atmospheric refraction

Atmospheric refraction. Refraction in the atmosphere causes the path of light from a star to be slightly curved, rather than straight. As a result, the star appears to be at a higher altitude above the horizon than it really is. The real scale of the effect is grossly exaggerated in this diagram. The small change in direction of light rays from astronomical objects as they pass through the Earth's atmosphere, making objects appear to be nearer the zenith than they really are. The degree of atmospheric refraction increases the nearer an object is to the horizon.