# Is the moon is moving away from us? or is the universe just expanding?

1. Jul 21, 2009

### bwana

is the moon is moving away from us?....or is the universe just expanding?

The 'measurement' of the radial expansion of the moon's orbit is 3.8 cm/yr
http://curious.astro.cornell.e...uestion.php?number=124 [Broken]

but that measurement is only slightly more than the hubble constant
(the rate that the universe is expanding)

Here is my calculation:
Hubble expansion
=70.8 km/sec / Megaparsec
=70.8 km/sec / 3.08x 10^19 km
= 2.3 km/sec / 1 x 10^18 km
= 2.3 m/s / 1 x 10^18 m

moon's radial expansion / dist to earth
3.8x10^-2 m/yr / 4 x 10^5 km
yr=365x24x60x60 sec
3.8x10^-2 m/3.15x10^7 sec / 4 x 10^5 km
3.8x10^-2 m/s / 1.26 x 10^13 km
3.8 m/s / 1.26 x 10^15 km
3.02 m/s / 1 x 10^18 m

The radial expansion of the moon was measured using the cube mirrors left by Apollo on the moon. Very simple. ---didnt they really just measure the Hubble constant? Isnt the moon really just moving away much more slowly than we measured because of the hubble constant?

This calculation is posted in memory of the 40th anniversary of homo sapiens visit to the moon.

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sorry i dont have references for this basic knowledge at my fingertips, but i did read it somewhere and dont remember the source.

Last edited by a moderator: May 4, 2017
2. Jul 21, 2009

### Sourabh N

Re: is the moon is moving away from us?....or is the universe just expanding?

The concept of expansion of universe, everything moving away from everything, doesn't make sense on such small scales as Earth-Moon (or for that matter, neither on the Earth-Sun scale, or even between Sun and nearby star scale). These systems are bound by gravitational relationships, making the effect of cosmological expansion negligible.

The real reason for the radial expansion is gravitational pull between Earth and moon and such.

Add : And obviously, hubble constant appears in cosmological expansion.

3. Jul 21, 2009

### Bob S

Re: is the moon is moving away from us?....or is the universe just expanding?

One possible reason for the moon moving away from the Earth is conservation of angular momentum of the Earth-moon system. Angular momentum of the Earth's rotational angular momentum plus the moon's angular momentum has to be conserved, even as the the moon-induced tides on the Earth dissipate the Earth's rotational energy. As the day on the Earth gets longer from the tidal energy loss, the moon has to take up the lost momentum of the Earth's rotation by moving away from the Earth.

4. Jul 21, 2009

### mgb_phys

Re: is the moon is moving away from us?....or is the universe just expanding?

As Sourabh says the Hubble expansion doesn't apply on the Earth Moon scale.

The moon is moving away from Earth at a couple of cm/year. the effect of the moon on Earth causes tides which slows the rotation of the Earth , in order to conserve angular momentum the moon has to move further away. the same effect has also slowed the moon's own rotation so that it always points the same face toward Earth.

5. Jul 21, 2009

### bwana

Re: is the moon is moving away from us?....or is the universe just expanding?

It seems there are two different issues being tossed around here in this word salad. First is the issue of the the coincidental similarity of the growing distance between the earth and the moon and the growing distance of objects in space. Second is the issue of the mechanism of the growing distance of the earth and the moon.

I did not consider why the earth moon distance is enlarging. Many of the posts above seem to address that interesting fact. Although conceptually, I have a hard time understanding why, now that you bring it up. Since it is a closed system, then a larger moon orbit requires more energy. The source of this energy is postulated to be a decreasing rotational velocity of the earth. Now, I have not done the calculation to determine whether the decrease in earth's momentum can completely account for the increased energy in a larger moon orbit. But it is an interesting one that has curious implications. What if the moon orbit is growing faster than can be accounted for by the slowing earth rotation?

But getting back to the point, which is, the Hubble constant has been measured and is slightly less than the expanding space between the earth and the moon. (According to my calculation above) Is my calculation wrong? If so, please point out where. If the calculation ignores a significant variable that alters the result, that would be helpful as well. If the calculation is correct, then similar rates of distance increases should be identifiable for the planets to earth as well. Too bad we dont have mirrors on those celestial bodies as well. Or have we put them there? Although there are other ways to measure interplanetary distances, it would be important to use the same technique to avoid introducing uncontrolled variables.

Alternatively, if the calculation above is correct, and the earth moon distance is not growing as fast as it should, then the Hubble constant would have to be negligible over short distances (earth to moon). It would therefore NOT be a constant, rather, it would be a function of distance-which is not the case based on the data found to date. Multiple measurements using different methods come up with similar numbers for the hubble constant.

Why do you say that the hubble constant is not valid over small distances? Just because large distances were used to measure it? Or are you saying that the sizes of the earth and the moon are also increasing as per the space expansion dictated by the hubble constant-thus the space between the two remains unaffected by this mechanism?

6. Jul 21, 2009

### negitron

Re: is the moon is moving away from us?....or is the universe just expanding?

It works like this: The tidal force of Moon's gravitation acting on the Earth pulls up a tidal bulge. Since the Earth is rotating much more rapidly than the Moon can orbit, the planet's rotation pulls the tidal bulge ahead of the Moon. The added mass of the bulge leading the Moon pulls on it gravitationally, imparting energy and resulting in an ever-increasing orbital radius; at the same time, the friction caused by the Earth rotating through the tidal bulge results in a continually-decreasing rotational period.

For your theory to work, we should see the Moon's rate of recession from us gradually increase in speed, but in point of fact we see the exact opposite.

7. Jul 21, 2009

### map19

Re: is the moon is moving away from us?....or is the universe just expanding?

Expansion according to the Hubble factor is easily overridden by the graviataional attraction of planets to suns, satellites to planets, and suns to their galaxy. Similarly at an atomic scale the Hubble factor is totally insignificant compared with electrostatic attraction and weak and strong forces.
http://en.wikipedia.org/wiki/Moon

8. Jul 21, 2009

### Janus

Staff Emeritus
Re: is the moon is moving away from us?....or is the universe just expanding?

The Earth loses more than enough to account for the increase in the moon's orbit.
Let's use an analogy. You have two people standing on a tile floor in their stocking feet. The floor is expanding. So just standing in place means that the two will move apart. But what if they grip hands? The strength of their grip is much stronger than the friction between their socks and the floor, so they don't move apart.

The same is true for the Earth and Moon, or the suns that make up galaxies; the gravity holding them together is much stronger than their tendency to move apart due to the expansion of the Universe, thus they don't.

9. Jul 21, 2009

### bwana

Re: is the moon is moving away from us?....or is the universe just expanding?

wow....just found some five year old threads that address this question as well...

Interestingly, the moderator (marcus) said this is one of the most asked questions here on pf. And I thought I had an original question--woe is me.

Elsewhere the question rises with regularity as well.

http://math.ucr.edu/home/baez/physics/Relativity/GR/expanding_universe.html

The explanation offered is always the same-space expands but atoms do not. Planets do not. Solar systems do not. nearby galaxies do not.

No redshift for us.

We are bound by the strong nuclear force, the electrostatic force, the gravitational force and it is postulated that these forces keep 'us together'. And we conclude this because redshift occurs for far away stars.

10. Jul 21, 2009

### sylas

Re: is the moon is moving away from us?....or is the universe just expanding?

The moon's orbit is not growing faster than can be accounted for by slowing Earth rotation. Just the reverse, in fact! The slowing of the Earth involves far more energy, which is dissipated as heat. Only a small fraction goes into the outward movement of the moon.

This has nothing to do with cosmological Hubble flow.

11. Jul 21, 2009

### George Jones

Staff Emeritus
12. Jul 22, 2009

### bwana

Re: is the moon is moving away from us?....or is the universe just expanding?

Thank you for the succinct replies. Again I read,

"My understanding is that a galaxy does not expand, even possibly contract due to gravitation.
However the space expansion is quite universal, so applys to each space in a galaxy. But in a galaxy after each tiny expansion between objects, they adjust their motion due to gravity, and we know in this case gravity influence is much much greater than space expansion in a galaxy distance. Therefore we can ignore space expansion within a galaxy."

The only problem I have now is understanding the energy balance. If space is expanding then the kinetic energy of everything in orbit is increasing. For objects to remain at their orbital locations, they have to shed their energy. Electrons do they same thing when they return to their 'proper' orbitals after they have been excited-they emit a photon. So what is it that atoms. planets, solar systems, and galaxies emit to remain in their gravitationally bound, spatially consistent array? gravity waves? dark energy?

13. Jul 23, 2009

### Chronos

Re: is the moon is moving away from us?....or is the universe just expanding?

Newtonian physics is sufficient to explain the moon's retreat from earth over time. You are attempting to play chess with dominos, bwana.

14. Jul 24, 2009

### Ich

Re: is the moon is moving away from us?....or is the universe just expanding?

Better read the post directly below the one you quoted.

15. Jul 24, 2009

### map19

Re: is the moon is moving away from us?....or is the universe just expanding?

bwana, there's no adjustment, they just don't move in the first place. The analogy of two people in socks on an expanding floor is excellent.
With the Hubble expansion there's no question of increase energy caused by the expansion. there's no cosmological reference frame to measure kinetic energy. If you measured kinetic energy of planets, for example, from Earth, and then from Mars, you get two sets of different figures. besides, if kinetic energy were increased by Hubble expansion it would stuff the whole energy balance of the Cosmos.