This has probably been answered before, but I couldn't find it online. So two spaceships leave Earth in opposite directions. Suppose they are both traveling away from Earth at .4c. From one of the ships, how fast would the other appear to be moving away? And then, same question, except both ships are going .8c away from Earth. If it would be impossible for the other ship to be moving 1.6c away, how fast would it be moving away (ignoring Earth)?