1. A spaceship is moving directly toward a planet at a speed of c/2. When the spaceship is 4.5E8 m from the planet a pulse of light is emitted by someone on the planet. As measured by someone on the spaceship, how long does it take the light pulse to travel from the planet to the ship? 2. I'm sure there are equations, but this question (from the explanation) relies upon Einstein's 2nd postulate of special relativity: That the speed of light is a universal constant regardless of the motion of the source or observer. The book states that because of this the occupants on the ship will still measure the speed of light to be c even though they are moving. As such the time it takes the pulse to travel 4.5E8 will be 1.5s given c=3E8 m/s 3. I accept that the speed of light is constant at 3E8 m/s. However my problem lies with the fact that the ship is moving TOWARDS the planet at half that speed. After 1 second, the pulse of light would have traveled 3E8 m as the beam will travel at the speed c, however the ship would have also moved towards the planet 1.5E8 m as it is also traveling towards the planet. After 1 second, wouldn't the ship and the pulse of light meet each other? I'm not sure if the book isn't taking into account the distance the ship moves or if I'm just not getting it.