Sometime, they know the way.
Introduction to Modern Astronomy …
For the medieval astronomer/astrologers the Universe was a small place, the Earth was the center, and events in the heavens were orderly and designed to benefit humanity. The only change that was deemed appropriate was cyclic change such as the (mostly) orderly motion of the planets on the sky or the daily travel of the sun around the heavens, for cyclic change returns one to the starting point and so is not really change at all. In Europe of the Middle Ages this belief was elevated to the level of religious dogma, and one dared challenge this world view at considerable personal peril.
However, the Copernican revolution began a long process that changed completely our perception of the heavens and humanity's place in the Universe. Beginning in the 16th and 17th centuries and continuing until today, observations and increased theoretical understanding demonstrated that the Universe is enormous, that it has existed for periods that dwarf human lifetimes, and that we do not occupy the center of the Universe (for there is no center).
Probably less appreciated is a change with antecedents in events observed hundreds of years ago, but that has accelerated at breathtaking pace over the last 30 years. As observational astronomy at wavelengths other than visible light (Radio-Frequency, X-Ray, Gamma-Ray, Ultraviolet, …) has become more commonplace, we have begun to appreciate that the Universe is party to scenes of unimaginable violence.
Far from an orderly stage for stately and gentle physical processes, the Universe at various times and various places undergoes violent cataclysms releasing energy on a scale to numb the mind of even the most analytic physical scientist.
The medieval natural philosopher would perhaps have had even greater difficulty accepting this insight than accepting the Copernican hypothesis that the Earth was not the center of the Universe, for it would have destroyed the strongly held belief that the Universe existed as a nurturing cocoon for humanity.
However, it is supremely ironic that these violent processes that on the surface seem hostile to the place of humanity in the Universe are in fact essential to the production of the present Universe. In particular, our modern understanding is that there would be no matter as we know it, no life as we know it, and no humanity to contemplate these questions, in the absence of violent processes that would, of themselves, destroy all life within countless light years.
The development of these ideas has been a truly remarkable odyssey in the history of human thought. These lectures represent an introduction to how this modern worldview has come about, and a survey of the often beautiful, sometimes astonishing, but never dull, Universe described by these evolving ideas.