I've been thinking about this an awful lot lately but I can't seem to get my head around something. Velocity is how fast an object moves. On Earth if you were travelling 600kmph that means you would cover 600km of distance over the Earth's surface in one hour's time. That much makes sense (if it didn't I'd really be ******ed). But if you look at the Earth in isolation of other celestial bodies (as in, only looking at objects travelling over the Earth's surface) it can be seen as a constant, a fixed surface area around which one can travel. But I've been thinking about how you calculate velocity in space. It seems different because you're travelling through a medium between points. Is speed only relative to objects around you? For example, if you were on a space ship travelling from Earth to Jupiter your velocity could only be determined as how fast you are travelling toward or away from something, as in you were travelling 600kmph away from Earth or 600kmph towards Jupiter. But, consider that objects in space move, and Jupiter is not always the same distance away from Earth at all times. For example, you're in a space ship and your destination is a comet. If you were travelling 600kmph away from Earth in the direction of the comet and the comet was travelling 600kmph away from Earth, would you not then be travelling 600kmph away from Earth but 0kmph towards the comet? Would you not need to travel 1200kmph away from Earth in order to travel 600kmph towards the comet? So, in my mind, velocity can only be calculated by measuring the distance you're travelling to or from an object, but even then you could be travelling at two different velocities depending on which objects you're using. So what about the speed of light? If you had two ships capable of travelling half the speed of light flying away from each other at that speed, would that not mean that each ship is travelling the speed of light in relation to the other? I'm using 600kmph as the velocity just to keep things simple. I realize that in space 600kmph is a relatively slow speed. **edit: Tried to explain how I see it in my mind it a little clearer in light of Jux's comment.