Hubble found that most galaxies are receding from us by measuring the redshift produced by this effect, which is also the basis for radar guns.
At relativistic velocities, the transverse form of this effect becomes appreciable and it is used to identify spectroscopic binaries. C. H. D. Buys-Ballot verified its existence in 1845, three years after its namesake predicted it. Hubble found that most galaxies are receding from us by measuring the redshift produced by this effect, which is also the basis for radar guns. For 10 points, name this effect in which the perceived frequency of a wave changes if the source or observer are moving relative to one another.
At relativistic velocities, the transverse form of this effect becomes appreciable and it is used to identify spectroscopic binaries.
{Doppler} effect or shift [prompt on {redshift} before mentioned]