Imagine an astronaut being pulled feet-first toward the Earth by gravity.
Newton's laws state that the gravitational force between two bodies is
directly proportional to the square of the distance which separates them;
the further apart they are, the less they attract one another. The force
on her head is therefore smaller than the force on her feet, as her head
is further away. From her point of view, the disparity in gravity makes
it seem like she is being pulled in both directions. Thus within her reference
frame, the effects of gravity are very much observable. These forces are
called tidal forces becuase they are the same forces which cause the ocean
to bulge out toward and away from the moon.