# True\False question: Newton's gravity would explain why Saturn, so far from the Sun, moves so slowly across the sky.

triteamdan | Teacher | (Level 2) Adjunct Educator

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Absolutely True!  Isaac Newton postulated that the force of gravity is a universal force that acts between any two masses in direct proportion to their masses and in inverse proportion to the distance between their centers.  His test case was the moon's orbittal motion around the Earth.  If his theory of gravity is correct, the force of gravity would equal in magnitude the centripetal force that keeps the moon circling the Earth.  Setting the two forces equal to each other yields:  Gm1m2/r² = m2v²/r  where G is the universal gravitational constant, m1 and m2 are the masses of the Earth and moon respectively.  It simplifies to v² = Gm1/r.  Solving for v, he found that it was exactly equal to the speed of the moon!  If you apply the same concept and formulas to Saturn as it orbits the sun, you get the speed of Saturn.  Ta-da! Saturn's slow speed... explained!

Johannes Kepler was looking for patterns and mathematical relationships that described and predicted those patterns.  His great achievement was his three laws as mentioned in the previous post which do in fact predict accurately how Saturn moves in its orbit.  However, he did not EXPLAIN WHY the planets move as they do because he did not understand gravity and how it was involved.  Isaac Newton described how forces accelerate objects and how masses exert gravitational forces on each other.  Again, Kepler used mathematics to accurately described WHAT we saw in the heavens, but it was Isaac Newton who explained WHY the planets and moons move due to the force of gravity.

enotechris | College Teacher | (Level 2) Senior Educator

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Absolutely false! Johannes Kepler (1571-1630) who lived a generation before Isaac Newton, was first to accurately describe the motions of the planets in his Three Laws of Planetary Motion in 1619.

Rejecting the Aristotelian concept of circular motion (in fact, the word "orbit" is Latin for circle) for planets, he instead, using data from Tycho Brahe, realized that planets travel in "flattened circles," or ellipses, which could elegantly account for all observed motions of the planets.

By Kepler's Second Law, ("Equal Areas Equal Time") if you connected an imaginary line to a given planet, the area that line would sweep in a given amount of time is exactly the same as a line drawn to another planet for that same amount of time  In other words, Mercury speeds around the Sun, but it has a relatively short line connecting it to the Sun.  Given the same amount of time, a line drawn to the much distant Saturn would cover the same amount of area as Mercury, but only by moving much more slowly.

The further an orbiting body is from the Sun, the slower it orbits.

See more details and all Three Laws at the link:

Sources:

astrosonuthird | Student, Kindergarten | (Level 1) Valedictorian

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It is false!

astrosonu | Student | (Level 1) Valedictorian

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Hello! This is a silly question. it is false. Scientist ''Johannes Kepler" lived before Issac Newton and he accurately explained the Three laws of planetary motion.

Riddhi don't feel bad! It was not a silly question.