This section title is identical to that of the first paper where the broadening of absorption lines in response to stellar rotation was discussed: Abney (1877).
More accurate measurements now reveal also the details of stellar line shapes, making it possible to segregate the signatures of rotational and other broadening mechanisms: e.g. Gray (1992) and Smith & Gray (1976). To determine the rotation, fits can be made to line profiles (e.g. Anders et al., 1993), to their Fourier transforms (e.g. Dravins et al. 1990;Smith & Gray 1976), or to extended spectral regions (e.g. Kurucz et al. 1977).
This review, however, concerns issues for [single-epoch] spectroscopic observations only, no temporal aspect will be discussed.
What is observed is a rotationally broadened profile, the accuracy begins to get limited by the incomplete physical understanding of stellarline profiles and of the nature of stellar rotation. In order to disentangle the rotational broadening from other effects, one needs to know the ‘intrinsic’ (i.e. rotationally unbroadened) profile of the non-rotating star. How does this profile change with latitude and longitude across the stellar disk? What effects besides rotation are broadening the lines? What about mass loss, radial pulsation, non-radial oscillations, magnetic fields, spots, etc.? And the star might not even rotate as a rigid body, but perhaps differentially with respect to latitude and/or atmospheric height. All this has to be deduced from the often blended lines in complex spectra.