Because there aren't many atoms in space,
there's not much to stop things from moving. For
this reason, the Earth keeps on spinning and
doesn't get much slower. (In truth, it's actually
slowing a little because it's exchanging energy
with the moon, which is why we have tides.) For
the same reason, gravity from the sun is
attracting the Earth towards the sun, and the
Earth is moving quickly, so it causes it to orbit.
Because there is nothing to apply an external
force on the Earth and change its momentum, it
continues in its stable orbit.
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