The light of distant galaxies | at least outside the Local Group | presents to the observer, specially to the spectroscopist, a singular feature discovered by the American astronomer Vesto Slipher (1875-1969) in the early decades of the XX century: the position of the spectral lines of the chemical elements are, in the great majority of cases, systematically displaced to wavelengths larger than those measured for the same elements (at rest) in earthly laboratories. Being the shifts in the direction of larger wavelengths, they are called redshifts because, in the visible solar spectrum, red has the larger wavelength. Such a nomenclature is adopted even when the object's spectrum is outside the visible range. Less frequently, there is also the blueshift, whose denition is analogous to the redshift.