Question

How do scientists know how long it takes light to reach the earth from the sun?

Answer 1

Assuming they know the distance `x` from the earth to the sun, (in reality around `1.44xx10^8km`) they can calculate the time `t` it takes light to travel from the sun to the earth since the speed of light is a constant `c=3xx10^8m//s` through air and vacuum and so they can use the formula that `t=x/c`.
In reality this comes to around 8 minutes.