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<!--Some redshifts are an example of the [[Effectus Doppler|Doppler effect]], familiar in the change of apparent pitches of sirens and frequency of the [[Sonus (physica)|sound waves]] emitted by speeding vehicles. A redshift occurs whenever a light source moves away from an observer. A special instance of this is the [[Cosmologia|cosmological]] redshift, which is due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million [[Annus luce mensus|light years]] away) show redshift corresponding to the rate of increase in their distance from Earth. Finally, gravitational redshift is a [[Relativitas generalis|relativistic]] effect observed in electromagnetic radiation moving out of gravitational fields. Conversely, a decrease in wavelength is called blueshift and is generally seen when a light-emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift.