Atmospheric Refraction

Atmospheric refraction is the bending of light or other electromagnetic radiation away from a straight line as it passes through the atmosphere, caused by air density changing with height and light slowing as it moves through denser air. Near the ground it produces mirages and can raise, lower, stretch or compress the apparent shape of distant objects even without a full mirage, and turbulent air makes distant objects twinkle or shimmer; because blue light bends more than red, high-resolution observation of astronomical objects can show them smeared into a small spectrum. It also lifts celestial objects higher in the sky than their true position, which is why sunrise and sunset are timed to when the upper edge of the sun appears at or vanishes from the horizon, even though the actual position of the sun is already below it at those moments.