Measurement of Kodaikanal white-light images. I - A comparison of 35 years of Kodaikanal and Mount Wilson sunspot data

Abstract

Areas and positions of individual sunspot umbrae measured in white-light solar images from Kodaikanal and Mount Wilson are compared and shown to be in quite good agreement. The agreement is sufficiently good that it is possible to measure motions and area changes of sunspots from one site to the next, involving time differences from about 12 hrs to about 36 hrs. This enables us to trace the motions of many more small sunspots than could be done from one site alone. Very small systematic differences in rotation rate between the two sites of about 0.4 percent are found. A portion of this discrepancy is apparently due to the difference in plate scales between the two sites. Another contributing factor in the difference is the latitude visibility of sunspots. In addition it is suggested that a small systematic difference in the measured radii at the two sites may contribute a small amount to this discrepancy, but it has not been possible to confirm this hypothesis. It is concluded that in general, when dealing with high precision rotation results of this sort, one must be extremely careful about subtle systematic effects.