Ok, thanks a lot, blue numbered dots below:
1: the path1 to follow: not an issue
2: the path2 that will follow: the path itself not an issue, but the motionpath is an issue (please look at the video at the start of this topic, and also attached here)
3. A fixed rotating line 900px (point N connected to the center: so the N positions are a circle), not an issue to let this rotate but spline 2 has to follow point N
4: the center: not an issue
5: a fixed length line 800px , which follows path1 at point V (blue dot6), pulling/pushing point W (which in se could be seen as path3, which is an 800px offset of path1)
6: 360 points on the path1
7: the distance between N and W never changes= 80px
The latter is my big problem.: N follows a circle-path, N-W= 80 px, AND W follows also a path3 (red spline2 is always relative to N-W)
I was able to let the rectangle follow the path : Motionpath Vawt under construction (codepen.io)
but this follows a linear center of the rectangle: how to do this with spline2, which has to follow the circlepath AND path3(not drawn but easy to calculate) in my case, (which is a simple absolute offset of green path1)
to conclude: two points of spline2 need to follow each another path
remark: the paths need of course geometrical correct (this I calculate) and doable.