at inertial motion is not absolute but relative has implications for an understanding of the behavior of bodies in motion both on earth and in space. Stwertka and Stwertka say that Newton's major contribution to modern science was the discovery of gravitation and "the relationship between gravity and acceleration" (33). In seeking to explain why the motion of bodies on earth could be controlled and to explain why the motion of celestial bodies, though not provably controlled, was not random, he came to the view that there had to be some governing influence on the motion of bodies both earthbound and celestial. As Sagan sums it up, Newton "was the first ever to figure out that these two phenomena [moon's earth orbit; ordinary gravity of falling objects on earth] were due to the same force" (69). But the behavior of objects in space and on earth affected by gravity was not absolutely identical, then, assuming gravity to be the common influence on them, it must exercise a relative rather than absolute influence. The force of gravity is what is un
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