Kepler's Laws of Planetary Motion Later, scientists found that this is a consequence of the conservation of angular momentum. The angular momentum of a planet is a measure of the amount of orbital motion it has and does NOT change as the planet orbits the Sun. It equals the (planet mass) × (planet's transverse speed) × (distance from the Sun). The transverse speed is the amount of the planet's orbital velocity that is in the direction perpendicular to the line between the planet and the Sun. If the distance decreases, then the speed must increase to compensate; if the distance increases, then the speed decreases (a planet's mass does not change). Finally, after several more years of calculations, Kepler found a simple, elegant equation relating the distance of a planet from the Sun to how long it takes to orbit the Sun (the planet's sidereal period). (One planet's sidereal period/another planet's sidereal period) 2 = (one planet's average distance from Sun/another planet's average distance from Sun)
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