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How Does Gravity & Inertia Keep the Planets in Orbit Around the Sun?

Gravity and inertia. Those are two of the many words that took on new depths of meaning after Isaac Newton was through with them. A related area that Newton also turned his considerable talents to was the motion of the planets. Newton fit the planets into a universe that operated the same way in the heavens as it did on the Earth. He formulated laws about the motion of physical bodies on Earth, then applied those laws to the heavens. His theories have seen some adjustment over the centuries, but for all practical purposes, Newton said everything that needed to be said about the motion of planets.

Inertia
- If he made this kick in an empty vacuum, the ball would travel forever according to Newton's first law of motion.
  
  "Every body continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state of motion by forces impressed upon it." Those are Newton's words for what is now known as Newton's first law of motion. This is often restated as "a body at rest stays at rest and a body in motion stays in motion." It defines what we commonly call inertia. If a soccer ball is kicked into space, in a vacuum far from any other masses, it will keep going and going and going.
Force and Motion
- "The change in motion is proportional to the motive force impressed and is made in the direction of the straight line in which that force is impressed." This statement, which we now know as Newton's second law of motion, says that the only way to change a body's motion is to apply a force to it. Or, put another way, if you apply force to an object, you will change its motion. Take that soccer ball and put it in a vacuum but within a couple hundred million miles of a massive object. The gravitational force between them will change their motion.
Gravity
- Gravity pulls this man towards the center of the Earth and also pulls the Earth towards the center of the man --- just a few billion times less powerfully.
  
  Newton stated that all objects with mass exert an attractive force on each other proportional to the product of their masses and inversely proportional to the distance between them. What's more, that force is on a line connecting the center of mass of the two bodies. This means that if you hold the soccer ball and let it drop, it will fall directly towards the center of the Earth. But it also means that if you kick the soccer ball horizontally, the Earth's gravitational attraction will not affect the ball's horizontal motion.
Orbits
- Put it all together. Take the soccer ball in space, all by itself, and make it six billion times heavier. Set it moving in a straight line. It will keep going forever. Now put it about 93 million miles away from an object that's a million times heavier than it. Line it up so that the heavy object is at a right angle with respect to the direction of the soccer ball's velocity. From Newton's laws, we know that the velocity perpendicular to the force of gravity will not change, but gravity will pull the soccer ball towards the big mass.
Planets
- Everything that orbits balances horizontal inertia with vertical gravitational acceleration.
  
  The huge soccer ball represents the Earth, and the big mass 93 million miles away is the sun. Because of gravity and inertia balancing each other, instead of traveling in a straight line, the Earth moves a little bit towards the sun with each moment. But it also keeps its perpendicular velocity, so it also moves "sideways" to the sun with each moment. If the velocity perpendicular to the sun equals the velocity the sun adds to the Earth, then the orbit will be perfectly circular. If it's a little off, the orbit is elliptical. If an object's initial velocity points more towards the sun, then you get parabolic or hyperbolic orbits, like some comets.

Astronomy