Why doesn't moon crash to Earth

[dannat (Daniel)]

I have asked this question to my year 8/9 sciecne class & none came up with a good answer - they all know gravity from Earth stops the moon from drifting away but why doesn't it crash into Earth if it is attracted by gravity?

[Chrissyo (Chris)]

I can answer, but it'll be a bit dodgy. I can't remember the exact terms and definitions.

It is because the Moon also has a velocity around the Earth. Basically, this motion 'cancels out' the force by gravity - the Moon is being pulled towards the Earth, but it's circular (well, elliptical) orbit is also pulling it away from the Earth. The centrifugal force balances out the centripetal force.

[fringe_dweller]

it is actually moving slowly away from earth in its orbit by a few centimetres each year atm

besides i think it is tethered to a mountain somewhere in peru? j/k

[ballaratdragons (Ken)]

and every time it moves out those few centimeteres each year, NASA or the Russians or someone else lands something on it and it nudges it back a few centimeteres again

But yes, Chris is right. The orbital velocity is throwing it out as it is being drawn in by gravity. Same as us around the sun.

[dannat (Daniel)]

you're right about it moving away slowly - shortly (in astro terms) the perfect solar eclipses we see will be a thing of the past as the moon will be smaller than the sun. I think today is the very small window where the sun and moon are the same size as viewed from Earth, when many years either side of present day the sizes did not match up at all, thanks for the responses
dan

[xelasnave]

I read that we of this time are lucky because in the future..sometime.. you wont see a total eclipse with the Moon totally covering the Sun ..

But if we are moving away from the Sun will it be smaller and so those of the future may indeed see an eclipse the same way as we do today.

How wonderful the children ask such questions ...what an opportunity to explain that there are still many mysteries yet to be explained and that is why Australia needs more scientists..simplistic but it may see one of them focus on a future in science.

Needless to say I see that gravity pushes and so of course the Moon will get pushed away.

And you would think that the Moon would slow up and move closer as the kids speculate no doubt, a simple question but not simple to answer. Kids just do that dont they.

But is it not wonderful how kids can ask questions like this.

alex

[ngcles]

Hi Dannat & All,

And ... you know the moment we stop making the repayments on the damn thing the aliens are gonna turn off the gravity, repossess it and tow it away for a mortgagee sale.


I say stop the payments now!!


Les Dalrymple
Contributing Editor
AS&T

[fringe_dweller]

Originally Posted by ngcles

Hi Dannat & All,

And ... you know the moment we stop making the repayments on the damn thing the aliens are gonna turn off the gravity, repossess it and tow it away for a mortgagee sale.

I say stop the payments now!!

yeh they can have it back, its all beaten up and damaged these days

'4 sale - 1 pre loved moon/satellite - has seen better days - renovators delight!'

[fringe_dweller]

Hi Danat, just mention the word 'equilibrium', that, and we see everything in extreme slow motion on our time scale :-) we're living in between the seconds of the universe

and its more than a small window for eclipses - ( as it is there is already differences in eclipses, due to moon's apogee and perigee and inbetween affect on apparent size - also the sun apparent size varies a little due to earths slightly ecliptic/egg shaped orbit ) it is many thousands of years since and into the future, natures grandest show still works, i think at least 4000 years in future from memory?

[robagar]

hi Dannat, short answer - "conservation of angular momentum". The Moon / Earth system has lots of it, which would have to be lost before they collide.

In the future I think tourists will come just to see eclipses. Can't be too many planets in the cosmos with this freaky coincidence in moon & sun sizes

[wasyoungonce (Brendan)]

It is classic circular motion. It is motion is the sum of it's velocity vectors one at a tangent to the motion and the other towards the earth.

Vector addition shows that the change in velocity is always at an angle to it's motion. The change in velocity is acceleration thus it is constantly accelerating at an angle to it's motion.

If it alters motion and moves away from earth it does work against gravity (a change in radius from earth means work is done against gravity by the body and thus the body looses some energy...some velocity, ek (energy kinetic)= 1/2mv^2 thus reduce Ek you must reduce Velocity (V), mass is constant) which will slow the body down altering it's velocity vectors making it move back in towards the earth. edit: errrr actually it will slow down and assume a new (slower) equilibrium of motion (velocity vectors).

If it alters it's motion and moves in it towards earth will gain energy (a change in radius from earth inwards means the body gains some potential energy (from gravity) and thus the body gains some velocity)) and speed up. The energy it had and the potential energy it gained equals new energy level for the body thus new velocity vectors. This again alters it's velocity vectors making it move back out again. edit: errrr actually it will speed up and assume a new equilibrium of motion (velocity vectors).

Thus it is maintaining equilibrium of it's motion.

edit:

edited for clarity.

This is a good question. In fact it's a similar one I posed to a yr 12 Physics teacher...whom couldn't answer it. I asked why a satellite cannot be at any velocity and also be geostationary orbit. In other words why cannot it be at a lower orbit and faster velocity but still be geostationary.

The answer is that at a faster velocity it's vector components will make it move at a tangent to it's current orbit (it has too much energy to remain geostationary). It will move outwards doing work against gravity (loosing energy) until such time as the energy of the satellite equals the energy of it's motion (eK=1/2mv^2)=(Ug=mv^2/R).

In other words the body (bodies) maintain an energy balance between themselves. Any movement of either body will alter this balance and the system bodies will move and re-balance energies. Thus equilibrium!

Hope this helps.

[bird (Anthony Wesley)]

The moon *is* falling towards the earth, but it's going so fast sideways that it's always missing, as the surface of the earth curves away from it at exactly the same rate as it's "falling" toward it.

Imagine throwing a ball as hard as you can horizontally (ignore air resistance and friction). It will travel maybe 30m before hitting the ground.

Ok, now imagine a bullet fired horizontally - it will go a lot further, maybe 500m.

If you could fire or throw something fast enough then it would fall toward the ground at the same rate that the ground curves away from it, and it would go all the way around the earth and hit you in the back of the head :-)

It helps to try this on the moon or somewhere that doesn't have an atmosphere to slow things down :-)

cheers, Bird

[middy]

Originally Posted by bird

The moon *is* falling towards the earth, but it's going so fast sideways that it's always missing, as the surface of the earth curves away from it at exactly the same rate as it's "falling" toward it.

Imagine throwing a ball as hard as you can horizontally (ignore air resistance and friction). It will travel maybe 30m before hitting the ground.

Ok, now imagine a bullet fired horizontally - it will go a lot further, maybe 500m.

If you could fire or throw something fast enough then it would fall toward the ground at the same rate that the ground curves away from it, and it would go all the way around the earth and hit you in the back of the head :-)

It helps to try this on the moon or somewhere that doesn't have an atmosphere to slow things down :-)

cheers, Bird

Someone has been listening to the Astronomy 161 podcasts.

[wasyoungonce (Brendan)]

And because the moon has mass it has gravity, thus the earth is accelerating (falling) towards the moon as well.

But at a much lower rate than the moon to the earth, because of their difference in respective masses.

Makes for interesting orbits.

[middy]

Originally Posted by wasyoungonce

And because the moon has mass it has gravity, thus the earth is accelerating (falling) towards the moon as well.

But at a much lower rate than the moon to the earth, because of their difference in respective masses.

Makes for interesting orbits.

That's right, the Earth and Moon orbit around their centre of mass.

[bojan]

Originally Posted by middy

That's right, the Earth and Moon orbit around their centre of mass.

And the centre of mass of Eart-Moon system is inside Earth, at ~1/3 Earth radius from the centre (if I remember correctly..)

[Blue Skies (Jacquie)]

I don't think anyone has mentioned the Roche limit yet, isn't that also a factor in the situation? The moon is well outside the Roche limit of the earth so it will tend to move away.

[robagar]

I think the roche limit is something else <quick check to wikipedia to refresh memory > ... yes, it's the limiting distance for an orbiting body not to be pulled apart by tidal forces. If the moon was inside the roche limit, we'd have rings not a moon

[Blue Skies (Jacquie)]

Ah, ok I knew the Roche limit had something to do with rings forming - now you mention it it seems rather obvious. oh well

[wasyoungonce (Brendan)]

Ahhh yes very interesting.

Something i did not know...but now do. All stands to reason though.

Thanks gents.