SOLAR ACTIVITY IN 1955

Abstract

During 1955, observations of the sun were obtained at Mount Wilson on 333 days. Of the 32 completely cloudy days, only five were in the six months May through October. No spots were seen on the sun on 38 of the days on which observations were made; a long series of 20 spotless days occurred from March 9 through 28 inclusive. January 1955 was the first month in the new cycle with no spotless days ; the last month of the old cycle with no spotless days was September 1953. The total number of sunspot groups observed at Mount Wilson in 1955 was 208, compared with 46 in 1954, and 93 in 1953. Ten years ago at a similar phase in the cycle, the number of groups in 1945, 1944, and 1943 was 220, 72, and 89, respectively. The northern hemisphere was again the more active with 127 groups compared with 81 in the southern hemisphere. Two of the 208 groups certainly belonged to the old cycle, the last one having appeared on November 24. The average latitude of the new cycle groups in the northern hemisphere was 26? 6 and in the southern hemisphere 25? 1. In 1954 the corresponding latitudes were +27?7 and 26?5. Solar activity has increased during 1955 at a rate slightly greater than at the same phase in the previous cycle. This rapid increase since the minimum in April 1954 is shown in Figure 1. The first long-lived active region of the new cycle began its activity in April 1955 in latitude about 25° south. A total of eight groups, two of which were quite active, occurred in this region. Chromospheric activity was in evidence on every return from April through November. Monthly means of the number of groups observed daily at Mount Wilson in the last three years are given in Table I.