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.