The apparent motions of the celestial bodies on the sky are a consequence of their motion in a three-dimensional inertial space combined with that of the observer. The earth is a platform that spins with one rotation per day, orbits the sun once per year, and precesses with a period of 25 770 yr. The motions of the celestial bodies include the orbiting of the planets about the sun, random motions of stars relative to the sun, and the rotations of stars about the center of the (MW) Galaxy with rotation speed dependent on distance from the center.

This results in considerable apparent motions of the stars and planets from our viewpoint. The driving force for all these motions is gravity; it can truly be said that gravity drives the universe. We therefore begin with a brief discussion of the role of gravity in astrophysics. We will describe how the motions of the moon and earth relative to the sun lead to solar and lunar eclipses. Finally, we conclude with a discussion of time standards which play an essential part in studies of celestial motions.

Telescopes Mastery

Telescopes Mastery

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