Solar rotation studies using sunspot data (1967-1974)

The solar rotation rate during 1967-1974 was measured from photographic observations of sunspots. The rates derived from isolated single spots and from bipolar groups were 14.38 +- 0.02 and 14.71 +- 0.05 deg per day equatorial sidereal, respectively. Year-to-year fluctuations in the bipolar group rates correlate with fluctuations in the Mt. Wilson spectroscopic rotation rates, while the isolated single spots show smaller, uncorrelated variations. A possible explanation for the fluctuations in the bipolar rates is year-to-year changes in the separation rates of the bipolar groups, rather than changes in the global solar rotation rate. The latter interpretation requires caution because (1) the sunspot rotation rates were derived from a limited amount of data (one month per year) and (2) the rotation rates were reduced to equatorial values assuming a differential rotation law ωsub(cq) = ω(phi) + 2.77 sin2phi. (orig.)