This is a followup to another thread I started, on the current status of Newton's theory of gravity as compared with Einstein's. I asked this further question there in the last reply I made, but nobody answered, so I started this thread to ask it here. Newton's theory as I said there is still accurate enough to plot orbits in the solar system, and is in fact used for this because GR is so complicated to use. I think that Newton uses the geometry of flat Euclidian space and GR uses a geometry that handles curved space-time. 1. Now if someone were plotting a course for a ship traveling millions, or hundreds of millions, of light years to another star, would Newton's theory be able to handle it accurately? Or would the use of GR be necessary? 2. What I'm really trying for here is to learn how geometry applies in the real universe. Let's modify the above so that the ship is heading from Earth to a planet millions of light years away which is somehow traveling exactly parallel to ours in every way (motion around its star, motion of its star in its galaxy, motion of its galaxy, etc.) so that it can be thought of as in our reference frame. Could a straight line be drawn (let's posit instantaneously) between Earth and that planet, or must that line necessarily be curved? Please indulge me here, because although a layman, I am really trying to conceive what geometries mean in the real universe. I am like the Flatlander in that Euclid's space seems natural and real to me. Curved space-time, although I understand its structure and implications, is a little in the realm of the fantastic for me.