Since February 1974 when the final Skylab crew splashed down in Pacific waters, thorough analysis of film and other solar data from the three Skylab missions has produced many significant advances in our understanding of the Sun. Hundreds of articles have already appeared in scientific journals and will become the basis of a modern reappraisal of our nearest and most important star. Many more will follow. This first phase of interpretation has involved not only the corps of solar scientists that took part in the operational phases of the Skylab program, but nearly all the solar observatories on Earth, and many of the world's solar astronomers. Because changes on the Sun are felt on Earth as well, the new solar results have affected areas of Earth science as well as the basic study of other stars.
Among the important results from Skylab's telescopes are new insights into the basic processes of solar behavior, into interplanetary and terrestrial studies, and into the theoretical understanding of magnetic plasmas and atomic spectra. Other important results are told more easily in pictorial form. The remainder of this book displays a number of such pictures. These illustrated results of Skylab are divided into two chapters. Chapter 6 tells Skylab's findings of the quiet, or inactive Sun, and chapter 7 of its more violent and active features. In each chapter the results are divided into several subject areas, selected from the areas of solar physics in which Skylab made significant advances.
The division into quiet and active solar behavior recognizes the fact- known long before Skylab- that the atmosphere of the Sun is ever changing. In the long term, of years and decades, solar behavior follows a cycle of about 11 years, with the last maximum in 1968 and 1969, as shown in the graph of annual averaged sunspot number at center left on the next page. During the 9 months of Skylab's intensive observation of the Sun-an inset circle on the same graph-solar activity was in the long term falling, to a minimum that was reached in 1976-77. The Sun, in the words of solar astronomers, was getting more and more "quiet."
Such descriptions apply only on the average, and although 1973 and 1974 were years of declining action on the Sun, there were days and weeks when the Sun was far from quiet. A daily measure of solar activity during the 251 days of active Skylab operation is shown at center right on the next page. High values indicate days when there were many sunspots and other signs of solar activity; when the plotted values are low, the Sun was quiet. The three periods when Skylab was manned by astronaut crews are black.
Solar scientists had hoped to observe both active  and quiet periods during Skylab, to sample the full range of solar behavior. As we can see, the Sun cooperated fully. When astronauts Conrad, Kerwin, and Weitz first docked with Skylab, on May 25, the Sun was active; solar behavior then calmed during the middle of their 28-day mission and rose again toward its end. During the second manned mission, after an initial burst of solar activity, astronauts Bean, Garriott, and Lousma saw the spectacle of an absolutely quiet solar face for several days. There were no sunspots. Then, in early September, the Sun surged to levels of activity more like the years of maximum in the solar cycle. A daily diary of the Sun showing the changing solar activity during their 59-day mission is shown on the following pages. During the long third mission, Astronauts Carr, Gibson, and Pogue watched the Sun alternate between equal intervals of action and quiet-the solar surface divided down the middle in two halves of opposite character. With them, when they returned to the surface of Earth at Skylab's end, were rolls of film that told of both quiet and active Suns.