Atmospheric refraction

Refraction changes the direction of a wave when it travels through regions with different refractive properties. Earth's atmosphere is not uniform: its density and refractive index generally decrease with altitude. As a ray of light from space passes through these layers, it bends gradually rather than at a single sharp boundary. For light arriving from a celestial object, the curved path makes the object appear in a direction somewhat closer to the zenith than its true geometric direction. The angular difference is called astronomical refraction.

The apparent displacement is small for objects high in the sky and grows as they approach the horizon, where their light travels through a longer, more oblique path in the atmosphere. Near the zenith it approaches zero. The exact amount depends on the vertical profile of temperature, pressure, humidity and air density, as well as wavelength. Refraction close to the horizon is especially sensitive to local atmospheric structure, so a standard correction is an average rather than a precise prediction for every observation.

Near the horizon, refraction raises the apparent position of the Sun or Moon relative to its geometric position. Standard sunrise and sunset calculations account for both this effect and the apparent radius of the solar disk; consequently, sunrise is conventionally listed when the Sun's upper edge appears at the horizon, even though its center is still geometrically below it. Refraction can also alter the apparent shape of a low celestial disk, and changing air conditions can make its appearance fluctuate.

Astronomers correct observed positions for refraction when accurate celestial coordinates are needed. The correction is most reliable for objects well above the horizon and becomes more uncertain at very low altitudes because atmospheric conditions vary along the long path through the air. Measurements of refraction also have scientific value: radio signals from spacecraft passing behind a planet can be bent by that planet's atmosphere, and the resulting changes help researchers infer atmospheric structure and composition.