How do you measure relative velocity/?

Question

The velocity of light relative to the sun? and relative to the earth?

if light bends it has changed direction is so why not its velocity?

Answer 1

Use the relative velocity formula:

V(b) = V(a) + V(b/a)

or:
V(light) = V(sun) + V(light rel. to sun)

and rearrange:
V(light/sun) = V(light) - V(sun)

You have to know the speed of light, first of all. Then you have to know whether the sun is considered an inertial non-moving reference point. If it's not, you have to know the velocity of the sun.

The same thing goes for the earth. You have to know its tangential velocity in its orbit at the point you're looking at.

Answer 2

For starters, there is no such thing as light velocity since light is not a particle, so you should use the term speed to talk about it's wave propagation speed.

In relativistic physics, when astronomers or physicists evaluate light's speed, they compare readings from the same light source at different points to analyze how it is affected by bodies like the sun for instance. Kind in mind that light goes the same speed everywhere and if you were on the Sun it would appear to go the same speed as if you're on Earth.

The difference can only be noticeable by an external observer.

Answer 3

In every reference frame, light always travels at the same speed...the speed of light.
The velocity of light relative to the sun is the same as the velocity of light relative to Earth is the same as the velocity of light relative to anything else in the universe.

The speed of light in a vacuum is DEFINED to be EXACTLY 299,792,458 m/s.
There is no need to measure the relative velocity of light, we know it by definition in all reference frames.