# Thread: Why doesn't the moon crash into the earth, or the earth into the sun?

1. ## Why doesn't the moon crash into the earth, or the earth into the sun?

If gravity is pulling on the moon and holding it in orbit around the Earth, then what is the moon doing (or anything else) that is stopping the moon from colliding into the earth, or the earth into the sun?

2. If you have a ball on a string, and you hold the other end of the string and spin the ball around your head, you are pulling on the ball with the string. Why doesn't the ball come and hit you on the head?

3. For a circular orbit, the orbital rotation rate is such that the centrifugal force (yes, I know, a pseudo-force. I'm looking at things in a rotating frame) balances the force of gravity. Or, in a non-rotating frame pov, the curvature of the path produced by the gravitatonal force is insufficient to have the orbit intersect the surface of the planet.

Actually, the stability of an orbit gets trickier when you include other bodies. It may be that the orbit of our moon is unstable in the longest possible terms due to the gravitational influence of the sun and jupiter, or due to drag produced by our oceans leaching energy.

4. They're moving. The direction of movement at all times would actually take them right past the objects they're orbiting instead of into them. Gravity only manages to balance this just enough to keep the them from flying past and then getting farther and farther away from then on.

Think of centrifugal force, the thing that makes your hands or something you're holding feel heavier if you spin yourself around, or spin a small object around yourself on a stick/string. Centrifugal force makes the thing seem to be trying to pull away from the center of spin. The reason is that momentum can only be along a straight line, so it's always different from any curve, and the difference between the straight momentum vector and the curved travel path is the centrifugal force's vector.

Because all objects would travel the straight line if no other forces acted on them, any curved travel path must be a balance between the object's momentum and some other force that causes its path to curve. If the object is a rock on a string that you're spinning around yourself, that extra force that it takes to curve the rock's path is applied by you pulling on the string to keep it from falling away while you spin it; if you let go, that force isn't there anymore, so the rock travels a straight path after that. And we all know a moon's or planet's orbit is curved, so what's the other force involved here, the equivalent of the string? Gravity.

An orbit is a balance between two opposite "forces": gravity pulling the planet/moon in, and the planet's own momentum/movment perpendicular to gravity (including the centrifugal force that this movement generates). If the planet/moon isn't moving fast enough (all other things like masses and distance being equal), it falls in. If it's moving too fast (all other things like masses and distance being equal), it goes right by and drifts away. An orbit is the Goldilocks solution between those extremes.

5. Work through the thought prosess... slowly. This planet Earth is Orbiting the solar mass, ( SUN ) It has only 365.25 Days to do this. Knowing the distances involved and the rate of Orbital velocity allows us to calculate the actual real 'speed' of planet Earth. Also knowing the actual mass of this planet allowed a very clear image of what is actually happening to be made. This planet is in a perfectly stable orbit of the parent star The Sun.
The same can be said for Earths moon. However the tidal effect has been observed to have a destabilizing influence and given time the moon will slowly recede away from us to a point where the solar eclipse will no longer be total. This has been calculated to be at a rate if approximately 30cm per year. Consequentially the moon will move away from earth eventually but, by the time this actually happens the sun will have begun its own destabilizing demise and will engulf the inner planets anyway. 4.5 to 5.5 billion years into our future.
Forget the experiments with bits of string... Its called gravity. It dictates that any object that has mass will have gravity. That gravity will attract objects to move toward each other is a fact. Orbital velocity prevents the collision you seem to be so concerned about.

6. Order of Kilopi
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Mark,

antoniseb's question about spinning a ball on a string is an
excellent place to start "working through the thought process",
and actually gets right to the reason gravity doesn't cause the
Moon to come crashing down on the Earth or the Earth to fall
into the Sun.

My own web page about how speed keeps things in orbit:

http://www.freemars.org/jeff/speed/index.htm

-- Jeff, in Minneapolis

7. I've got a question. Just what are they teaching in schools nowadays?
Last edited by Occam; 2007-Jan-14 at 09:32 AM. Reason: removed superfluous question mark

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Newton's laws of motion were taught in 11th-grade physics when
I was in high school. The poster of the original post is in 9th or
10th grade, if I recall correctly.

-- Jeff, in Minneapolis

9. ## Tidal Forces

If we ignore tidal forces, then nothing prevents the moon from colliding with Earth, and nothing prevents Earth from falling into the sun. But, then again, nothing is forcing the moon into Earth, nor is anything forcing Earth into the sun. So, since there is no reason for the moon to fall into Earth, or Earth to fall into the sun, there is nothing to prevent, which is why we don't need anything to prevent it from happening. The fact that something is in orbit around something else is an indication that its motion (momentum), and the force of gravity, are in balance. So, in principle, it will just keep orbiting forever, or until something comes along to disturb the equilibrium. But this is all true, if and only if, we pretend that all of the masses are true point masses. Real objects are not, and their real extended sizes, and real uneven mass distributions, imbalance the equilibrium via tidal forces.

Tidal forces between Earth & the moon currently cause the moon to move away from Earth, not fall towards it. At the moment, the moon recedes from Earth at a measured rate of 3.82±0.07 cm/year. In about 50 billion years, the moon will reach it's maximum distance, where both Earth & moon will be tidally locked (each will always show the same face to the other). If Earth-moon were a 2-body system, it would stop there. But it's not. It's really a 3-body system, Earth-moon-sun (it's really a many body system Earth-moon-sun-planets, but we can safely ignore planets in this case). Because of solar tides, once the Earth-moon system is locked, the moon will start its journey back to Earth. After another 50 billion years or so, the moon will indeed collide with Earth. It just takes a long time to happen. For more, see The Recession of the Moon and the Age of the Earth-moon System. The Martian moon Phobos is already on the return trip, and is bound to collide with Mars, but I can't remember the timescale.

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Tim,

You posted about tidal forces because ASEI and astromark brought
up that subject. But with respect to the original question, you said:
Originally Posted by Tim Thompson
nothing is forcing the moon into Earth, nor is anything forcing
Earth into the sun. So, since there is no reason for the moon to
fall into Earth, or Earth to fall into the sun, there is nothing to
prevent, which is why we don't need anything to prevent it
from happening.
That is obviously untrue-- Gravity is pulling the Earth and Moon
and Sun together, as said in the original post. I don't understand
why you said the opposite. Hey??

-- Jeff, in Minneapolis

11. I quite like the idea that an orbit is a object falling towards the centre of what it is orbiting, but moving fast enough sideways to keep on missing.

Originally Posted by Jeff Root
You posted about tidal forces because ASEI and astromark brought up that subject. But with respect to the original question, you said:

Originally Posted by Tim Thompson
nothing is forcing the moon into Earth, nor is anything forcing Earth into the sun. So, since there is no reason for the moon to fall into Earth, or Earth to fall into the sun, there is nothing to prevent, which is why we don't need anything to prevent it from happening.
That is obviously untrue-- Gravity is pulling the Earth and Moon
and Sun together, as said in the original post. I don't understand
why you said the opposite. Hey??
Well, i did say, in the very next sentence ...

Originally Posted by Tim Thompson
The fact that something is in orbit around something else is an indication that its motion (momentum), and the force of gravity, are in balance. So, in principle, it will just keep orbiting forever, or until something comes along to disturb the equilibrium. But this is all true, if and only if, we pretend that all of the masses are true point masses. Real objects are not, and their real extended sizes, and real uneven mass distributions, imbalance the equilibrium via tidal forces.
I meant it in that context. Of course gravity pulls the moon to Earth, and Earth to the sun. However, that pull is balanced by the orbital momentum. So, in the absence of any additional forces, aside from the point source gravity, there is no reason for Earth to fall into the sun, nor for the moon to fall to Earth.

13. Please feel free to correct this, but it is to the best of my understanding; the moon is actually pulling away from the Earth at about 2.5 cm/yr

14. Originally Posted by closetgeek
Please feel free to correct this, but it is to the best of my understanding; the moon is actually pulling away from the Earth at about 2.5 cm/yr
Yes. It's actually a bit more than the value you quoted. See Tim Thompson's post in this thread.

15. how did I not see that?
Originally Posted by Hamlet
Yes. It's actually a bit more than the value you quoted. See Tim Thompson's post in this thread.

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Originally Posted by Occam
I've got a question. Just what are they teaching in schools nowadays?
Virtually nothing. I just recently graduated high school (last year), taking my first physics class as a senior. It was truly the best class I had ever taken, and when I went home each day I was inspired to read virtually everything related to science. The realization sunk in, most of our text books in schools give satisfying answers- not correct ones. Now I am an autodidact, fully embracing the power of self-education over the reliance of a perceived 'expert.' That's not to say guidance isn't needed, however...

To answer the thread question in the most simplest terms... What prevents the moon from colliding with earth? Nothing, it already did. Now it is slowly distancing itself from our gravitational pull.

17. What bothers me about the difference is that I learned that from a question on an ask an astronomer page. The person asks "I heard that the moon moves away from the Earth at about 2.5 cm/yr. Why is that happening?" The professor answers the question why, but never corrects the distance. I have gone under that assumption ever since.

http://www.astronomy.com/asy/default.aspx?c=a&id=2195

Originally Posted by Hamlet
Yes. It's actually a bit more than the value you quoted. See Tim Thompson's post in this thread.

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Originally Posted by closetgeek
What bothers me about the difference is that I learned that from a
question on an ask an astronomer page. The person asks "I heard
that the moon moves away from the Earth at about 2.5 cm/yr.
Why is that happening?" The professor answers the question why,
but never corrects the distance.
Two possible reasons spring to mind, and both are likely.

First, the figure of "2.5 cm" is an approximation of "an inch".
"An inch" is a good approximation of the correct value. The
difference between "an inch" and 3.82&#177;0.07 cm isn't of much
consequence.

Second, the person answering the question may not have
noticed that the number provided in the question is off.
Another thread here had a figure of 8 seconds for the time
required for light to travel from the Sun to the Earth, and
several posts went by before I happened to catch the error.
I almost posted a reply without noticing that the unit should
have been minutes, not seconds. 500 seconds or 8 minutes,
20 seconds is almost exact.

-- Jeff, in Minneapolis

19. Originally Posted by Jeff Root
Two possible reasons spring to mind, and both are likely.
...
Or the third is: Yes, we may silently agree, or not care at this point, that the numbers are off, but lets address the important issue before we cloud the answer with nitpickiness.

20. Hey nitpicking is always good. It provides a great distraction from the bigger issues.

Originally Posted by NEOWatcher
Or the third is: Yes, we may silently agree, or not care at this point, that the numbers are off, but lets address the important issue before we cloud the answer with nitpickiness.

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Originally Posted by Knowledge_Seeker
If gravity is pulling on the moon and holding it in orbit around the Earth, then what is the moon doing (or anything else) that is stopping the moon from colliding into the earth, or the earth into the sun?
It's the finger of God.

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The finger of whom?

23. What is the distance from earth that the moon will break free of earths orbit? Or begin its decent back to earth?

Originally Posted by Crux Australis
The finger of whom?
Nope, they don't teach that in schools anymore either.
Last edited by daxloves; 2007-Feb-10 at 07:34 AM. Reason: Spelling

24. Originally Posted by daxloves
What is the distance from earth that the moon will break free of earths orbit? Or begin its decent back to earth?
I'll take a quick stab at it since it's layman's understanding. I'm sure it will prompt some good clarification from some experts.

Given the nature of an orbit, there is no "break free" point between two bodies. Now add a third influence and you change things a bit. The two should continue to orbit until a third body exerts a stronger pull than the first.
In Earth-Moon, that would be the sun, and the point at which that happens would be around L1 which is about 1.5M km.

That's my short answer, corrections and nitpickies always welcome.

25. What is the 3rd force that is currently causing the moon to move away form Earth at an inch a year?

26. Tidal dragging.

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The Moon's gravity raises a tidal bulge on the Earth. Because the
Earth is spinning faster than the Moon goes around it, the bulge
moves ahead of the point directly under the Moon. A balance is
established such that the bulge stays a constant distance ahead
of the Moon. The gravitational attraction between the bulge and
the Moon pulls the Moon forward, increasing its angular momentum,
and pulls the bulge back, decreasing Earth's angular momentum.

-- Jeff, in Minneapolis

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