The motion of astronomical objects due solely to this expansion is known as the Hubble flow. It is often expressed by the equation , with "H"0 the constant of proportionality—Hubble constant—between the "proper distance" "D" to a galaxy, which can change over time, unlike the comoving distance, and its speed of separation "v", i.e. the derivative of proper distance with respect to cosmological time coordinate. (See "uses of the proper distance" for some discussion of the subtleties of this definition of 'velocity'.)