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This question already has an answer here:

Today one question pop up into my mind... What will happen if Earth spinning about its axis and around the sun will start moving in reverse direction... Whether it will affect us... What are all things will change?

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marked as duplicate by AccidentalFourierTransform, ZeroTheHero, Qmechanic May 4 '18 at 14:30

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The change in direction itself will not effect us at all. To achieve this, you just rotate the system in your head by 180°. There is no global "up" or "down" in the universe, so in free space the direction of rotation is a matter of convention. If, however, you wanted to physically change the direction of rotation/orbit, there are two cases to consider:

If you were to very quickly (or almost instantaneously) change the direction of motion, we would die - everything that is not "fixed" on earth would continue moving on its original trajectory. If the change in motion would be very slow, i.e. the angular velocity would slowly decrease to zero and then increase in the opposite direction, this would be different. We would not smash into the next wall, because the forces acting on us would be small enough for us to "counteract". Days would get longer at first, then shorter again, but the sun would rise on the opposite side of the horizon. For the motion of earth around the sun, there is something else to consider:

On a circular orbit the centripetal force $F_Z$ is equal to the gravitational force $F_G$, so we have $F_Z-F_G = 0$. Since $F\propto v^2$, reducing the velocity of earth will result in a negative net force, so earth would spiral in towards the sun. So if we decelerate too slowly, earth might get too close to the sun, again leading to death and doom.

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  • $\begingroup$ Reversing the direction of rotation and slowing the Earth in its orbit around the sun are different things. Spinning it backwards would have a lot of consequences, but falling into the sun isn't one of them. $\endgroup$ – zeta-band May 4 '18 at 18:41
  • $\begingroup$ I totally agree, but as far as I understood he is interested in both, the earth's rotation about its axis (answered in the question linked as duplicate) and the orbital motion around the sun. $\endgroup$ – scaphys May 5 '18 at 11:00

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