The statement that light travels more slowly through fog than through a clear environment is not accurate. In reality, the speed of light in a vacuum is constant and is approximately 299,792,458 meters per second (or about 186,282 miles per second). This speed is denoted by the symbol 'c' in physics.
However, light can be affected by the medium through which it propagates. In substances such as fog, water, or glass, light travels at a slower speed compared to its speed in a vacuum. This phenomenon is called "refraction," and it occurs due to the interaction of light with the atoms or molecules in the medium.
When we talk about the distance to a planet or any astronomical object in terms of light years, we are not considering the speed of light through the medium of fog or any other intervening substance. Instead, we are referring to the distance that light would travel in a vacuum over the course of one year.
A light year is defined as the distance that light travels in one year, approximately 9.461 trillion kilometers (or about 5.878 trillion miles). It is a unit of astronomical distance, representing the vast distances between celestial objects.
When astronomers estimate the distance to a planet in another solar system, they use various methods based on observations and measurements. These methods include parallax, spectroscopic analysis, and other techniques. They do not directly measure the time it takes for light to travel from the planet to Earth through any intervening medium.
To summarize, the concept of light years and the measurement of astronomical distances are based on the assumption that light travels in a vacuum with a constant speed. The presence of a medium like fog does not affect the calculation of distances in terms of light years to celestial objects.