Only two kinds of motion of geodetic importance affect: the rotation of the Earth which causes diurnal aberration, and the revolution of the Earth, which causes annual aberration. The part of the aberration attributable to motion of the source alone is known as planetary aberration

Aberration, annual

The apparent change in direction of stars, planets, and other celestial bodies, caused principally by the changing direction in which the Earth is moving, in its revolution about the Sun, with respect to the line connecting the Earth to the celestial object

Aberration, diurnal

The apparent change in direction to a star or other celestial object caused by the combination of the velocity of light and the velocity of the observer on the rotating Earth

Aphelion

The point in an orbit where a planet is at its furthest distance from the Sun. Opposite of perihelion

Apogee

The point in its orbit where the Moon is at its furthest distance from the Earth. Opposite of perigee

Apollo

The American space program in the late 1960's and early 1970's which sent six successful manned missions to the surface of the Moon

Astronomic azimuth

The angle measured to the right from the vertical plane through the celestial pole and the observer to the vertical plane through observer and the observed object

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