navigator will tell the pilot to fly along.  In general relativity, bodies always follow straight lines in four-dimensional space- time, but they nevertheless appear to us to move along curved paths in our three- dimensional space.  (This is rather like watching an airplane flying over hilly ground.  Although it follows a straight line in three-dimensional space, its shadow follows a curved path on the two- dimensional ground.)

     The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space.  In fact, the orbits of the planets predicted by general relativity are almost exactly the same as those predicted by the Newtonian theory of gravity.  However, in the case of