I have only taken my highschool Physics course so excuse me if I don't understand some things right away. I looked at the two stickies and neither seemed to explain my question. I have been trying to understand why the moon's orbit's apogee moves around Earth over a period of time. The moon's orbit isn't quite an ellipse because of the Sun's gravitational pull. I want to understand how the three bodies (the Sun, moon and Earth) influence eachother. I have been told by one person that astro-physicists have yet to understand this and I have been told by someone else that they understand it but are just having a hard time making accurate calculations partly because they have to take into account Einstein's theory of relativity. However, I can't find any articles on the internet that attempt to explain the orbit of the moon in detail and the reasons for the peculiarity in it's orbit.