Atmospheric refraction

When looking at something, we not unnaturally assume that what we see is exactly as we see it and precisely where we see it. In almost all situations in which we find ourselves this assumption is perfectly justified because we are close enough to the things that we see for their light to reach us by travelling in a straight line to our eyes through a homogeneous atmosphere.

Over larger distances, however, the atmosphere is not homogeneous because its density is not uniform. As you might expect, the density of air decreases with height because the lower layers are compressed by the weight of those above.