Gravity, and the conservation of momentum -
you have to apply a force to stop something from
moving, or cause something to change direction.
The sun exerts gravity that pulls the planets
toward it, but the planets are already moving, so
the pulling causes them to circle the sun rather
than just fall in.
Rotation is a consequence of the conservation
of angular momentum - you have to apply a torque
in order to slow down and stop something from
spinning. Nothing is applying such a torque on
planets, and they were spinning when they formed,
so they're still spinning now.
The reason why planets, such as Earth orbit
around the sun is because of the pull of the sun's
gravity. And the reason why it travels around the
sun is due to Earth's velocity which is in the
direction perpendicular to the force of the sun's
pull. If we didn't have a sun, the Earth would
just travel in a straight line. Imagine if
superman were to throw a football with all his
strength, it would keep going straight across the
Earth which circles the Earth and will eventually
hit him, that is basically what we are doing to
the sun, we are the football and the earth's
velocity is superman's strength. And the strength
is hard enough so that gravity doesn't pull the
football down to the ground. I am not sure, but I
think the reason why we rotate is due to the Earth
being at some certain angle.
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