There are at least two fields of study where a spectroscopic precision of 10 cm s$-1 is needed in astronomy. One involves the detection of exoplanets via the changing radial velocity of the parent star. The other, at the other end of the astronomical scale, is cosmology. Due to the expansion of the Universe, which is small but detectable over a decade or so, one might expect the redshift z to change by about $10-10 paper in MNRAS.