From what I understand (and also assuming that the mass of one body is much greater than the other), if we had a body performing a circular orbit, and we put more (or less?) energy into the orbit by changing the speed of the body, the orbit will become more eccentric, i.e. elliptical. My question is: If this is the case, then why is it so, instead of the orbit remaining circular but with a different radius? I have a feeling that I sort of have the answer, but I'm not entire sure about it: That would be the case if the acceleration was at a certain angle, with a radial component. Is this right? I also have another question relating to elliptical orbits: since the speed of bodies in elliptical orbits isn't constant, it implies that the body is always being accelerated. With Earth itself being in an elliptical orbit, shouldn't we be able to measure this acceleration? Is it because its magnitude is too small (since the Earth's orbit is huge and rather circular anyway), or is there some other reason? Thanks!