Rotation of the Sun is generally inferred from the movement of sunspots. It is possible however to determine the speed of rotation directly from the difference in Doppler shift in high resolution spectra taken close to opposite edges of the Sun’s disc.An attempt at this using a LHIRES III spectrograph gave a figure at the equator of 1.90 km/s Close to, but slightly lower than published figures based on sunspots.
While watching the recent partial eclipse in largely cloudy skies I wondered if it might be possible to follow an eclipse even in cloudy conditions in spectra of the sky. The shape of the spectrum line profiles should alter slightly as different parts of the rotating solar disc are covered. (Similar to the Rossiter-McLaughlin effect see in the spectrum of eclipsing binaries) Does anyone know if this type of observation has ever been attempted ?