MEASUREMENT OF KODAIKANAL WHITE-LIGHT IMAGES .1. A COMPARISON OF 35 YEARS OF KODAIKANAL AND MOUNT-WILSON SUNSPOT DATA

A program of digitization of the daily white-light solar images from t he Kodaikanal station of the Indian Institute of Astrophysics is in pr ogress. A similar set of white-light data from the Mount Wilson Observ atory was digitized some years ago. In both cases, areas and positions of individual sunspot umbrae are measured. In this preliminary report , comparisons of these measurements from the two sites are made. It is shown that both area and position measurements are in quite good agre ement. The agreement is sufficiently good that it is possible to measu re motions and area changes of sunspots from one site to the next, inv olving time differences from about 12 hours to about 36 hours. This en ables us to trace the motions of many more small sunspots than could b e done from one site alone. Very small systematic differences in rotat ion rate between the two sites of about 0.4% are found. A portion of t his discrepancy is apparently due to the difference in plate scales be tween 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 m ust be extremely careful about subtle systematic effects.