quite gullible enough to fall for that one, GoW... /phpBB/images/smiles/icon_biggrin.gif

GrapesOfWrath
2002-Apr-07, 02:55 AM
No one else was either, which is a good sign. (MHS is excused, as he points out) No doubt there is a high level of quality on this board.

Still, the usual friction explanation is common enough that I'm not at all surprised that it appears in dictionaries. I did a quick altavista search on "friction ablation meteors", and all the sites I checked seemed to attribute the glowing of meteors to friction. Here's some of my results: Astronomer James Schombert at Univ of Oregon (http://zebu.uoregon.edu/~js/glossary/ablation.html), Science at NASA (http://science.msfc.nasa.gov/headlines/y2001/ast27jul_1.htm), Spaceguard (http://spaceguard.ias.rm.cnr.it/tumblingstone/dictionary/fragmentation.htm), and Adam Block at the National Optical Astronomy Observatory (http://www.noao.edu/outreach/nop/nophigh/leonid.html). I tried to include the name of the author when I could find it, but I don't know these guys.

Clearly, the shockwave that is built up in front of the meteor is caused by the meteor actually hitting the atmospheric atoms, but the distinction is interesting. How high can we actually see meteors?

MHS
2002-Apr-07, 08:29 AM
On 2002-04-06 21:55, GrapesOfWrath wrote:
No one else was either, which is a good sign. (MHS is excused, as he points out) No doubt there is a high level of quality on this board.


Damn you guys. Wait till I trick you in Dutch /phpBB/images/smiles/icon_wink.gif

Hat Monster
2002-Apr-08, 03:33 AM
Posted by The BA:


"Meteoroids heat up because the rammed air in front of them heats up (Boyle's law, I think... any chemists here?)"

Yep. Part of ideal gas law. Compress a gas, it heats up. The same principle makes a pump warm after you've inflated a bicycle tube, for instance.
At the height of a meteor, I wouldn't imagine there'd be enough air, though?

Kaptain K
2002-Apr-11, 08:22 AM
At the height of a meteor, I wouldn't imagine there'd be enough air, though?
You'd be surprised at how little air is necessary when the piston in your pump is moving at double digit KPS.

GrapesOfWrath
2002-Apr-12, 08:07 AM
On 2002-04-07 23:33, Hat Monster wrote:
At the height of a meteor, I wouldn't imagine there'd be enough air, though?

At that James Schombert link, he talks about meteors at 100km, in his first paragraph, then mentions that at "deeper levels in the atmosphere, a shock wave may be produced".

Another Phobos
2002-Apr-16, 08:21 PM
You should try some dictionary definitions for Big Bang! Those can get pretty bad (especially older dictionaries).