This book-like set of lecture notes resulted from a course at the Sterrekundig Instituut Utrecht during the first months of 1999. The motivation for this "nakandidaats" (masters) course was the presence of S.K. Solanki as Minnaert Guest Professor. The lecturers were S.K. Solanki, J.P. Goedbloed, P. Hoyng, and myself. The students were asked to produce lecture notes, including S.K. Solanki's formal Minnaert Lecture for a wider audience, splitting the lectures between them. Their writeups were checked by the teachers and then edited and collected in this volume by J.M. Krijger and me.
This scanned copy uncompresses to hundreds of megabytes postscript in a printer (black & white suffices). Printing the full text takes ages, better do so in parts with the from-to print option.