An Etymological Dictionary of Astronomy and Astrophysics

In the → Sobolev approximation, the length over which the conditions of the gas do not change and
the approximation is valid. It is expressed by: l_s = v_th/(dv/dr), where v_th is the thermal line width and (dv/dr) the velocity gradient. In other words, the length over which the profile function of a line is shifted through a distance equal to its own width by the macroscopic velocity gradients that exist in the moving medium.

See also: → Sobolev approximation; → length.