What is meaning of atmospheric refraction ?

A light ray coming from a celestial body is slightly deflected toward the earth when passing obliquely through the atmosphere. This phenomenon is called refraction, and occurs always when light enters matter of different density at an angle smaller than 90°.

Since the eye cannot detect the curvature of the light ray, the body appears to be at the end of a straight line tangent to the light ray at the observer’s eye and thus appears to be higher in the sky. R is the angular distance between apparent and true position of the body at the observer’s eye.