The Ionospheric Institute of the National Observatory of Athens has had two interests in recent years: the study of the ionosphere and the study of the sun. In our previous Advanced Study Institutes in 1960, 1961, and 1962, we have em phasized the ionosphere. For the Advanced Study Institute of 1964, however, we invited Dr. Jules Aarons of the Air Force Cambridge Research Laboratories to collaborate in preparing and directing a program of studies of the sun, the moon, the planets, and the interplanetary medium. The lectures of this Advanced Study Institute form essentially an advanced course...