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The Limber approximation is a formula (the Limber formula or Limber equation) used for relating observation with the density of galaxies at distances, i.e., relating the statistics of the angles on the celestial sphere between galaxies with the statistics of the galaxies' actual intervening distances. It was developed for galaxies, but applies to other distant objects. It incorporates mathematical approximations, making some kinds of analysis tractable, and has been used for the last few decades in cosmological studies of distant galaxies, including analysis of the effects of gravitational lensing. The relativistic Limber equation is a version adjusted to reflect relativity.