  With an aim to examine how much information of solar rotation can be obtained
purely spectroscopically by observing the sun-as-a-star during the 2012 May 21
eclipse at Okayama Astrophysical Observatory, we studied the variation of
radial velocities (V_r), which were derived by using the iodine-cell technique
based on a set of 184 high-dispersion spectra consecutively obtained over the
time span of ~4 hours. The resulting V_r(t) was confirmed to show the
characteristic variation (Rossiter-McLaughlin effect) caused by time-varying
visibility of the solar disk. By comparing the observed V_r(t) curve with the
theoretical ones, which were simulated with the latitude (psi) dependent solar
rotation law omega(psi) = A + B sin^2(psi) (deg/day), we found that the
relation B = -5.5 A + 77 gives the best fit, though separate determinations of
A and B were not possible. Since this relationship is consistent with the real
values known for the sun (A = 14.5, B = -2.8), we may state that our analysis
yielded satisfactory results. This consequence may provide a prospect of
getting useful information on stellar rotation of eclipsing binaries from
radial-velocity studies during eclipse, if many spectra of sufficiently high
time-resolution are available.
