The statement that light travels more slowly through fog than through a clear environment might be a bit misleading. In reality, light always travels at the same speed in a vacuum, which is approximately 299,792,458 meters per second (often rounded to 300,000 kilometers per second).
However, light can be affected by the medium through which it propagates. When light passes through a medium like fog or any other material medium, it can experience a phenomenon called "refraction," which can alter its direction and apparent speed. This refraction occurs due to the interaction of light with the atoms and molecules in the medium.
In the case of fog, the water droplets suspended in the air can cause light to scatter and refract, leading to a slower apparent speed. The speed of light in the fog is still incredibly fast compared to everyday objects, but it is slower than the speed of light in a vacuum.
When astronomers measure the distance to celestial objects, such as planets in other solar systems or galaxies, they don't rely on the speed of light in a medium like fog. Instead, they use the speed of light in a vacuum as a constant reference point.
The measurement of astronomical distances, particularly on a large scale, relies on a concept called "light-years." A light-year is a unit of distance equal to the distance that light travels in one year in a vacuum. Since the speed of light in a vacuum is constant, astronomers can calculate the distance to a celestial object by multiplying the speed of light by the time it takes for light to travel from that object to Earth.
So, when astronomers claim that a planet in another solar system is located, for example, 10 light-years away, they are referring to the distance that light has traveled in 10 years at the speed of light in a vacuum. The presence of a medium like fog or any other substance between Earth and the distant planet would not affect this measurement, as long as the medium does not significantly impede the propagation of light or alter its speed in a vacuum.