This is a well established experimental fact: the speed of light in vacuum is constant.
Any measurement will always show that a light beam travels 299,792,458 meters/second (this is pretty close to a billion feet per second, or one foot per nanosecond).
This is true even if the source
of light is moving at high speed towards (or away from) the observers
making the measurements:
In the diagram, the Nostromo is outbound towards the moon at high speed. Midway between the earth and the moon, a small explosion on board the Nostromo releases a flash of light which can be seen by observers both on the earth and on the moon.
Even though the Nostromo is traveling towards the moon, light from the explosion which heads towards the moon takes the same amount of time to arrive as light received on earth.
Snowballs thrown from cars don't behave this way!
