# Speed astronauts measure moving at the speed of light

Lets suppose a spaceship travels with v = 0.9c relative to the Earth. The time inside the spaceship would pass slower than on Earth. Would the astronauts measure a different speed (that means, a different one that the observer on Earth does) in relation to the same reference frame (Earth)?

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Speed of what? What are these astronauts measuring? – Chris White Jan 12 '13 at 1:32
@Chris White: I think OP is essentially asking Is the velocity of $A$ relative to $B$ the opposite of the velocity of $B$ relative to $A$? – Qmechanic Jul 7 '13 at 9:21

$$v=\frac{v_1+v_2}{1+\frac{v_1v_2}{c_0^2}}$$
So, if $v_1$ is different, $v$ is also different. So, the spaceship will observe a different speed, of the flying garbage bin.
The time inside spaceship would pass slower than Earth and distance measured by astronauts would be less than distance measured by Earth scientists. As $speed=distance/time$, they would observe different speed of something other than light. Speed of light would be observed same by all.