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.