The present book is a translation of the original German edition (published in 1982) with some minor corrections and improvements. The guide to sup- plementary and advanced literature given in the Appendix, however, has been brought up to date. This book is addressed primarily to students taking astronomy as a prin- cipal or subsidiary subject, and to scientists of related fields, but amateur as- tronomers should also be able to profit from it. For most chapters an elementary knowledge of mathematics and physics will be sufficient, however, Chaps. 5 and 6 impose somewhat greater requirements....