Question
Air has a small, regularly negligible index of refraction. It is 1.0002926. This causes the Sun to actually be below the horizon when it appears to be just on the edge of sinking below it. Assume you are on the sea-shore watching the Sun actually sinking into the ocean. When only its upper tip is still visible, by what fraction of the diameter of the Sun is that tip actually already below the surface? As estimation, take the earth's atmosphere as being of uniform density out to a thickness of 7.500 km, beyond which there is no atmosphere. This means that, with the Earth's radius being 6400. Km, your line of sight due West along ocean surface to the horizon will intersect this upper surface of atmosphere at about 309.9 km from your eye. (The diameter of the sun subtends 0.5000 degrees at your eye.)