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View Full Version : They can be taught! (maybe we're not doomed)

SharkByte
2008-Jun-15, 06:27 PM
I am a Staff Seargent in the US Air Force currently deployed to a remote Army camp in Afghanistan. A few minutes ago I was sitting outside my hut smoking a cigar and watching the sky when a few of the other guys who live in the huts around me wandered up.

One of the guys started looking up trying to figure out what I was looking at and finaly eventually said: "What are you looking at? I don't see anything." I simply told him he just didn't know where to look and started pointing things out. "Right up there (pointing) you can see two stars close together in line with each other. The bottom one is Regulus, the top one is the planet Saturn. Right over there (pointing again) is a small, kind of dim red star. Thats Mars. Behind us there's one really, really brigh star. That would be the planet Venus. At the moment though, I'm just watching the moon because I can see it with the naked eye pretty well." This got us onto the topic of the moon and the several nations that are talking about trying to go back to the moon. So the guy says "back to the moon implies that we actually went there in the first place. We didn't."

The conversation drew on for a while and somehowgot into explaining gravity and orbital mechanics. He knew that the moon is slowly drifting away from the Earth and lept to the (incorrect i believe) conclusion that it would some day spiral off into space when it got far enough away to break out of earth orbit. I asked how we could possibly know how how far the moon was away with enough accuracy to see a few inches of drift per year on average and he admitted that he didn't know.

I explained that the speed of light is finite and constant. He agreed. I suggested that if you could place a mirror on the moon and angle it just right, that you could bounce a laser beam of the mirror from earth, time how long it took the beam to get to the moon and back, do some simple math and calculate the distance the light traveled. Once again he agreed. I told him that that was exactly how we knew the distance from the earth to the moon with the kind of accuracy that would show a few inches of drift per year. I let him think about it a few seconds then asked: "So in order for that all to work, the mirror first has to get to the moon, then it has to be placed just right and angled to return the beam to its starting point so we can bounce a laser off of it. How do you think it got there and in the precise correct position to make that possible?"

"someone had to place it there..."

He went into his room for a while and the rest of us continued chatting. Eventually he came back out and told us he had just looked it up online and decided we must have actually gone to the moon. He couldn't think of any other explaination for the mirriors... :)

My explainations may not have been exactly on the mark all around and I may have gotten some facts wrong but thats my one convert for the week. :lol:

Kaptain K
2008-Jun-15, 06:52 PM
The Moon will spiral out, and the Earth's rotation will slow down, until the month and day are equal. The Moon will then spiral back in until it reaches Roche's limit. Then, like Saturn, the Earth will have rings.

SharkByte
2008-Jun-15, 06:58 PM
Thats pretty much what i thought. I figured it would spiral out until its velocity was to low to maintain its orbit then spiral back in until it gained enough velocity to start spiralling back out again and continue the cycle until it created enough drag on the earth to slow the earths rotation etc. eventually I figured the earth/moon would always present the same face to each other. I'm not sure where the rings fit in though?

The Supreme Canuck
2008-Jun-15, 07:07 PM
When a moon reaches the Roche Limit, it breaks up due to tidal forces.

Kaptain K
2008-Jun-15, 07:09 PM
The rings fit in because at Roche's limit the tides will tear the Moon apart:

http://en.wikipedia.org/wiki/Roche_limit

RalofTyr
2008-Jun-15, 07:29 PM
The Moon will spiral out, and the Earth's rotation will slow down, until the month and day are equal. The Moon will then spiral back in until it reaches Roche's limit. Then, like Saturn, the Earth will have rings.

By that time, the Moon and Earth may not exist as the sun will have already went red giant and may swallow them both up.

Kaptain K
2008-Jun-15, 07:59 PM
By that time, the Moon and Earth may not exist as the sun will have already went red giant and may swallow them both up.

As best we can project, Venus is doomed, but Earth is safe from being swallowed, not that anything inside of Jupiter will be habitable! :whistle:

Delvo
2008-Jun-15, 08:05 PM
What would bring the moon back after the two things are "tide locked"?

Swift
2008-Jun-15, 08:40 PM
I am a Staff Seargent in the US Air Force currently deployed to a remote Army camp in Afghanistan...

My explainations may not have been exactly on the mark all around and I may have gotten some facts wrong but thats my one convert for the week. :lol:
I've heard we have been making progress in Afghanistan, but I didn't think that's what was meant. ;)
Nice going SharkByte. :clap:

And stay safe over there!

Kaptain K
2008-Jun-15, 08:58 PM
What would bring the moon back after the two things are "tide locked"?

They will not be tidally locked with the Sun! :whistle:

mugaliens
2008-Jun-15, 10:57 PM
When a moon reaches the Roche Limit, it breaks up due to tidal forces.

That works great for the gas giants which have strong gravities and equally strong gravitational gradients per radial unit of altitude.

Does the Earth have strong enough of a gradient to cause the Moon to actually break up?

I recall reading that during Moonquakes, the Moon "rings like a bell," implying it's a fairly solid object.

Is there a mathematical way to determine the Roche limit, or does it depend on the integrity of the orbiting object?

Grashtel
2008-Jun-15, 11:14 PM
That works great for the gas giants which have strong gravities and equally strong gravitational gradients per radial unit of altitude.

Does the Earth have strong enough of a gradient to cause the Moon to actually break up?

I recall reading that during Moonquakes, the Moon "rings like a bell," implying it's a fairly solid object.

Is there a mathematical way to determine the Roche limit, or does it depend on the integrity of the orbiting object?
The answer to both your questions is yes, as is explained in the Wikipedia article on it (http://en.wikipedia.org/wiki/Roche_limit) that was linked earlier in the thread.

Maksutov
2008-Jun-16, 05:40 AM
I've heard we have been making progress in Afghanistan, but I didn't think that's what was meant. ;)
Nice going SharkByte. :clap:

And stay safe over there!I second Swift's remarks. Excellent work in putting out a few bits of information and then letting the person work out the rest. Best way to convince folks is to structure it so they convince themselves.

Stay safe and hope you quickly return home healthy.

Tobin Dax
2008-Jun-17, 06:07 AM
Great story, but I have to nitpick since I checked this last week (and just now) on Starry Night. The bright planet is Jupiter. It's distant position from the sun should give that away, too, since Venus will never rise after sunset.

(Just by coincidence, Venus is apparently within a degree or two of the sun right now.)

AndreasJ
2008-Jun-17, 11:34 AM
As best we can project, Venus is doomed, but Earth is safe from being swallowed, not that anything inside of Jupiter will be habitable! :whistle:

According to Schroder & Smith, 2008 (http://arxiv.org/abs/0801.4031), not so - they find that tidal interactions will drag the Earth down into the red giant Sun shortly before it reaches maximum size at the tip of the RGB.

Amber Robot
2008-Jun-17, 01:24 PM
What's interesting to me is that he unquestioningly believed that the Moon was moving away from the Earth. If he gave up that idea, then he could still maintain that no one ever walked on the Moon.

SharkByte
2008-Jun-17, 06:03 PM
Great story, but I have to nitpick since I checked this last week (and just now) on Starry Night. The bright planet is Jupiter. It's distant position from the sun should give that away, too, since Venus will never rise after sunset.

(Just by coincidence, Venus is apparently within a degree or two of the sun right now.)

Opps, chalk that up to a typo. Sometimes my fingers move faster then my brain does:D

Disinfo Agent
2008-Jun-17, 10:38 PM
What's interesting to me is that he unquestioningly believed that the Moon was moving away from the Earth. If he gave up that idea, then he could still maintain that no one ever walked on the Moon.You just never know which premises people will assume, and which ones they will question.