Anisotropic effective redshift and evolving clustering amplitude
  A typical galaxy survey geometry results in galaxy pairs of different
separation and angle to the line-of-sight having different distributions in
redshift and consequently a different effective redshift. However, clustering
measurements are analysed assuming that the clustering is representative of
that at a single effective redshift. We investigate the impact of variations in
the galaxy-pair effective redshift on the large-scale clustering measured in
galaxy surveys. We find that galaxy surveys spanning a large redshift range
have different effective redshifts as a function of both pair separation and
angle. Furthermore, when considering tracers whose clustering amplitude evolves
strongly with redshift, this combination can result in an additional
scale-dependent clustering anisotropy. We demonstrate the size of this effect
on the eBOSS DR16 Quasar sample and show that, while the impact on monopole is
negligible, neglecting this effect can result in a large-scale tilt of $\sim
4\%$ and $\sim40\%$ in quadrupole and hexadecapole, respectively. We discuss
strategies to mitigate this effect when making measurements.
