# Orbits: elliptical and circular

Hi all,

I found a post from last March ('06) but it didn’t answer my question, so I thought I’d start a new post.

I’m in the process of building a variety of astronomy-related educational films. My current project is to show the Moon orbiting Earth, relating to eclipses and the phases of the Moon.

I’ve tried setting the Moon’s center at Earth’s center and using a z-rotation to produce orbital motion. However, the Moon’s orbit is not in the same plane as Earth’s so I’m trying to figure out how to tilt the Moon’s orbit. I can tilt the Earth and the child-Moon will follow. But Earth has a different tilt (23 degrees) than the Moon’s orbit (5 degrees) and in a different direction, so I’m not sure how to produce a tilted Moon and a tilted Earth. Any ideas?

Ideally, I’d like to use a path. This would allow elliptical orbits as well as allow non-uniform speeds. I know how to build an orbit path and get the Moon to follow the path. But I cannot figure out how to get the Moon (or any object) to follow a path repeatedly. Changing the pathlength doesn’t do what I want and I haven’t come up with other solutions. Any ideas?

Anyone else have any nifty ideas for building orbits?

Thanks a bunch. As always, you guys rock.

The Astrogeek

Parent an empty to the earth. Parent the moon to the empty. Tilting the empty tilts the moon’s plane and the empty follows the earth.

To make a circular path repeat you have to make the speed ipo cyclic.

1. open the speed ipo of the curve and add 3 points.
2. in edit mode in the ipo window selet the first point and use “n” to set it to 1,0, second point to 100,1, third point to 101,0
3. exit edit mode and press the “t” key and select linear.
4. set the extrapolation mode to cyclic

GreyBeard

Hi Greybeard,

Thanks a bunch. I found a bit of info on another post about a camera path and figured out how to set up the Speed IPO. It’s beautiful and exactly what I needed.

I wonder if there is a good doc somewhere describing the details of path follow and Speed IPOs. I’ve read/watched many tutorials (including several of your own) and read docs on Blender and hadn’t come across this orbit technique. I’d definitely like to learn more.

Thanks again for your help. I’ll try out the empty method next.

The Astrogeek