Earth’s Orbit

Earth’s elliptical orbit is not fixed in orientation with respect to the stars. Why not?

Answer

Although the general theory of relativity dictates a precession of the perihelion for all planets, including Earth, this effect is very small compared to perturbations provided by gravitational influences of all the planets. Apparently Earth’s elliptical orbit will pass through a repetitive cycling about every 93,000 years from its present ellipse and orientation with respect to the stars, to a circle, then back to an ellipse with an orientation perpendicular to the present orientation, to a circle again, back to an ellipse perpendicular again, etc., until the present elliptical orientation is recovered approximately. Of course, all the planets are experiencing these perturbation effects simultaneously, so the detailed calculations become quite interesting!