Mercury's orbit is absolutely an ellipse. It is incredibly elliptical, with an apogee of 69.8 million km, and a perigee of 46 million km. The relativistic effe

Mercury's orbit is absolutely an ellipse. It is incredibly elliptical, with an apogee of 69.8 million km, and a perigee of 46 million km. The relativistic effects on Mercury concern its precession - the way that elliptical orbit rotates [1] around the sun. And it's not caused by Mercury's speed (Which is only ~59 km/s at its maximum, compared to the Earth's 30 km/s). It's caused by spacetime being curved by the immense gravitational field of the sun. If Mercury had a circular orbit, it would have no precession. [1] Precession is akin to spinning a hula hoop around your body - with the hula hoop representing an orbit. https://en.wikipedia.org/wiki/Apsidal_precession https://en.wikipedia.org/wiki/Apsidal_precession