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. Measuring this to any degree of precision is not straightforward however as the total shift is small ( ~1 part per 10⁵ ), comparable to the stability of the LHIRES III spectrograph.

By aligning the slit along the solar equator however, both edges can be measured simultaneously and any geometric distortion can be corrected for by taking spectra of the sunlit sky (which shows no net shift) before and after, measured at the same places in the spectrograph field. Shifts between the spectra can then be measured to high precision using cross correlation.

After correcting for the difference between the full diameter and the distance between the two measurement points, the surface speed of the photosphere at the equator was measured as 1.90 km/s with an excellent 2 sigma precision of 60m/s or 3% based on the average of 17 repeat measurements. The resulting estimated equatorial sidereal rotation period of 26.7 days is slightly longer than published figure of 25 days based on sunspot movement. The difference is statistically significant at the 3 sigma level based on the measured uncertainty in surface speed but it is not clear if this is real or due to some unidentified systematic error.