  We present a fully relativistic calculation of the observed galaxy number
counts in the linear regime. We show that besides the density fluctuations and
redshift-space distortions, various relativistic effects contribute to
observations at large scales. These effects all have the same physical origin:
they result from the fact that our coordinate system, namely the galaxy
redshift and the incoming photons' direction, is distorted by inhomogeneities
in our universe. We then discuss the impact of the relativistic effects on the
angular power spectrum and on the two-point correlation function in
configuration space. We show that the latter is very well adapted to isolate
the relativistic effects since it naturally makes use of the symmetries of the
different contributions. In particular, we discuss how the Doppler effect and
the gravitational redshift distortions can be isolated by looking for a dipole
in the cross-correlation function between a bright and a faint population of
galaxies.
