[Fyron]

The speed of light is the same here as it is near Jupiter. It is not relative to the nearest large body. That is the point; from all reference frames, the speed of light is the same.

This contradicts what you would expect, given the behavior of matter. eg: If I throw a ball, its speed of travel is just the speed at which I threw it, when we are discussing the reference frame of sitting here an Earth. However, the ball is really moving a lot faster than that. Its absolute speed is the speed at which I threw it, plus the speed of the Earth's rotation, plus the speed of the Earth's orbit around the sun, plus the speed of the suns movement in the galaxy, plus the movement of the galaxy... (though we really should be talking about velocities, not speeds.)

But with light, none of that is relevant; it travels at the same speed regardless of reference frame. Shine a flashlight from the surface of the earth, the light moves at c. Shine a light from (a stable, geosynchronous) orbit, the light moves at c. Shine a light from orbiting Jupiter, where you are moving at a different speed than orbiting Earth, the light still moves at c.