The second edition of this popular text provides undergraduates with a quantitative yet accessible introduction to the physical principles underlying the collection & analysis of observational data in contemporary optical & infrared astronomy The text clearly links recent developments in ground- & space-based telescopes observatory & instrument design adaptive optics & detector technologies to the more modest telescopes & detectors that students may use themselves Beginning with reviews of the most relevant physical concepts & an introduction to elementary statistics students are given the firm theoretical foundation they need New topics including an expanded treatment of spectroscopy Gaia the Large Synoptic Survey Telescope & photometry at large redshifts bring the text up to date Historical development of topics "ations emphasize that astronomy is both a scientific & a human endeavour while extensive end-of-chapter exercises facilitate the students' practical learning experience