I know that the force of gravity is:

$$F = GMm/R^2$$

Where $G$ is gravitational constant, $M$ is mass of sun, $m$ is mass of the orbiting planet and $R$ is the distance between the center of their masses.

Would I be right to assume that the gravitational pull on the planet due to the sun, changes because the distance between the sun and the planet changes over time?

  • 2
    $\begingroup$ absolutely right. $\endgroup$ – user108787 May 22 '16 at 19:53
  • $\begingroup$ It is, in principle, possible for a body to have a perfectly circular orbit, in which case the force remains constant. $\endgroup$ – WhatRoughBeast May 22 '16 at 21:34

Just to more fully answer your question, here is an example of what differences in distances can mean mean as far as they affect gravitational forces.

The planet Jupiter is extremely massive, and one side of one of it's moons, Io, feels a slightly larger gravitational pull than the opposite side. This difference in distance results in a gravitational force producing distortion of Io’s shape. Io’s orbit is elliptical and the distance between Jupiter and Io constantly changes, meaning that Io undergoes friction-induced heating and results in a lot of volcanic activity on it's surface.

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