When a space ship accelerates to 1.000 kmh, the effect of time dilation is tiny. But what happens if this space ship accelerates in an inertial frame whose total speed is much slower or whose time goes much faster, will the time dilation effect be the same? If the spaceship travels in a inertial frame that has a speed of 1.000.000 kmh an additional speed of 1.000 kmh is just 0,1 of the total speed. But if the speed of the inertial frame is 1.000 and the spaceship travels with an additional speed of 1.000 kmh, that doubles the total speed. Will the time dilation be the same? I think that time dilation is: Total speed of the observer through the total speed of the spaceship So in the first example, 1.000.000 / 1.001.000 = 0,999 (that is how much time goes by in the spaceship) For the second example 1000 / 2000 = 0,5 (that is how much time goes by in the spaceship) Wouldn't it be possible based of the amount of time dilation to determine how fast the earth is moving?