I have recently heard of Apsidal Vectors. I was searching about it through internet. I had found [the video](https://www.youtube.com/watch?v=gjSdIwOsnic) in YT. I had found similar question in [PF](https://www.physicsforums.com/threads/apsidal-vectors.187586/). But, the PF answer wasn't clear to me. I am just going to state what I understood from that YT video.
If I think of a elliptical or any kind of circle, and imagine a center line (related to centroid) in that elliptical than that center line is Apsidal Distance or line.
It might be related to following center line.