Question about the speed of light https://www.youtube.com/watch?v=CYv5GsXEf1o&t=4m54s
The speed of light is constant as long as you are measuring it from a reference frame moving at a constant velocity, such as earth
What do we have to measure it from a reference frame moving at a constant velocity ? Isn't the speed of light always constant?
I don't really understand..Please explain.
Also sorry if this question seems dumb, im a 10th grader..
 A: Two points:


*

*The speed of light is not constant for inertial reference frames - it is easy to show that the speed of light relative to the observer varies with the speed of the observer. But since this is not an answer to your question, I am not going to elaborate.

*According to general relativity, the speed of light is variable for non-inertial systems. For instance, the speed of photons falling towards the source of gravity DECREASES - their acceleration is NEGATIVE (in the gravitational field of the Earth the acceleration is -2g). This is a fudge factor which is absurd but indispensable - without it the gravitational time dilation fabricated by Einstein in 1911 would be incompatible with the gravitational redshift. See more discussion here:
http://www.speed-light.info/speed_of_light_variable.htm 
 "Einstein wrote this paper in 1911 in German. [...] ...you will find in section 3 of that paper Einstein's derivation of the variable speed of light in a gravitational potential, eqn (3). The result is: c'=c0(1+φ/c^2) where φ is the gravitational potential relative to the point where the speed of light c0 is measured. Simply put: Light appears to travel slower in stronger gravitational fields (near bigger mass). [...] You can find a more sophisticated derivation later by Einstein (1955) from the full theory of general relativity in the weak field approximation. [...] Namely the 1955 approximation shows a variation in km/sec twice as much as first predicted in 1911."
Speed of light in a gravitational field? 
 (John Rennie's answer)
