Question: Earth orbits the sun all due to gravity. So, is the gravitational pull on Earth continuous and is it effected by centrifugal force- because of it circling the sun ?

Hannah Middleton answered on 8 Nov 2017:

The orbit of the Earth is not quite a perfect circle, it is slightly elliptical. So sometimes we are a little bit closer to the Sun and sometimes a bit further away. Because the strength of gravity is dependent on the distance from a mass (as well as the mass itself), this means that at some times in the year, the Earth experiences slightly less gravitational pull from the Sun than at other times.

As for the centrifugal force, this is what’s known as an “inertial” or “fictitious” force. Inertia is an object’s resistance to motion; the more massive an object, the more difficult it is to make it move – it’s harder to push a full shopping trolley than an empty one. When you’re rotating something in a circle, the object is constantly being pulled inwards to keep going around in the circle. If you imagine swinging a yoyo on a string above your head, then the yoyo will unravel more with each rotation. So surely there must be some force pulling outwards on the yoyo?

But what’s really going on is the yoyo is trying its hardest not to be accelerated because of it’s inertia. If the string were to break, then the yoyo would just fly off in a straight line at a constant speed (forgetting air resistance, which would slow it down). So the centrifugal force is this apparent outward force which results from rotating an object. The actual force acting here is the inward force from the string on the yoyo – that is the centripetal force.

Back to the Earth and the Sun: the Earth is trying to go in a straight line, but it is getting pulled round by the gravitational pull of the Sun. So it feels an outwards centrifugal force similar to the yoyo.