Firstly, just to confirm, the value for velocity in the equations to calculate time dilation is the speed at which two things move away or towards each other? So if some object was moving parallel to the other, they would measure no time dilation on each other? But if one was moving perpendicular to the other, would the first measure time dilation based on there speed or how fast they travel away from them? Secondly, in the time dilation equations what is the velocity based on? I ask because, if something is moving at a speed which display time dilation for someone else, that would also affect the velocity of that thing. If your time slows down 50% relative to the observer than your velocity would then be 50% slower. So how can the velocity of something be used to calculate time dilation when it is constantly slowing down the faster it goes. - Thanks in advance.