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View Full Version : Moon's Departing From Us!



Spica__16
2004-May-06, 05:29 AM
Our very own satellite, Moon brings brilliance amidst the darkness that envelopes the night sky.... It may not be that obvious but the Moon actually moves away from the earth at about 3.8 centimetres every year.... I really can't believe this for I know that Earth has this gravitational force that keeps our satellite from getting astrayed from us, plus the centrifugal force (Moon), the force that makes a rotating body tend to pull away from the center which it rotates.. Thus, it's safe to say that Moon could definitely be kept itself in its orbit... Ummm... Having those prevailing forces, how come the Moon has now its tendency to go astray from us???

StarLab
2004-May-06, 06:05 AM
Let's look at it this way: the moon began when an object (Mars-sized) crashed into earth, some earth crap was ejected into space, and formed the moon. In essence, the moon has ALWAYS been going away from us, from when the time it was ejected, since it was ejected AWAY. Of course, the gravity of earth is such that the moon's momentum is decreasing. However, the moon will eventually get so far away such that its own velocity overpowers earth's gravitational power, and the moon will REALLY start moving away!!! :( :o ;) :D

One thing, though: what's the difference btwn. p (momentum) and v(velocity)? :blink: :unsure:

antoniseb
2004-May-06, 09:16 PM
Originally posted by Spica__16@May 6 2004, 05:29 AM
how come the Moon has now its tendency to go astray from us???
The short answer is tides.
The moon lifts tides [both in water and rocks], and the spinning Earth advances the tide a little bit beyond the direction to the moon. The gravity of this leading lump pulls the moon a bit, and allows the Earth to lose some of its rotational energy by giving orbital energy to the moon.

GOURDHEAD
2004-May-07, 12:31 AM
One thing, though: what's the difference btwn. p (momentum) and v(velocity)?

v = time rate of change in position (dx/dt)

p = product of v and mass (p = v*m)

m = some sort of modulation of spacetime

TheThorn
2004-May-07, 02:42 AM
What antoniseb said.

By coincidence, in another thread, I just posted this Link (http://antwrp.gsfc.nasa.gov/apod/ap010818.html) which talks about the exact opposite situation in respect of Phobos. It is slowly losing altitude, and will crash into Mars in about 100 million years, for exactly the same reason as our moon is moving away from us.

Any moon that has an orbital period less than the rotational period of its planet will transfer angular momentun to the planet through tides. The planet will "spin up" and the moon will slowly orbit closer. Eventually it will crash. That's Phobos problem.

Any moon that has an orbital period longer than the rotational period of the planet, the tides transfer angular momentum the other direction - from the planet to the moon. The planet slows down, and the moon moves out. Eventually either the planet's rotation will slow down so much that it will equal the moon's orbital period and the process will stop, or the moon will move out far enough to escape. That's what's going on with our moon, but I doubt that there is enough angular momentum in the Earth's spin to lift the moon to escape distance (probably about 1 million miles), so we're doomed to eventually have days that are over a month long.

Probably not something my kids have to worry about, though.

ASEI
2004-May-07, 03:18 AM
Wouldn't friction or heating losses with our liquid oceans here on earth cause the moon to lose orbital energy through the gravitational field, rather than gain it?

StarLab
2004-May-07, 03:52 AM
so we're doomed to eventually have days that are over a month long.
Big bummer, but not something to worry about terribly even when it DOES happen

TheThorn
2004-May-07, 03:52 PM
Originally posted by ASEI@May 7 2004, 03:18 AM
Wouldn't friction or heating losses with our liquid oceans here on earth cause the moon to lose orbital energy through the gravitational field, rather than gain it?
If I understand correctly (and I'm still learning)....

The friction in the earths oceans and rocks that is caused by the tides causes the Earth to lose rotational energy rather than causing the moon to lose orbital energy. So we rotate more slowly.

But angular momentum also must be conserved. So while we slow down, the moon moves farther out.

I stand to be corrected... (that's how I learn).

Tim Thompson
2004-May-13, 01:18 AM
Try this: The Recession of the Moon and the Age of the Earth-Moon System (http://www.talkorigins.org/faqs/moonrec.html). It is written in response to young-Earth creationist claims that the Earth-moon system cannot be "old". But it does explain in some detail how the tidal interaction transfers energy from Earth to the moon. The current recession rate, about 3.82 cm/year, is not constant. It will decrease as the moon gets farther away, and in about 50 billion years the moon will reach its maximum distance from Earth (I don't know what that distance will be). After that, because of solar tides on Earth, the moon will reverse directions, and move inward towards Earth, just as Phobos approaches Mars. Eventually (maybe another 50 billion years or so), the moon will rejoin Earth. It'll be a honey of a show I guess.

kashi
2004-May-13, 01:31 AM
Of course in 50 billion years, the Earth moon system will be long gone!

Nuradnan
2004-Jun-13, 06:19 PM
If the Moon is going away from Earth, then how is the effect to the Earth movement? I mean, is there any change in rotational period, the orbit curve, etc, caused by this thing? Well, I don't know why (I'm still learning), but I saw in Discovery Channel that it will give disastrous effect on Earth. Any explanation?

GOURDHEAD
2004-Jun-13, 06:57 PM
Of course in 50 billion years, the Earth moon system will be long gone!

How so?

The sun should be a white dwarf (possibly black dwarf if it is no longer luminous) and perhaps still the most massive member of the solar system, Jupiter may have gained enough mass from accretion to light up (highly speculative) as may have Saturn (assuming we or our progeny haven't disassembled them), and the earth-moon system should have put on a lot of mass also by accretion (maybe a jovian mass or so). The moon may have been transformed into something like Saturn's rings but less icy. As the moon approaches the earth, it will impart angular momentum to the earth which will act to slow down the approach and eventually cause the moon to recede thus participating in something not unlike damped oscillation.

On the other hand, if the rate of cosmological expansion continues to increase (as opposed to oscillate), the earth-moon may be the size of the milky way and quarks the size of a basketball--all right golfball or small marble (hyper-speculative).

Back to the sun and setting aside cosmological expansion and the sun becoming a tidy morsel for some interloping black hole, might it accrete enough mass over 50 billion years to regain luminous status? Our progeny can argue whether the sun has a carbon (diamond) center mediated to some extent by their advanced knowledge and more voluminous data base as well as their lack of memory of us.

StarLab
2004-Jun-13, 10:55 PM
Well, ignoring the obvious idea of a possibly dense sun core, when the sun becomes a dwarf star - well, y'all should know what that means...a nova explosion unlike all others...whether or not it would be powerful to rip the moon away from the earth, I cannot assess...

galaxygirl
2004-Jun-13, 11:17 PM
The sun doesnt have enough mass to go supernova. Instead, it will become a red giant (and engulf the earth moon system), then become a planetary nebula, and go on to its white to black dwarf phase. Like Kashi said, we will be long gone before 50 billion years.

Tom2Mars
2004-Jun-13, 11:42 PM
Can we remove material from the moon and reduce its mass in such a way as to change its orbital velocity so the the orbit will remain somewhat stable?

kashi
2004-Jun-14, 06:47 AM
Originally posted by Tom2Mars@Jun 14 2004, 10:42 AM
Can we remove material from the moon and reduce its mass in such a way as to change its orbital velocity so the the orbit will remain somewhat stable?
It's not high on my "to do list".

damienpaul
2004-Jun-14, 09:19 AM
maybe not your list kashi...

I am intrigued, is what Tom said at all possible?

rahuldandekar
2004-Jun-14, 12:44 PM
It may be , but where would we keep the extra mass? We would have to make it orbit around the earth, Since if we keep it on the earth, the earth's mass would increase, and maybe it's rotational motion would slow down at a faster rate, making the moon's orbital radius increase faster. And we would have to constantly keep reducing it's mass, until the rate of cahnge of orbital velocity is zero.
Or we could just take a lot of mass out of the moon, so that it starts coming towards us, and then put in back on after some years so that it starts moving away. We could repeat this process and make the moon perform sort of oscillations.


If the rate of cosmological expansion continues to increase (as opposed to oscillate), the earth-moon may be the size of the milky way and quarks the size of a basketball--all right golfball or small marble (hyper-speculative).

I don't think that the size of any object in the unuiverse increases. These objects are bound by forces stronger than the expansion force. the earth will remain the size of the earth, but it will be smaller compared to the universe ( as opposed to the same size with respect to the universe).

GOURDHEAD
2004-Jun-14, 03:27 PM
I don't think that the size of any object in the unuiverse increases. These objects are bound by forces stronger than the expansion force. the earth will remain the size of the earth,

How can we be sure that cosmological (spacetime) expansion does not operate within gravitational or quantum chromodynamic fields above a certain intensity or determine what such intensity is?

The earth, moon, Mars, and every object in the universe above a certain (unknown to me) mass, let's arbitrarily pick the moon, even though I'm sure smaller masses pertain, are constantly increasing in mass due to accretion. The earth , for example, gains several thousands kilograms per day by vacuuming out dust and gas from interplantetary and interstellar space (as the whole solar system orbits the galaxy). It also collects a few comets and asteroids now and then. There are also losses from the occasional collision, but on net each item constantly is being increased in mass. In most cases this will be accompanied by an increase in volume. Increase in size is a certainity from gravitational effects and not yet ruled out from cosmological expansion.

The distance between the earth and the moon is controlled more by transfers of energy by exchanges of angular momentum between the rotation of the earth on its axis and the orbiting of the moon about the system than by adding and subtracting mass to and from the moon.

StarLab
2004-Jun-14, 03:44 PM
Whatever the case, let&#39;s be sure our descendants don&#39;t pick or mine the moon to pieces...imagine the tidal impact on Gaea&#33; The better solution our great-great-grandkids should take is just making sure the moon never ends up colliding with earth, or getting closer, for that matter. :o :D :rolleyes: <_< :ph34r: :unsure:

Tom2Mars
2004-Jun-15, 01:25 AM
rahuldandekar-
but where would we keep the extra mass?

I was just thinking we could make some space habitats out of the mass, a lot of really big habitats.

galaxygirl
2004-Jun-15, 01:34 AM
How would we take large quantities of mass off the Moon in the first place?

Tom2Mars
2004-Jun-15, 05:00 AM
GalaxyGirl...Well, considering what Gourdhead said-
The distance between the earth and the moon is controlled more by transfers of energy by exchanges of angular momentum between the rotation of the earth on its axis and the orbiting of the moon about the system than by adding and subtracting mass to and from the moon. and also remembering about the astronaut on the moon doing Galileo&#39;s experiment with the feather and hammer masses dropping at the same rate(in a vacuum on the moon&#33;) and how that relates to orbiting being like free fall, maybe removing mass off the moon won&#39;t keep the moon in a stable orbit.

But that shouldn&#39;t stop us from being able to remove Lunar Mass with:

Electromagnetic Mass launchers*

Chemical Propulsion using electrolyzed* Lunar polar water(ices) in the form of Hydrogen and Oxygen.

Lunar Space Elevator*

And, making some fun things anyway&#33;



(*)Of course powering as much as possible with Solar generated electricity.

B)

antoniseb
2004-Jun-15, 10:05 AM
Originally posted by Tom2Mars@Jun 15 2004, 05:00 AM
Lunar Space Elevator*
A lunar space elevator would have some advantages over a terestrial one in that there is no weather to ruin the bottom of it, and little space junk to cut through it. It would have a big problem: How high an orbit would it need to be in lunar synchronous orbit? I think the elevator would have to be a quarter million miles long with a quarter million mile extension for the counter-weight, going through either L2, L4 or L5.

GOURDHEAD
2004-Jun-15, 01:02 PM
How high an orbit would it need to be in lunar synchronous orbit? I think the elevator would have to be a quarter million miles long with a quarter million mile extension for the counter-weight, going through either L2, L4 or L5.

Since the earth is in approximate synchronous orbit with respect to the moon, we would probably have to locate the elevator above the moon&#39;s surface that faces away from earth to avoid unpleasant gravitational interaction with the earth. Maybe we could use a relatively much heavier counter weight and avoid the extra quarter million miles of structure. Which L is diametrically opposite from the direction to earth?

Sp1ke
2004-Jun-15, 11:34 PM
How about putting the space elevator between the earth and the moon? That would give you two large objects attracting it so could that keep it in place? If you weighted each end accurately, would it stay in place? Or would the gravitational forces or relative movement mess things up?

I guess it&#39;d be easier to keep it clear of earth&#39;s atmosphere to reduce sideways forces. Maybe start with it based on the moon then have a counterweight outside the earth&#39;s atmosphere. Then it&#39;s just a "short" jump to the start and an easy ride the rest of the way to the moon.

Tom2Mars
2004-Jun-17, 02:13 AM
antoniseb-
How high an orbit would it need to be in lunar synchronous orbit?How high an orbit would it need to be in lunar synchronous orbit?

I guess I don&#39;t understand the dynamics. So, an elevator at the Moon with lower gravity would be longer than one at Earth(with a higher gravity). Is that because the rotation of the Earth comes into play? :blink:

antoniseb
2004-Jun-17, 10:50 AM
Originally posted by Tom2Mars@Jun 17 2004, 02:13 AM
So, an elevator at the Moon with lower gravity would be longer than one at Earth(with a higher gravity). Is that because the rotation of the Earth comes into play?
A space elevator needs to extend both up and down from a synchronous orbit of the body in question. Yes having a giant counter-weight would reduce the extension required above the synchronous orbit. The elevator can only be held aloft if the centripidal acceleration of the counterweight exceeds the gravitational attraction for an object that does not move relative to a fixed spot on the planet. Because the moon spins about a thirtieth as fast as the Earth, its elevator would have to be much longer.

L2 is the place opposite the other large body. It is an unstable balance point in the Earth Moon system, but could be maintained for this purpose.

It would take some new technology to have the elevator go to a spot between the Moon and the Earth since the Earth Moon distance changes by about 10% over the course of the month.