What is Relative motion? - Imagine two spacecraft approaching one another. Each spacecraft travels at 20,000 kilometers per hour. Their closing speed, relative to one another is twice as great: 40,000 kilometer per hour. If one scientist on Earth and another scientist on one of the speeding spacecraft were to measure the speed of the light reaching then from the Sun, they would obtain the same figure which is around 300,000 kilometers per second. This example illustrates two points central to Einstein’s theories about relativity: firstly, all motion is relative, and secondly, the velocity of light is always the same.
