# Moon's path/orbit - is it changing?

1. Sep 15, 2009

### fawk3s

Hi

Today, in my physics class, a guy brought out something interesting.

We were talking about gravity and somehow the talk got to the moon. I have always thought that our moon's path is constant. That its not changing. Yet that "guy" said that every year (or so) the moons path has changd about 1-3 mm.
Im abit confused, since all the gravities are constant. I would understand IF
1) an object with the needed gravity to affect the moons path, and which is not yet affecting the moon, would start affecting it
2) every year different planets in our solar system would affect the moon differently (different speed on revolving around the sun, ending up affecting the moon gravitationally from different places).

Our teacher was also abit confused, but said he wouldnt start arguing about it since he hasnt hear of it really.

So my questions would be: Is the moons path changing? And if it is, why?

fawk3s

2. Sep 15, 2009

### sylas

Yes, the Moon's orbit is getting larger, by about 3cm per year. This caused by tidal interactions. The tides on Earth act to slow down the Earth's rotation, and (conserving angular momentum) the Moon is also pulled along in its orbit just a little bit; enough to move it further out. The tides cause a transfer of energy and angular momentum from the Earth to the Moon. Much more energy is dissipated within the Earth, as heat.

This has been discussed in several threads here. See, for example: [thread=67691]"why is the moon drifting into space"[/thread], or just search the forum for "tides" for many other threads on the subject.

One of the experiments left behind on the Moon by Apollo 11 was a mirror for accurate range finding, to measure this more precisely.

Cheers -- sylas

3. Sep 15, 2009

### D H

Staff Emeritus
I can understand students being confused by this, but a physics teacher? The status of science education in the US education reaches another low.

Ok. Got that off my chest. Carry on with the main topic.

4. Sep 15, 2009

### mgb_phys

Most school physics teachers are biologists.

In the UK a survey by the Royal Society of Chemists found science classes were taught by people with a first degree in
Biology 44%
Chemistry 25%
Physics 19%
No-science 8%

And across schools 1% had no biology teacher, 12% no chemist and 26% no physicist.

ps. Nothing against biologists, but there are a lot of maths-free 'life sciences' courses that count as biology for a science specialist teacher. So the kid was probably being taught orbital mechanics by a gym teacher with a degree in sports science or chiropody.

5. Sep 15, 2009