Kepler's Three Laws of Planetary Motion:
I. The planets move in ellipses with the sun at one focus.
II. The radius vector of a planet (line adjoining sun and planet) sweeps over equal areas in equal times.
III. The square of the time of revolution (the year) of each planet is proportional to the cube of its mean distance from the sun.
Sir Isaac Newton discovered that the law of gravitation extends to the stars. That is, every mass in the universe attracts every other mass with an attraction directly proportional to the product of the masses and inversely proportional to the square of the distances between them.
Ocean tides are caused by the difference between the attraction of the sun and moon for the main body of the earth and their attraction for different particles of the earth's surface. The tide-raising force of the disturbing body is proportional to its mass and inversely proportional to the cube of its distance. The tides produced by the sun are, therefore, only two-fifths as great as the tides produced by the moon.
The celestial sphere is an imaginary sphere of infinite radius, with the earth at its center, upon which the celestial bodies are considered to be projected for convenience in determining their positions with respect to fixed points of reference in the heavens.