Orbit modelling: Earth-Moon Like

Hello

I'm hoping to make a small mechanical demo (~10cm tall) that models an orbit that is similar to the Earth-Moon orbit - that is, only one side of the Moon faces Earth. I'm hoping to keep it simple, so here's my idea so far

where the blue and yellow represent Earth and Moon, respectively. These are meant to represent a different planet and its moon though.

The orbit of the Planet (blue) and its Moon (yellow) has the following differences:

- Planet is eccentric
- Only one face of the Planet and see its Moon, i.e. the horizontal bar on each disc should remain parallel. The bars are for explanatory purposes only.
- Planet's rotation axis may shift (vertically along the green bar), but the Moon's orbital axis is fixed, i.e. Moon should be always in contact with the outer ring housing

My question is: how should the two discs be connected in order to to have the above properties? I've thought about using springs, but I think it'd be quite flimsy.

Thank you for the help in advance!

Reading this might be a good starting point.

Russell

Sure, I know orreries. However, I just wonder if there's a simpler way to achieve something similar in a more compact housing if the two discs can be directly joined up. I'm only trying to model the motion of the objects on one plane, so there doesn't need to be empty space between the discs.

Also, I'm not sure traditional orreries can dynamically adapt/self-adjust to objects shifting axis.

Thanks

John

Edited By John Munroe on 06/02/2016 09:54:12

Edited By John Munroe on 06/02/2016 09:56:05

Edited By John Munroe on 06/02/2016 09:56:21

The moon actually 'librates' and so the exact spot facing the earth changes slightly, you could say ant small errors in you device carefully model this extra motion

Yeah, I'm aware of that. I'm having problem with even just connecting the two discs together!

Fix a long flat bar to the blue disk. The bar rotates with the blue disk.

Put a bush on a round yellow shaft that goes into the yellow disk. The bush is free to rotate on the yellow shaft. The outside of the bush is square. The bush is fixed to the yellow disk. The yellow shaft does not rotate the yellow disk, but moves it in a circular orbit.

Cut a long slot in the flat bar, at the end furthest from the blue disk. The square portion of the bush is free to slide in the slot of the flat bar.

I've no idea if this will work, but it works in my mind at the moment.

Steve

John,

This may be of interest:

**LINK**

MichaelG.

It might be worth you watching https://www.youtube.com/watch?v=nZXjUqLMgxM

It is quite long but worth it. They discovered how the designer had use a gear with a slot to plot the elliptical orbit of the Moon (about 38 minutes in).

I must get hold of some drawings of it!

Regards.

David