  The double pulsar PSR J0737-3039A/B consists of two neutron stars in a highly
relativistic orbit that displays a roughly 30-second eclipse when pulsar A
passes behind pulsar B. Describing this eclipse of pulsar A as due to
absorption occurring in the magnetosphere of pulsar B, we successfully use a
simple geometric model to characterize the observed changing eclipse morphology
and to measure the relativistic precession of pulsar B's spin axis around the
total orbital angular momentum. This provides a test of general relativity and
alternative theories of gravity in the strong-field regime. Our measured
relativistic spin precession rate of 4.77 (+0.66,-0.65) degrees per year (68%
confidence level) is consistent with that predicted by general relativity
within an uncertainty of 13%.
