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Recessional velocity
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<p>Recessional velocity is the rate at which an <a href="/facts/Extragalactic_astronomy/WtKKEWru">extragalactic</a> <a href="/facts/Astronomical/0ArJVM1p">astronomical</a> object recedes (becomes more distant) from an observer as a result of the <a href="/facts/Expansion_of_the_universe/f6Uvt79B">expansion of the universe</a>. It can be measured by observing the wavelength shifts of <a href="/facts/Spectral_line/HuI4Bj5f">spectral lines</a> emitted by the object, known as the object's cosmological redshift. </p>