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**"Kepler's Laws"**Three fundamental laws of planetary motion first observed by Johannes Kepler in the late sixteenth century. The laws state: First Law: The orbit of a planet is an ellipse with the Sun at one focus. Second Law: The radius vector sweeps over equal areas in equal time intervals. Third Law: The period of a planet squared is proportional to the orbital distance cubed. What this law means is that the further you are from the Sun, the longer it takes to orbit it.

Kepler proposed three laws of planetary motion based on his analysis of Tycho Brahe's long and detailed observations of the orbits of the planets. The laws are: The planets orbit the Sun in elliptical orbits, with the Sun as one common focus. The line between a planet and the Sun (the radius vector) sweeps out equal areas in equal periods of time (sometimes called the Law of Equal Areas). The square of a planet's period, , is directly proportional to the cube of its average distance from the Sun, : T2 âˆ r3. This law, the Law of Periods or Harmonic Law also applies to other orbital systems from the moons of Jupiter through to binary star systems. Astronomers use it to calculate the masses of stars in binary systems.

rules for the orbital motion of planets or anything else bound by gravity. The law of most interest here is that the square of the orbital period is proportional to the cube of the orbital separation, and inversely proportional to the mass. Thus, if we see an orbital period, we can estimate the mass or orbital separation and therefore constrain the mass and radius of a neutron star.