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 \sim 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 _ { sidereal } ( \psi ) = A + B \sin ^ { 2 } \psi ( deg day ^ { -1 } ) , we found that the relation B \simeq - 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 \simeq 14.5 , B \simeq - 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 .